diff --git a/toolchain/check/impl.cpp b/toolchain/check/impl.cpp
index 6393d654fe72..db7854887abf 100644
--- a/toolchain/check/impl.cpp
+++ b/toolchain/check/impl.cpp
@@ -198,9 +198,10 @@ static auto BuildInterfaceWitness(
   }
 
   auto table_id = context.inst_blocks().Add(table);
-  return context.AddInst(SemIR::LocIdAndInst::NoLoc<SemIR::InterfaceWitness>(
+  return context.AddInstReusingLoc<SemIR::InterfaceWitness>(
+      context.insts().GetLocId(impl.definition_id),
       {.type_id = context.GetBuiltinType(SemIR::BuiltinInstKind::WitnessType),
-       .elements_id = table_id}));
+       .elements_id = table_id});
 }
 
 auto BuildImplWitness(Context& context, SemIR::ImplId impl_id)
diff --git a/toolchain/check/member_access.cpp b/toolchain/check/member_access.cpp
index 5f90056aff54..d65a59f01b57 100644
--- a/toolchain/check/member_access.cpp
+++ b/toolchain/check/member_access.cpp
@@ -196,11 +196,10 @@ static auto PerformImplLookup(Context& context, Parse::NodeId node_id,
   auto subst_type_id = SemIR::GetTypeInSpecific(
       context.sem_ir(), interface_type.specific_id, assoc_type.entity_type_id);
 
-  return context.AddInst(
-      SemIR::LocIdAndInst::NoLoc<SemIR::InterfaceWitnessAccess>(
-          {.type_id = subst_type_id,
-           .witness_id = witness_id,
-           .index = assoc_entity->index}));
+  return context.AddInstReusingLoc<SemIR::InterfaceWitnessAccess>(
+      node_id, {.type_id = subst_type_id,
+                .witness_id = witness_id,
+                .index = assoc_entity->index});
 }
 
 // Performs a member name lookup into the specified scope, including performing
diff --git a/toolchain/check/testdata/function/builtin/method.carbon b/toolchain/check/testdata/function/builtin/method.carbon
index c3f8f1da32fb..7154329e135e 100644
--- a/toolchain/check/testdata/function/builtin/method.carbon
+++ b/toolchain/check/testdata/function/builtin/method.carbon
@@ -73,10 +73,10 @@ var arr: [i32; 1.(I.F)(2)];
 // CHECK:STDOUT:   %.loc19_16: i32 = int_literal 1 [template = constants.%.6]
 // CHECK:STDOUT:   %I.ref.loc19: type = name_ref I, %I.decl [template = constants.%.1]
 // CHECK:STDOUT:   %F.ref: %.3 = name_ref F, @I.%.loc12_40 [template = constants.%.4]
-// CHECK:STDOUT:   %.1: %F.type.1 = interface_witness_access @impl.%.1, element0 [template = constants.%F.2]
-// CHECK:STDOUT:   %.loc19_17: <bound method> = bound_method %.loc19_16, %.1 [template = constants.%.7]
+// CHECK:STDOUT:   %.loc19_17.1: %F.type.1 = interface_witness_access @impl.%.loc15, element0 [template = constants.%F.2]
+// CHECK:STDOUT:   %.loc19_17.2: <bound method> = bound_method %.loc19_16, %.loc19_17.1 [template = constants.%.7]
 // CHECK:STDOUT:   %.loc19_24: i32 = int_literal 2 [template = constants.%.8]
-// CHECK:STDOUT:   %int.sadd: init i32 = call %.loc19_17(%.loc19_16, %.loc19_24) [template = constants.%.9]
+// CHECK:STDOUT:   %int.sadd: init i32 = call %.loc19_17.2(%.loc19_16, %.loc19_24) [template = constants.%.9]
 // CHECK:STDOUT:   %.loc19_11.1: type = value_of_initializer %int.make_type_32.loc19 [template = i32]
 // CHECK:STDOUT:   %.loc19_11.2: type = converted %int.make_type_32.loc19, %.loc19_11.1 [template = i32]
 // CHECK:STDOUT:   %.loc19_26: type = array_type %int.sadd, i32 [template = constants.%.10]
@@ -127,11 +127,11 @@ var arr: [i32; 1.(I.F)(2)];
 // CHECK:STDOUT:     %.loc16_34.2: type = converted %int.make_type_32.loc16_34, %.loc16_34.1 [template = i32]
 // CHECK:STDOUT:     %return.var: ref i32 = var <return slot>
 // CHECK:STDOUT:   }
-// CHECK:STDOUT:   %.1: <witness> = interface_witness (%F.decl) [template = constants.%.5]
+// CHECK:STDOUT:   %.loc15: <witness> = interface_witness (%F.decl) [template = constants.%.5]
 // CHECK:STDOUT:
 // CHECK:STDOUT: !members:
 // CHECK:STDOUT:   .F = %F.decl
-// CHECK:STDOUT:   witness = %.1
+// CHECK:STDOUT:   witness = %.loc15
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
 // CHECK:STDOUT: generic fn @F.1(@I.%Self: %.1) {
diff --git a/toolchain/check/testdata/function/builtin/no_prelude/call_from_operator.carbon b/toolchain/check/testdata/function/builtin/no_prelude/call_from_operator.carbon
index 631efa53fc92..f8732017f652 100644
--- a/toolchain/check/testdata/function/builtin/no_prelude/call_from_operator.carbon
+++ b/toolchain/check/testdata/function/builtin/no_prelude/call_from_operator.carbon
@@ -152,9 +152,9 @@ var arr: [i32; 1 + 2] = (3, 4, 3 + 4);
 // CHECK:STDOUT:   %int.make_type_32.loc10: init type = call constants.%Int32() [template = i32]
 // CHECK:STDOUT:   %.loc10_16: i32 = int_literal 1 [template = constants.%.4]
 // CHECK:STDOUT:   %.loc10_20: i32 = int_literal 2 [template = constants.%.5]
-// CHECK:STDOUT:   %.1: %Op.type.2 = interface_witness_access @impl.%.1, element0 [template = constants.%Op.1]
-// CHECK:STDOUT:   %.loc10_18: <bound method> = bound_method %.loc10_16, %.1 [template = constants.%.8]
-// CHECK:STDOUT:   %int.sadd: init i32 = call %.loc10_18(%.loc10_16, %.loc10_20) [template = constants.%.9]
+// CHECK:STDOUT:   %.loc10_18.1: %Op.type.2 = interface_witness_access @impl.%.loc6, element0 [template = constants.%Op.1]
+// CHECK:STDOUT:   %.loc10_18.2: <bound method> = bound_method %.loc10_16, %.loc10_18.1 [template = constants.%.8]
+// CHECK:STDOUT:   %int.sadd: init i32 = call %.loc10_18.2(%.loc10_16, %.loc10_20) [template = constants.%.9]
 // CHECK:STDOUT:   %.loc10_11.1: type = value_of_initializer %int.make_type_32.loc10 [template = i32]
 // CHECK:STDOUT:   %.loc10_11.2: type = converted %int.make_type_32.loc10, %.loc10_11.1 [template = i32]
 // CHECK:STDOUT:   %.loc10_21: type = array_type %int.sadd, i32 [template = constants.%.10]
@@ -180,11 +180,11 @@ var arr: [i32; 1 + 2] = (3, 4, 3 + 4);
 // CHECK:STDOUT:     %Self.ref.loc7_37: type = name_ref Self, i32 [template = i32]
 // CHECK:STDOUT:     %return.var: ref i32 = var <return slot>
 // CHECK:STDOUT:   }
-// CHECK:STDOUT:   %.1: <witness> = interface_witness (%Op.decl) [template = constants.%.3]
+// CHECK:STDOUT:   %.loc6: <witness> = interface_witness (%Op.decl) [template = constants.%.3]
 // CHECK:STDOUT:
 // CHECK:STDOUT: !members:
 // CHECK:STDOUT:   .Op = %Op.decl
-// CHECK:STDOUT:   witness = %.1
+// CHECK:STDOUT:   witness = %.loc6
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
 // CHECK:STDOUT: fn @Int32() -> type = "int.make_type_32";
@@ -203,9 +203,9 @@ var arr: [i32; 1 + 2] = (3, 4, 3 + 4);
 // CHECK:STDOUT:   %.loc10_29: i32 = int_literal 4 [template = constants.%.12]
 // CHECK:STDOUT:   %.loc10_32: i32 = int_literal 3 [template = constants.%.9]
 // CHECK:STDOUT:   %.loc10_36: i32 = int_literal 4 [template = constants.%.12]
-// CHECK:STDOUT:   %.1: %Op.type.2 = interface_witness_access @impl.%.1, element0 [template = constants.%Op.1]
-// CHECK:STDOUT:   %.loc10_34: <bound method> = bound_method %.loc10_32, %.1 [template = constants.%.13]
-// CHECK:STDOUT:   %int.sadd: init i32 = call %.loc10_34(%.loc10_32, %.loc10_36) [template = constants.%.14]
+// CHECK:STDOUT:   %.loc10_34.1: %Op.type.2 = interface_witness_access @impl.%.loc6, element0 [template = constants.%Op.1]
+// CHECK:STDOUT:   %.loc10_34.2: <bound method> = bound_method %.loc10_32, %.loc10_34.1 [template = constants.%.13]
+// CHECK:STDOUT:   %int.sadd: init i32 = call %.loc10_34.2(%.loc10_32, %.loc10_36) [template = constants.%.14]
 // CHECK:STDOUT:   %.loc10_37.1: %.15 = tuple_literal (%.loc10_26, %.loc10_29, %int.sadd)
 // CHECK:STDOUT:   %.loc10_37.2: i32 = int_literal 0 [template = constants.%.16]
 // CHECK:STDOUT:   %.loc10_37.3: ref i32 = array_index file.%arr.var, %.loc10_37.2
diff --git a/toolchain/check/testdata/impl/compound.carbon b/toolchain/check/testdata/impl/compound.carbon
index 5b44bc32961d..297885338b5b 100644
--- a/toolchain/check/testdata/impl/compound.carbon
+++ b/toolchain/check/testdata/impl/compound.carbon
@@ -158,12 +158,12 @@ fn InstanceCallIndirect(p: i32*) {
 // CHECK:STDOUT:     %self.loc18_8.1: i32 = param self
 // CHECK:STDOUT:     %self.loc18_8.2: i32 = bind_name self, %self.loc18_8.1
 // CHECK:STDOUT:   }
-// CHECK:STDOUT:   %.1: <witness> = interface_witness (%F.decl, %G.decl) [template = constants.%.7]
+// CHECK:STDOUT:   %.loc16: <witness> = interface_witness (%F.decl, %G.decl) [template = constants.%.7]
 // CHECK:STDOUT:
 // CHECK:STDOUT: !members:
 // CHECK:STDOUT:   .F = %F.decl
 // CHECK:STDOUT:   .G = %G.decl
-// CHECK:STDOUT:   witness = %.1
+// CHECK:STDOUT:   witness = %.loc16
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
 // CHECK:STDOUT: generic fn @F.1(@Simple.%Self: %.1) {
@@ -188,8 +188,8 @@ fn InstanceCallIndirect(p: i32*) {
 // CHECK:STDOUT:   %n.ref: i32 = name_ref n, %n
 // CHECK:STDOUT:   %Simple.ref: type = name_ref Simple, file.%Simple.decl [template = constants.%.1]
 // CHECK:STDOUT:   %F.ref: %.3 = name_ref F, @Simple.%.loc12 [template = constants.%.4]
-// CHECK:STDOUT:   %.1: %F.type.1 = interface_witness_access @impl.%.1, element0 [template = constants.%F.2]
-// CHECK:STDOUT:   %F.call: init %.2 = call %.1()
+// CHECK:STDOUT:   %.loc22: %F.type.1 = interface_witness_access @impl.%.loc16, element0 [template = constants.%F.2]
+// CHECK:STDOUT:   %F.call: init %.2 = call %.loc22()
 // CHECK:STDOUT:   return
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
@@ -198,9 +198,9 @@ fn InstanceCallIndirect(p: i32*) {
 // CHECK:STDOUT:   %n.ref: i32 = name_ref n, %n
 // CHECK:STDOUT:   %Simple.ref: type = name_ref Simple, file.%Simple.decl [template = constants.%.1]
 // CHECK:STDOUT:   %G.ref: %.5 = name_ref G, @Simple.%.loc13_21 [template = constants.%.6]
-// CHECK:STDOUT:   %.1: %G.type.1 = interface_witness_access @impl.%.1, element1 [template = constants.%G.2]
-// CHECK:STDOUT:   %.loc26: <bound method> = bound_method %n.ref, %.1
-// CHECK:STDOUT:   %G.call: init %.2 = call %.loc26(%n.ref)
+// CHECK:STDOUT:   %.loc26_4.1: %G.type.1 = interface_witness_access @impl.%.loc16, element1 [template = constants.%G.2]
+// CHECK:STDOUT:   %.loc26_4.2: <bound method> = bound_method %n.ref, %.loc26_4.1
+// CHECK:STDOUT:   %G.call: init %.2 = call %.loc26_4.2(%n.ref)
 // CHECK:STDOUT:   return
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
@@ -209,9 +209,9 @@ fn InstanceCallIndirect(p: i32*) {
 // CHECK:STDOUT:   %p.ref: %.8 = name_ref p, %p
 // CHECK:STDOUT:   %Simple.ref: type = name_ref Simple, file.%Simple.decl [template = constants.%.1]
 // CHECK:STDOUT:   %F.ref: %.3 = name_ref F, @Simple.%.loc12 [template = constants.%.4]
-// CHECK:STDOUT:   %.loc30: ref i32 = deref %p.ref
-// CHECK:STDOUT:   %.1: %F.type.1 = interface_witness_access @impl.%.1, element0 [template = constants.%F.2]
-// CHECK:STDOUT:   %F.call: init %.2 = call %.1()
+// CHECK:STDOUT:   %.loc30_4.1: ref i32 = deref %p.ref
+// CHECK:STDOUT:   %.loc30_4.2: %F.type.1 = interface_witness_access @impl.%.loc16, element0 [template = constants.%F.2]
+// CHECK:STDOUT:   %F.call: init %.2 = call %.loc30_4.2()
 // CHECK:STDOUT:   return
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
@@ -221,10 +221,10 @@ fn InstanceCallIndirect(p: i32*) {
 // CHECK:STDOUT:   %Simple.ref: type = name_ref Simple, file.%Simple.decl [template = constants.%.1]
 // CHECK:STDOUT:   %G.ref: %.5 = name_ref G, @Simple.%.loc13_21 [template = constants.%.6]
 // CHECK:STDOUT:   %.loc34_4.1: ref i32 = deref %p.ref
-// CHECK:STDOUT:   %.1: %G.type.1 = interface_witness_access @impl.%.1, element1 [template = constants.%G.2]
-// CHECK:STDOUT:   %.loc34_4.2: <bound method> = bound_method %.loc34_4.1, %.1
-// CHECK:STDOUT:   %.loc34_4.3: i32 = bind_value %.loc34_4.1
-// CHECK:STDOUT:   %G.call: init %.2 = call %.loc34_4.2(%.loc34_4.3)
+// CHECK:STDOUT:   %.loc34_4.2: %G.type.1 = interface_witness_access @impl.%.loc16, element1 [template = constants.%G.2]
+// CHECK:STDOUT:   %.loc34_4.3: <bound method> = bound_method %.loc34_4.1, %.loc34_4.2
+// CHECK:STDOUT:   %.loc34_4.4: i32 = bind_value %.loc34_4.1
+// CHECK:STDOUT:   %G.call: init %.2 = call %.loc34_4.3(%.loc34_4.4)
 // CHECK:STDOUT:   return
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
diff --git a/toolchain/check/testdata/impl/empty.carbon b/toolchain/check/testdata/impl/empty.carbon
index ee82e9ce6161..1dccf7ed4d6e 100644
--- a/toolchain/check/testdata/impl/empty.carbon
+++ b/toolchain/check/testdata/impl/empty.carbon
@@ -63,10 +63,10 @@ impl i32 as Empty {
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
 // CHECK:STDOUT: impl @impl: i32 as %.1 {
-// CHECK:STDOUT:   %.1: <witness> = interface_witness () [template = constants.%.3]
+// CHECK:STDOUT:   %.loc14: <witness> = interface_witness () [template = constants.%.3]
 // CHECK:STDOUT:
 // CHECK:STDOUT: !members:
-// CHECK:STDOUT:   witness = %.1
+// CHECK:STDOUT:   witness = %.loc14
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
 // CHECK:STDOUT: fn @Int32() -> type = "int.make_type_32";
diff --git a/toolchain/check/testdata/impl/extend_impl.carbon b/toolchain/check/testdata/impl/extend_impl.carbon
index 5934a6c68f16..270d930c53f4 100644
--- a/toolchain/check/testdata/impl/extend_impl.carbon
+++ b/toolchain/check/testdata/impl/extend_impl.carbon
@@ -85,11 +85,11 @@ fn G(c: C) {
 // CHECK:STDOUT:
 // CHECK:STDOUT: impl @impl: %C as %.1 {
 // CHECK:STDOUT:   %F.decl: %F.type.2 = fn_decl @F.2 [template = constants.%F.2] {}
-// CHECK:STDOUT:   %.1: <witness> = interface_witness (%F.decl) [template = constants.%.5]
+// CHECK:STDOUT:   %.loc16: <witness> = interface_witness (%F.decl) [template = constants.%.5]
 // CHECK:STDOUT:
 // CHECK:STDOUT: !members:
 // CHECK:STDOUT:   .F = %F.decl
-// CHECK:STDOUT:   witness = %.1
+// CHECK:STDOUT:   witness = %.loc16
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
 // CHECK:STDOUT: class @C {
@@ -116,12 +116,12 @@ fn G(c: C) {
 // CHECK:STDOUT: !entry:
 // CHECK:STDOUT:   %C.ref: type = name_ref C, file.%C.decl [template = constants.%C]
 // CHECK:STDOUT:   %F.ref.loc22: %.3 = name_ref F, @HasF.%.loc12 [template = constants.%.4]
-// CHECK:STDOUT:   %.1: %F.type.1 = interface_witness_access @impl.%.1, element0 [template = constants.%F.2]
-// CHECK:STDOUT:   %F.call.loc22: init %.2 = call %.1()
+// CHECK:STDOUT:   %.loc22: %F.type.1 = interface_witness_access @impl.%.loc16, element0 [template = constants.%F.2]
+// CHECK:STDOUT:   %F.call.loc22: init %.2 = call %.loc22()
 // CHECK:STDOUT:   %c.ref: %C = name_ref c, %c
 // CHECK:STDOUT:   %F.ref.loc23: %.3 = name_ref F, @HasF.%.loc12 [template = constants.%.4]
-// CHECK:STDOUT:   %.2: %F.type.1 = interface_witness_access @impl.%.1, element0 [template = constants.%F.2]
-// CHECK:STDOUT:   %F.call.loc23: init %.2 = call %.2()
+// CHECK:STDOUT:   %.loc23: %F.type.1 = interface_witness_access @impl.%.loc16, element0 [template = constants.%F.2]
+// CHECK:STDOUT:   %F.call.loc23: init %.2 = call %.loc23()
 // CHECK:STDOUT:   return
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
diff --git a/toolchain/check/testdata/impl/fail_call_invalid.carbon b/toolchain/check/testdata/impl/fail_call_invalid.carbon
index b9e3e8e8e6cd..8bf5f199795f 100644
--- a/toolchain/check/testdata/impl/fail_call_invalid.carbon
+++ b/toolchain/check/testdata/impl/fail_call_invalid.carbon
@@ -101,11 +101,11 @@ fn InstanceCall(n: i32) {
 // CHECK:STDOUT:     %self.loc19_8.1: <error> = param self
 // CHECK:STDOUT:     %self.loc19_8.2: <error> = bind_name self, %self.loc19_8.1
 // CHECK:STDOUT:   }
-// CHECK:STDOUT:   %.1: <witness> = interface_witness (<error>) [template = <error>]
+// CHECK:STDOUT:   %.loc15: <witness> = interface_witness (<error>) [template = <error>]
 // CHECK:STDOUT:
 // CHECK:STDOUT: !members:
 // CHECK:STDOUT:   .G = %G.decl
-// CHECK:STDOUT:   witness = %.1
+// CHECK:STDOUT:   witness = %.loc15
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
 // CHECK:STDOUT: generic fn @G.1(@Simple.%Self: %.1) {
@@ -123,8 +123,8 @@ fn InstanceCall(n: i32) {
 // CHECK:STDOUT:   %n.ref: i32 = name_ref n, %n
 // CHECK:STDOUT:   %Simple.ref: type = name_ref Simple, file.%Simple.decl [template = constants.%.1]
 // CHECK:STDOUT:   %G.ref: %.3 = name_ref G, @Simple.%.loc12_21 [template = constants.%.4]
-// CHECK:STDOUT:   %.1: %G.type.1 = interface_witness_access @impl.%.1, element0 [template = <error>]
-// CHECK:STDOUT:   %.loc23: <bound method> = bound_method %n.ref, %.1
+// CHECK:STDOUT:   %.loc23_4.1: %G.type.1 = interface_witness_access @impl.%.loc15, element0 [template = <error>]
+// CHECK:STDOUT:   %.loc23_4.2: <bound method> = bound_method %n.ref, %.loc23_4.1
 // CHECK:STDOUT:   return
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
diff --git a/toolchain/check/testdata/impl/fail_extend_impl_forall.carbon b/toolchain/check/testdata/impl/fail_extend_impl_forall.carbon
index 00dd7bdbd2d8..862eece9c988 100644
--- a/toolchain/check/testdata/impl/fail_extend_impl_forall.carbon
+++ b/toolchain/check/testdata/impl/fail_extend_impl_forall.carbon
@@ -100,11 +100,11 @@ class C {
 // CHECK:STDOUT:     %x.loc20_10.1: @F.2.%T (%T) = param x
 // CHECK:STDOUT:     %x.loc20_10.2: @F.2.%T (%T) = bind_name x, %x.loc20_10.1
 // CHECK:STDOUT:   }
-// CHECK:STDOUT:   %.1: <witness> = interface_witness (%F.decl) [template = constants.%.5]
+// CHECK:STDOUT:   %.loc19: <witness> = interface_witness (%F.decl) [template = constants.%.5]
 // CHECK:STDOUT:
 // CHECK:STDOUT: !members:
 // CHECK:STDOUT:   .F = %F.decl
-// CHECK:STDOUT:   witness = %.1
+// CHECK:STDOUT:   witness = %.loc19
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
 // CHECK:STDOUT: class @C {
diff --git a/toolchain/check/testdata/impl/fail_extend_impl_scope.carbon b/toolchain/check/testdata/impl/fail_extend_impl_scope.carbon
index c1824f324f8d..65cc2d583c7d 100644
--- a/toolchain/check/testdata/impl/fail_extend_impl_scope.carbon
+++ b/toolchain/check/testdata/impl/fail_extend_impl_scope.carbon
@@ -64,10 +64,10 @@ extend impl i32 as I {}
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
 // CHECK:STDOUT: impl @impl: i32 as %.1 {
-// CHECK:STDOUT:   %.1: <witness> = interface_witness () [template = constants.%.3]
+// CHECK:STDOUT:   %.loc16: <witness> = interface_witness () [template = constants.%.3]
 // CHECK:STDOUT:
 // CHECK:STDOUT: !members:
-// CHECK:STDOUT:   witness = %.1
+// CHECK:STDOUT:   witness = %.loc16
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
 // CHECK:STDOUT: fn @Int32() -> type = "int.make_type_32";
diff --git a/toolchain/check/testdata/impl/fail_extend_impl_type_as.carbon b/toolchain/check/testdata/impl/fail_extend_impl_type_as.carbon
index c602ecf2872b..d6d0f2aef00c 100644
--- a/toolchain/check/testdata/impl/fail_extend_impl_type_as.carbon
+++ b/toolchain/check/testdata/impl/fail_extend_impl_type_as.carbon
@@ -92,19 +92,19 @@ class E {
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
 // CHECK:STDOUT: impl @impl.1: i32 as %.1 {
-// CHECK:STDOUT:   %.1: <witness> = interface_witness () [template = constants.%.3]
+// CHECK:STDOUT:   %.loc18: <witness> = interface_witness () [template = constants.%.3]
 // CHECK:STDOUT:
 // CHECK:STDOUT: !members:
-// CHECK:STDOUT:   witness = %.1
+// CHECK:STDOUT:   witness = %.loc18
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
 // CHECK:STDOUT: impl @impl.2: %D as %.1;
 // CHECK:STDOUT:
 // CHECK:STDOUT: impl @impl.3: %E as %.1 {
-// CHECK:STDOUT:   %.1: <witness> = interface_witness () [template = constants.%.3]
+// CHECK:STDOUT:   %.loc39: <witness> = interface_witness () [template = constants.%.3]
 // CHECK:STDOUT:
 // CHECK:STDOUT: !members:
-// CHECK:STDOUT:   witness = %.1
+// CHECK:STDOUT:   witness = %.loc39
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
 // CHECK:STDOUT: class @C {
diff --git a/toolchain/check/testdata/impl/fail_impl_as_scope.carbon b/toolchain/check/testdata/impl/fail_impl_as_scope.carbon
index a7bad951c49b..bf3cdde26367 100644
--- a/toolchain/check/testdata/impl/fail_impl_as_scope.carbon
+++ b/toolchain/check/testdata/impl/fail_impl_as_scope.carbon
@@ -71,11 +71,11 @@ impl as Simple {
 // CHECK:STDOUT:
 // CHECK:STDOUT: impl @impl: <error> as %.1 {
 // CHECK:STDOUT:   %F.decl: %F.type.2 = fn_decl @F.2 [template = constants.%F.2] {}
-// CHECK:STDOUT:   %.1: <witness> = interface_witness (%F.decl) [template = constants.%.5]
+// CHECK:STDOUT:   %.loc18: <witness> = interface_witness (%F.decl) [template = constants.%.5]
 // CHECK:STDOUT:
 // CHECK:STDOUT: !members:
 // CHECK:STDOUT:   .F = %F.decl
-// CHECK:STDOUT:   witness = %.1
+// CHECK:STDOUT:   witness = %.loc18
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
 // CHECK:STDOUT: generic fn @F.1(@Simple.%Self: %.1) {
diff --git a/toolchain/check/testdata/impl/fail_impl_bad_assoc_fn.carbon b/toolchain/check/testdata/impl/fail_impl_bad_assoc_fn.carbon
index 1208e0d61434..3225f3857720 100644
--- a/toolchain/check/testdata/impl/fail_impl_bad_assoc_fn.carbon
+++ b/toolchain/check/testdata/impl/fail_impl_bad_assoc_fn.carbon
@@ -432,29 +432,29 @@ class SelfNestedBadReturnType {
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
 // CHECK:STDOUT: impl @impl.1: %NoF as %.1 {
-// CHECK:STDOUT:   %.1: <witness> = interface_witness (<error>) [template = <error>]
+// CHECK:STDOUT:   %.loc21: <witness> = interface_witness (<error>) [template = <error>]
 // CHECK:STDOUT:
 // CHECK:STDOUT: !members:
-// CHECK:STDOUT:   witness = %.1
+// CHECK:STDOUT:   witness = %.loc21
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
 // CHECK:STDOUT: impl @impl.2: %FNotFunction as %.1 {
 // CHECK:STDOUT:   %F.decl: type = class_decl @F.16 [template = constants.%F.2] {}
-// CHECK:STDOUT:   %.1: <witness> = interface_witness (<error>) [template = <error>]
+// CHECK:STDOUT:   %.loc25: <witness> = interface_witness (<error>) [template = <error>]
 // CHECK:STDOUT:
 // CHECK:STDOUT: !members:
 // CHECK:STDOUT:   .F = %F.decl
-// CHECK:STDOUT:   witness = %.1
+// CHECK:STDOUT:   witness = %.loc25
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
 // CHECK:STDOUT: impl @impl.3: %FAlias as %.1 {
 // CHECK:STDOUT:   %PossiblyF.ref: %PossiblyF.type = name_ref PossiblyF, file.%PossiblyF.decl [template = constants.%PossiblyF]
 // CHECK:STDOUT:   %F: %PossiblyF.type = bind_alias F, file.%PossiblyF.decl [template = constants.%PossiblyF]
-// CHECK:STDOUT:   %.1: <witness> = interface_witness (<error>) [template = <error>]
+// CHECK:STDOUT:   %.loc41: <witness> = interface_witness (<error>) [template = <error>]
 // CHECK:STDOUT:
 // CHECK:STDOUT: !members:
 // CHECK:STDOUT:   .F = %F
-// CHECK:STDOUT:   witness = %.1
+// CHECK:STDOUT:   witness = %.loc41
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
 // CHECK:STDOUT: impl @impl.4: %FExtraParam as %.1 {
@@ -465,11 +465,11 @@ class SelfNestedBadReturnType {
 // CHECK:STDOUT:     %b.loc62_10.1: bool = param b
 // CHECK:STDOUT:     %b.loc62_10.2: bool = bind_name b, %b.loc62_10.1
 // CHECK:STDOUT:   }
-// CHECK:STDOUT:   %.1: <witness> = interface_witness (<error>) [template = <error>]
+// CHECK:STDOUT:   %.loc54: <witness> = interface_witness (<error>) [template = <error>]
 // CHECK:STDOUT:
 // CHECK:STDOUT: !members:
 // CHECK:STDOUT:   .F = %F.decl
-// CHECK:STDOUT:   witness = %.1
+// CHECK:STDOUT:   witness = %.loc54
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
 // CHECK:STDOUT: impl @impl.5: %FExtraImplicitParam as %.1 {
@@ -478,11 +478,11 @@ class SelfNestedBadReturnType {
 // CHECK:STDOUT:     %self.loc75_10.1: %FExtraImplicitParam = param self
 // CHECK:STDOUT:     %self.loc75_10.2: %FExtraImplicitParam = bind_name self, %self.loc75_10.1
 // CHECK:STDOUT:   }
-// CHECK:STDOUT:   %.1: <witness> = interface_witness (<error>) [template = <error>]
+// CHECK:STDOUT:   %.loc67: <witness> = interface_witness (<error>) [template = <error>]
 // CHECK:STDOUT:
 // CHECK:STDOUT: !members:
 // CHECK:STDOUT:   .F = %F.decl
-// CHECK:STDOUT:   witness = %.1
+// CHECK:STDOUT:   witness = %.loc67
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
 // CHECK:STDOUT: impl @impl.6: %FExtraReturnType as %.1 {
@@ -492,11 +492,11 @@ class SelfNestedBadReturnType {
 // CHECK:STDOUT:     %.loc89_15.2: type = converted %bool.make_type, %.loc89_15.1 [template = bool]
 // CHECK:STDOUT:     %return.var: ref bool = var <return slot>
 // CHECK:STDOUT:   }
-// CHECK:STDOUT:   %.1: <witness> = interface_witness (<error>) [template = <error>]
+// CHECK:STDOUT:   %.loc81: <witness> = interface_witness (<error>) [template = <error>]
 // CHECK:STDOUT:
 // CHECK:STDOUT: !members:
 // CHECK:STDOUT:   .F = %F.decl
-// CHECK:STDOUT:   witness = %.1
+// CHECK:STDOUT:   witness = %.loc81
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
 // CHECK:STDOUT: impl @impl.7: %FMissingParam as %.6 {
@@ -511,11 +511,11 @@ class SelfNestedBadReturnType {
 // CHECK:STDOUT:     %.loc104_27.2: type = converted %bool.make_type.loc104_27, %.loc104_27.1 [template = bool]
 // CHECK:STDOUT:     %return.var: ref bool = var <return slot>
 // CHECK:STDOUT:   }
-// CHECK:STDOUT:   %.1: <witness> = interface_witness (<error>) [template = <error>]
+// CHECK:STDOUT:   %.loc96: <witness> = interface_witness (<error>) [template = <error>]
 // CHECK:STDOUT:
 // CHECK:STDOUT: !members:
 // CHECK:STDOUT:   .F = %F.decl
-// CHECK:STDOUT:   witness = %.1
+// CHECK:STDOUT:   witness = %.loc96
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
 // CHECK:STDOUT: impl @impl.8: %FMissingImplicitParam as %.6 {
@@ -530,11 +530,11 @@ class SelfNestedBadReturnType {
 // CHECK:STDOUT:     %.loc117_22.2: type = converted %bool.make_type.loc117_22, %.loc117_22.1 [template = bool]
 // CHECK:STDOUT:     %return.var: ref bool = var <return slot>
 // CHECK:STDOUT:   }
-// CHECK:STDOUT:   %.1: <witness> = interface_witness (<error>) [template = <error>]
+// CHECK:STDOUT:   %.loc109: <witness> = interface_witness (<error>) [template = <error>]
 // CHECK:STDOUT:
 // CHECK:STDOUT: !members:
 // CHECK:STDOUT:   .F = %F.decl
-// CHECK:STDOUT:   witness = %.1
+// CHECK:STDOUT:   witness = %.loc109
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
 // CHECK:STDOUT: impl @impl.9: %FMissingReturnType as %.6 {
@@ -550,11 +550,11 @@ class SelfNestedBadReturnType {
 // CHECK:STDOUT:     %b.loc130_22.1: bool = param b
 // CHECK:STDOUT:     %b.loc130_22.2: bool = bind_name b, %b.loc130_22.1
 // CHECK:STDOUT:   }
-// CHECK:STDOUT:   %.1: <witness> = interface_witness (<error>) [template = <error>]
+// CHECK:STDOUT:   %.loc122: <witness> = interface_witness (<error>) [template = <error>]
 // CHECK:STDOUT:
 // CHECK:STDOUT: !members:
 // CHECK:STDOUT:   .F = %F.decl
-// CHECK:STDOUT:   witness = %.1
+// CHECK:STDOUT:   witness = %.loc122
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
 // CHECK:STDOUT: impl @impl.10: %FDifferentParamType as %.6 {
@@ -572,11 +572,11 @@ class SelfNestedBadReturnType {
 // CHECK:STDOUT:     %.loc143_34.2: type = converted %bool.make_type.loc143_34, %.loc143_34.1 [template = bool]
 // CHECK:STDOUT:     %return.var: ref bool = var <return slot>
 // CHECK:STDOUT:   }
-// CHECK:STDOUT:   %.1: <witness> = interface_witness (<error>) [template = <error>]
+// CHECK:STDOUT:   %.loc135: <witness> = interface_witness (<error>) [template = <error>]
 // CHECK:STDOUT:
 // CHECK:STDOUT: !members:
 // CHECK:STDOUT:   .F = %F.decl
-// CHECK:STDOUT:   witness = %.1
+// CHECK:STDOUT:   witness = %.loc135
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
 // CHECK:STDOUT: impl @impl.11: %FDifferentImplicitParamType as %.6 {
@@ -594,11 +594,11 @@ class SelfNestedBadReturnType {
 // CHECK:STDOUT:     %.loc156_34.2: type = converted %bool.make_type.loc156_34, %.loc156_34.1 [template = bool]
 // CHECK:STDOUT:     %return.var: ref bool = var <return slot>
 // CHECK:STDOUT:   }
-// CHECK:STDOUT:   %.1: <witness> = interface_witness (<error>) [template = <error>]
+// CHECK:STDOUT:   %.loc148: <witness> = interface_witness (<error>) [template = <error>]
 // CHECK:STDOUT:
 // CHECK:STDOUT: !members:
 // CHECK:STDOUT:   .F = %F.decl
-// CHECK:STDOUT:   witness = %.1
+// CHECK:STDOUT:   witness = %.loc148
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
 // CHECK:STDOUT: impl @impl.12: %FDifferentReturnType as %.6 {
@@ -616,11 +616,11 @@ class SelfNestedBadReturnType {
 // CHECK:STDOUT:     %Self.ref: type = name_ref Self, constants.%FDifferentReturnType [template = constants.%FDifferentReturnType]
 // CHECK:STDOUT:     %return.var: ref %FDifferentReturnType = var <return slot>
 // CHECK:STDOUT:   }
-// CHECK:STDOUT:   %.1: <witness> = interface_witness (<error>) [template = <error>]
+// CHECK:STDOUT:   %.loc161: <witness> = interface_witness (<error>) [template = <error>]
 // CHECK:STDOUT:
 // CHECK:STDOUT: !members:
 // CHECK:STDOUT:   .F = %F.decl
-// CHECK:STDOUT:   witness = %.1
+// CHECK:STDOUT:   witness = %.loc161
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
 // CHECK:STDOUT: impl @impl.13: %FDifferentParamName as %.6 {
@@ -640,11 +640,11 @@ class SelfNestedBadReturnType {
 // CHECK:STDOUT:     %.loc183_38.2: type = converted %bool.make_type.loc183_38, %.loc183_38.1 [template = bool]
 // CHECK:STDOUT:     %return.var: ref bool = var <return slot>
 // CHECK:STDOUT:   }
-// CHECK:STDOUT:   %.1: <witness> = interface_witness (<error>) [template = <error>]
+// CHECK:STDOUT:   %.loc175: <witness> = interface_witness (<error>) [template = <error>]
 // CHECK:STDOUT:
 // CHECK:STDOUT: !members:
 // CHECK:STDOUT:   .F = %F.decl
-// CHECK:STDOUT:   witness = %.1
+// CHECK:STDOUT:   witness = %.loc175
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
 // CHECK:STDOUT: impl @impl.14: %SelfNestedBadParam as %.9 {
@@ -667,11 +667,11 @@ class SelfNestedBadReturnType {
 // CHECK:STDOUT:     %.loc200_81: type = array_type %.loc200_80, %SelfNestedBadParam [template = constants.%.21]
 // CHECK:STDOUT:     %return.var: ref %.21 = var <return slot>
 // CHECK:STDOUT:   }
-// CHECK:STDOUT:   %.1: <witness> = interface_witness (<error>) [template = <error>]
+// CHECK:STDOUT:   %.loc192: <witness> = interface_witness (<error>) [template = <error>]
 // CHECK:STDOUT:
 // CHECK:STDOUT: !members:
 // CHECK:STDOUT:   .F = %F.decl
-// CHECK:STDOUT:   witness = %.1
+// CHECK:STDOUT:   witness = %.loc192
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
 // CHECK:STDOUT: impl @impl.15: %SelfNestedBadReturnType as %.9 {
@@ -692,11 +692,11 @@ class SelfNestedBadReturnType {
 // CHECK:STDOUT:     %.loc212_106: type = array_type %.loc212_105, %SelfNestedBadParam [template = constants.%.21]
 // CHECK:STDOUT:     %return.var: ref %.21 = var <return slot>
 // CHECK:STDOUT:   }
-// CHECK:STDOUT:   %.1: <witness> = interface_witness (<error>) [template = <error>]
+// CHECK:STDOUT:   %.loc205: <witness> = interface_witness (<error>) [template = <error>]
 // CHECK:STDOUT:
 // CHECK:STDOUT: !members:
 // CHECK:STDOUT:   .F = %F.decl
-// CHECK:STDOUT:   witness = %.1
+// CHECK:STDOUT:   witness = %.loc205
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
 // CHECK:STDOUT: class @NoF {
diff --git a/toolchain/check/testdata/impl/fail_redefinition.carbon b/toolchain/check/testdata/impl/fail_redefinition.carbon
index 91ddbee26111..0badf34f8bb2 100644
--- a/toolchain/check/testdata/impl/fail_redefinition.carbon
+++ b/toolchain/check/testdata/impl/fail_redefinition.carbon
@@ -76,7 +76,7 @@ impl i32 as I {}
 // CHECK:STDOUT:
 // CHECK:STDOUT: impl @impl: i32 as %.1 {
 // CHECK:STDOUT: !members:
-// CHECK:STDOUT:   witness = <unexpected>.inst+18
+// CHECK:STDOUT:   witness = <unexpected>.inst+18.loc13_15
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
 // CHECK:STDOUT: fn @Int32() -> type = "int.make_type_32";
diff --git a/toolchain/check/testdata/impl/impl_as.carbon b/toolchain/check/testdata/impl/impl_as.carbon
index 05e1276a8895..d8fb643d388d 100644
--- a/toolchain/check/testdata/impl/impl_as.carbon
+++ b/toolchain/check/testdata/impl/impl_as.carbon
@@ -76,11 +76,11 @@ class C {
 // CHECK:STDOUT:
 // CHECK:STDOUT: impl @impl: %C as %.1 {
 // CHECK:STDOUT:   %F.decl: %F.type.2 = fn_decl @F.2 [template = constants.%F.2] {}
-// CHECK:STDOUT:   %.1: <witness> = interface_witness (%F.decl) [template = constants.%.5]
+// CHECK:STDOUT:   %.loc16: <witness> = interface_witness (%F.decl) [template = constants.%.5]
 // CHECK:STDOUT:
 // CHECK:STDOUT: !members:
 // CHECK:STDOUT:   .F = %F.decl
-// CHECK:STDOUT:   witness = %.1
+// CHECK:STDOUT:   witness = %.loc16
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
 // CHECK:STDOUT: class @C {
diff --git a/toolchain/check/testdata/impl/impl_forall.carbon b/toolchain/check/testdata/impl/impl_forall.carbon
index 216f023f189f..4ff834c2e6e8 100644
--- a/toolchain/check/testdata/impl/impl_forall.carbon
+++ b/toolchain/check/testdata/impl/impl_forall.carbon
@@ -72,11 +72,11 @@ impl forall [T:! type] T as Simple {
 // CHECK:STDOUT:
 // CHECK:STDOUT: impl @impl: %T as %.1 {
 // CHECK:STDOUT:   %F.decl: %F.type.2 = fn_decl @F.2 [template = constants.%F.2] {}
-// CHECK:STDOUT:   %.1: <witness> = interface_witness (%F.decl) [template = constants.%.5]
+// CHECK:STDOUT:   %.loc15: <witness> = interface_witness (%F.decl) [template = constants.%.5]
 // CHECK:STDOUT:
 // CHECK:STDOUT: !members:
 // CHECK:STDOUT:   .F = %F.decl
-// CHECK:STDOUT:   witness = %.1
+// CHECK:STDOUT:   witness = %.loc15
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
 // CHECK:STDOUT: generic fn @F.1(@Simple.%Self: %.1) {
diff --git a/toolchain/check/testdata/impl/lookup/alias.carbon b/toolchain/check/testdata/impl/lookup/alias.carbon
index 4d6a0097561d..589432b50164 100644
--- a/toolchain/check/testdata/impl/lookup/alias.carbon
+++ b/toolchain/check/testdata/impl/lookup/alias.carbon
@@ -91,11 +91,11 @@ fn G(c: C) {
 // CHECK:STDOUT:
 // CHECK:STDOUT: impl @impl: %C as %.1 {
 // CHECK:STDOUT:   %F.decl: %F.type.2 = fn_decl @F.2 [template = constants.%F.2] {}
-// CHECK:STDOUT:   %.1: <witness> = interface_witness (%F.decl) [template = constants.%.6]
+// CHECK:STDOUT:   %.loc19: <witness> = interface_witness (%F.decl) [template = constants.%.6]
 // CHECK:STDOUT:
 // CHECK:STDOUT: !members:
 // CHECK:STDOUT:   .F = %F.decl
-// CHECK:STDOUT:   witness = %.1
+// CHECK:STDOUT:   witness = %.loc19
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
 // CHECK:STDOUT: class @C {
@@ -122,12 +122,12 @@ fn G(c: C) {
 // CHECK:STDOUT: !entry:
 // CHECK:STDOUT:   %C.ref: type = name_ref C, file.%C.decl [template = constants.%C]
 // CHECK:STDOUT:   %G.ref.loc24: %.3 = name_ref G, @C.%G [template = constants.%.4]
-// CHECK:STDOUT:   %.1: %F.type.1 = interface_witness_access @impl.%.1, element0 [template = constants.%F.2]
-// CHECK:STDOUT:   %F.call.loc24: init %.2 = call %.1()
+// CHECK:STDOUT:   %.loc24: %F.type.1 = interface_witness_access @impl.%.loc19, element0 [template = constants.%F.2]
+// CHECK:STDOUT:   %F.call.loc24: init %.2 = call %.loc24()
 // CHECK:STDOUT:   %c.ref: %C = name_ref c, %c
 // CHECK:STDOUT:   %G.ref.loc25: %.3 = name_ref G, @C.%G [template = constants.%.4]
-// CHECK:STDOUT:   %.2: %F.type.1 = interface_witness_access @impl.%.1, element0 [template = constants.%F.2]
-// CHECK:STDOUT:   %F.call.loc25: init %.2 = call %.2()
+// CHECK:STDOUT:   %.loc25: %F.type.1 = interface_witness_access @impl.%.loc19, element0 [template = constants.%F.2]
+// CHECK:STDOUT:   %F.call.loc25: init %.2 = call %.loc25()
 // CHECK:STDOUT:   return
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
diff --git a/toolchain/check/testdata/impl/lookup/fail_todo_undefined_impl.carbon b/toolchain/check/testdata/impl/lookup/fail_todo_undefined_impl.carbon
index d886359da3ac..a05f039ac324 100644
--- a/toolchain/check/testdata/impl/lookup/fail_todo_undefined_impl.carbon
+++ b/toolchain/check/testdata/impl/lookup/fail_todo_undefined_impl.carbon
@@ -99,11 +99,11 @@ impl C as I {
 // CHECK:STDOUT:
 // CHECK:STDOUT: impl @impl: %C as %.1 {
 // CHECK:STDOUT:   %F.decl: %F.type.3 = fn_decl @F.3 [template = constants.%F.3] {}
-// CHECK:STDOUT:   %.1: <witness> = interface_witness (%F.decl) [template = constants.%.7]
+// CHECK:STDOUT:   %.loc27: <witness> = interface_witness (%F.decl) [template = constants.%.7]
 // CHECK:STDOUT:
 // CHECK:STDOUT: !members:
 // CHECK:STDOUT:   .F = %F.decl
-// CHECK:STDOUT:   witness = %.1
+// CHECK:STDOUT:   witness = %.loc27
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
 // CHECK:STDOUT: class @C {
diff --git a/toolchain/check/testdata/impl/lookup/import.carbon b/toolchain/check/testdata/impl/lookup/import.carbon
index 6c4fc8f8249d..69d8470c1198 100644
--- a/toolchain/check/testdata/impl/lookup/import.carbon
+++ b/toolchain/check/testdata/impl/lookup/import.carbon
@@ -87,11 +87,11 @@ fn G(c: Impl.C) {
 // CHECK:STDOUT:
 // CHECK:STDOUT: impl @impl: %C as %.1 {
 // CHECK:STDOUT:   %F.decl: %F.type.2 = fn_decl @F.2 [template = constants.%F.2] {}
-// CHECK:STDOUT:   %.1: <witness> = interface_witness (%F.decl) [template = constants.%.6]
+// CHECK:STDOUT:   %.loc10: <witness> = interface_witness (%F.decl) [template = constants.%.6]
 // CHECK:STDOUT:
 // CHECK:STDOUT: !members:
 // CHECK:STDOUT:   .F = %F.decl
-// CHECK:STDOUT:   witness = %.1
+// CHECK:STDOUT:   witness = %.loc10
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
 // CHECK:STDOUT: class @C {
@@ -197,8 +197,8 @@ fn G(c: Impl.C) {
 // CHECK:STDOUT:   %Impl.ref: <namespace> = name_ref Impl, imports.%Impl [template = imports.%Impl]
 // CHECK:STDOUT:   %HasF.ref: type = name_ref HasF, imports.%import_ref.7 [template = constants.%.2]
 // CHECK:STDOUT:   %F.ref: %.5 = name_ref F, imports.%import_ref.3 [template = constants.%.6]
-// CHECK:STDOUT:   %.1: %F.type.1 = interface_witness_access imports.%import_ref.5, element0 [template = constants.%F.2]
-// CHECK:STDOUT:   %F.call: init %.3 = call %.1()
+// CHECK:STDOUT:   %.loc5: %F.type.1 = interface_witness_access imports.%import_ref.5, element0 [template = constants.%F.2]
+// CHECK:STDOUT:   %F.call: init %.3 = call %.loc5()
 // CHECK:STDOUT:   return
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
diff --git a/toolchain/check/testdata/impl/lookup/instance_method.carbon b/toolchain/check/testdata/impl/lookup/instance_method.carbon
index 6a7f3135a526..cd486826f966 100644
--- a/toolchain/check/testdata/impl/lookup/instance_method.carbon
+++ b/toolchain/check/testdata/impl/lookup/instance_method.carbon
@@ -113,11 +113,11 @@ fn F(c: C) -> i32 {
 // CHECK:STDOUT:     %.loc19_27.2: type = converted %int.make_type_32, %.loc19_27.1 [template = i32]
 // CHECK:STDOUT:     %return.var: ref i32 = var <return slot>
 // CHECK:STDOUT:   }
-// CHECK:STDOUT:   %.1: <witness> = interface_witness (%F.decl) [template = constants.%.5]
+// CHECK:STDOUT:   %.loc18: <witness> = interface_witness (%F.decl) [template = constants.%.5]
 // CHECK:STDOUT:
 // CHECK:STDOUT: !members:
 // CHECK:STDOUT:   .F = %F.decl
-// CHECK:STDOUT:   witness = %.1
+// CHECK:STDOUT:   witness = %.loc18
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
 // CHECK:STDOUT: class @C {
@@ -144,9 +144,9 @@ fn F(c: C) -> i32 {
 // CHECK:STDOUT: !entry:
 // CHECK:STDOUT:   %c.ref: %C = name_ref c, %c
 // CHECK:STDOUT:   %F.ref: %.3 = name_ref F, @I.%.loc14_28 [template = constants.%.4]
-// CHECK:STDOUT:   %.1: %F.type.1 = interface_witness_access @impl.%.1, element0 [template = constants.%F.2]
-// CHECK:STDOUT:   %.loc24_11: <bound method> = bound_method %c.ref, %.1
-// CHECK:STDOUT:   %F.call: init i32 = call %.loc24_11(%c.ref)
+// CHECK:STDOUT:   %.loc24_11.1: %F.type.1 = interface_witness_access @impl.%.loc18, element0 [template = constants.%F.2]
+// CHECK:STDOUT:   %.loc24_11.2: <bound method> = bound_method %c.ref, %.loc24_11.1
+// CHECK:STDOUT:   %F.call: init i32 = call %.loc24_11.2(%c.ref)
 // CHECK:STDOUT:   %.loc24_15.1: i32 = value_of_initializer %F.call
 // CHECK:STDOUT:   %.loc24_15.2: i32 = converted %F.call, %.loc24_15.1
 // CHECK:STDOUT:   return %.loc24_15.2
diff --git a/toolchain/check/testdata/impl/lookup/no_prelude/import.carbon b/toolchain/check/testdata/impl/lookup/no_prelude/import.carbon
index 96e82df6bb90..65c17f3ce5a2 100644
--- a/toolchain/check/testdata/impl/lookup/no_prelude/import.carbon
+++ b/toolchain/check/testdata/impl/lookup/no_prelude/import.carbon
@@ -73,11 +73,11 @@ fn G(c: Impl.C) {
 // CHECK:STDOUT:
 // CHECK:STDOUT: impl @impl: %C as %.1 {
 // CHECK:STDOUT:   %F.decl: %F.type.2 = fn_decl @F.2 [template = constants.%F.2] {}
-// CHECK:STDOUT:   %.1: <witness> = interface_witness (%F.decl) [template = constants.%.6]
+// CHECK:STDOUT:   %.loc10: <witness> = interface_witness (%F.decl) [template = constants.%.6]
 // CHECK:STDOUT:
 // CHECK:STDOUT: !members:
 // CHECK:STDOUT:   .F = %F.decl
-// CHECK:STDOUT:   witness = %.1
+// CHECK:STDOUT:   witness = %.loc10
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
 // CHECK:STDOUT: class @C {
@@ -172,8 +172,8 @@ fn G(c: Impl.C) {
 // CHECK:STDOUT:   %Impl.ref: <namespace> = name_ref Impl, imports.%Impl [template = imports.%Impl]
 // CHECK:STDOUT:   %HasF.ref: type = name_ref HasF, imports.%import_ref.7 [template = constants.%.2]
 // CHECK:STDOUT:   %F.ref: %.5 = name_ref F, imports.%import_ref.3 [template = constants.%.6]
-// CHECK:STDOUT:   %.1: %F.type.1 = interface_witness_access imports.%import_ref.5, element0 [template = constants.%F.2]
-// CHECK:STDOUT:   %F.call: init %.3 = call %.1()
+// CHECK:STDOUT:   %.loc5: %F.type.1 = interface_witness_access imports.%import_ref.5, element0 [template = constants.%F.2]
+// CHECK:STDOUT:   %F.call: init %.3 = call %.loc5()
 // CHECK:STDOUT:   return
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
diff --git a/toolchain/check/testdata/impl/no_prelude/basic.carbon b/toolchain/check/testdata/impl/no_prelude/basic.carbon
index 76b552190f2a..716e20c3cb6b 100644
--- a/toolchain/check/testdata/impl/no_prelude/basic.carbon
+++ b/toolchain/check/testdata/impl/no_prelude/basic.carbon
@@ -61,11 +61,11 @@ impl C as Simple {
 // CHECK:STDOUT:
 // CHECK:STDOUT: impl @impl: %C as %.1 {
 // CHECK:STDOUT:   %F.decl: %F.type.2 = fn_decl @F.2 [template = constants.%F.2] {}
-// CHECK:STDOUT:   %.1: <witness> = interface_witness (%F.decl) [template = constants.%.6]
+// CHECK:STDOUT:   %.loc17: <witness> = interface_witness (%F.decl) [template = constants.%.6]
 // CHECK:STDOUT:
 // CHECK:STDOUT: !members:
 // CHECK:STDOUT:   .F = %F.decl
-// CHECK:STDOUT:   witness = %.1
+// CHECK:STDOUT:   witness = %.loc17
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
 // CHECK:STDOUT: class @C {
diff --git a/toolchain/check/testdata/impl/no_prelude/fail_impl_bad_type.carbon b/toolchain/check/testdata/impl/no_prelude/fail_impl_bad_type.carbon
index c1fb472103d7..92f0c4543ba9 100644
--- a/toolchain/check/testdata/impl/no_prelude/fail_impl_bad_type.carbon
+++ b/toolchain/check/testdata/impl/no_prelude/fail_impl_bad_type.carbon
@@ -45,9 +45,9 @@ impl true as I {}
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
 // CHECK:STDOUT: impl @impl: <error> as %.1 {
-// CHECK:STDOUT:   %.1: <witness> = interface_witness () [template = constants.%.4]
+// CHECK:STDOUT:   %.loc16: <witness> = interface_witness () [template = constants.%.4]
 // CHECK:STDOUT:
 // CHECK:STDOUT: !members:
-// CHECK:STDOUT:   witness = %.1
+// CHECK:STDOUT:   witness = %.loc16
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
diff --git a/toolchain/check/testdata/impl/no_prelude/import_self.carbon b/toolchain/check/testdata/impl/no_prelude/import_self.carbon
index 8cc5771d5929..aaa75463106d 100644
--- a/toolchain/check/testdata/impl/no_prelude/import_self.carbon
+++ b/toolchain/check/testdata/impl/no_prelude/import_self.carbon
@@ -155,11 +155,11 @@ fn F(x: (), y: ()) -> () {
 // CHECK:STDOUT:     %Self.ref.loc7_37: type = name_ref Self, constants.%.1 [template = constants.%.1]
 // CHECK:STDOUT:     %return.var: ref %.1 = var <return slot>
 // CHECK:STDOUT:   }
-// CHECK:STDOUT:   %.1: <witness> = interface_witness (%Op.decl) [template = constants.%.3]
+// CHECK:STDOUT:   %.loc6: <witness> = interface_witness (%Op.decl) [template = constants.%.3]
 // CHECK:STDOUT:
 // CHECK:STDOUT: !members:
 // CHECK:STDOUT:   .Op = %Op.decl
-// CHECK:STDOUT:   witness = %.1
+// CHECK:STDOUT:   witness = %.loc6
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
 // CHECK:STDOUT: fn @Op.1[@impl.%self.loc7_9.2: %.1](@impl.%other.loc7_21.2: %.1) -> %.1 {
@@ -181,10 +181,10 @@ fn F(x: (), y: ()) -> () {
 // CHECK:STDOUT:   %x.ref: %.1 = name_ref x, %x
 // CHECK:STDOUT:   %Add.ref: type = name_ref Add, imports.%import_ref.1 [template = constants.%.2]
 // CHECK:STDOUT:   %Op.ref: %.4 = name_ref Op, imports.%import_ref.3 [template = constants.%.5]
-// CHECK:STDOUT:   %.1: %Op.type.2 = interface_witness_access @impl.%.1, element0 [template = constants.%Op.1]
-// CHECK:STDOUT:   %.loc11_11: <bound method> = bound_method %x.ref, %.1
+// CHECK:STDOUT:   %.loc11_11.1: %Op.type.2 = interface_witness_access @impl.%.loc6, element0 [template = constants.%Op.1]
+// CHECK:STDOUT:   %.loc11_11.2: <bound method> = bound_method %x.ref, %.loc11_11.1
 // CHECK:STDOUT:   %y.ref: %.1 = name_ref y, %y
-// CHECK:STDOUT:   %Op.call: init %.1 = call %.loc11_11(%x.ref, %y.ref)
+// CHECK:STDOUT:   %Op.call: init %.1 = call %.loc11_11.2(%x.ref, %y.ref)
 // CHECK:STDOUT:   %.loc11_20.1: ref %.1 = temporary_storage
 // CHECK:STDOUT:   %.loc11_20.2: ref %.1 = temporary %.loc11_20.1, %Op.call
 // CHECK:STDOUT:   %tuple: %.1 = tuple_value () [template = constants.%tuple]
diff --git a/toolchain/check/testdata/impl/no_prelude/interface_args.carbon b/toolchain/check/testdata/impl/no_prelude/interface_args.carbon
index d87e09dbe31b..8136db43cff8 100644
--- a/toolchain/check/testdata/impl/no_prelude/interface_args.carbon
+++ b/toolchain/check/testdata/impl/no_prelude/interface_args.carbon
@@ -163,11 +163,11 @@ fn MakeC(a: A) -> C {
 // CHECK:STDOUT:
 // CHECK:STDOUT: impl @impl: %A as %.6 {
 // CHECK:STDOUT:   %Op.decl: %Op.type.2 = fn_decl @Op.2 [template = constants.%Op.2] {}
-// CHECK:STDOUT:   %.1: <witness> = interface_witness (%Op.decl) [template = constants.%.9]
+// CHECK:STDOUT:   %.loc12: <witness> = interface_witness (%Op.decl) [template = constants.%.9]
 // CHECK:STDOUT:
 // CHECK:STDOUT: !members:
 // CHECK:STDOUT:   .Op = %Op.decl
-// CHECK:STDOUT:   witness = %.1
+// CHECK:STDOUT:   witness = %.loc12
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
 // CHECK:STDOUT: class @A {
@@ -203,8 +203,8 @@ fn MakeC(a: A) -> C {
 // CHECK:STDOUT:   %.loc16_23: init type = call %Action.ref(%B.ref) [template = constants.%.6]
 // CHECK:STDOUT:   %.loc16_26: %.7 = specific_constant @Action.%.loc5, @Action(constants.%B) [template = constants.%.8]
 // CHECK:STDOUT:   %Op.ref: %.7 = name_ref Op, %.loc16_26 [template = constants.%.8]
-// CHECK:STDOUT:   %.1: %Op.type.3 = interface_witness_access @impl.%.1, element0 [template = constants.%Op.2]
-// CHECK:STDOUT:   %Op.call: init %.1 = call %.1()
+// CHECK:STDOUT:   %.loc16_15: %Op.type.3 = interface_witness_access @impl.%.loc12, element0 [template = constants.%Op.2]
+// CHECK:STDOUT:   %Op.call: init %.1 = call %.loc16_15()
 // CHECK:STDOUT:   return
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
@@ -345,8 +345,8 @@ fn MakeC(a: A) -> C {
 // CHECK:STDOUT:   %.loc4_23: init type = call %Action.ref(%B.ref) [template = constants.%.4]
 // CHECK:STDOUT:   %.loc4_26: %.7 = specific_constant imports.%import_ref.9, @Action(constants.%B) [template = constants.%.8]
 // CHECK:STDOUT:   %Op.ref: %.7 = name_ref Op, %.loc4_26 [template = constants.%.8]
-// CHECK:STDOUT:   %.1: %Op.type.2 = interface_witness_access imports.%import_ref.13, element0 [template = constants.%Op.3]
-// CHECK:STDOUT:   %Op.call: init %.2 = call %.1()
+// CHECK:STDOUT:   %.loc4_15: %Op.type.2 = interface_witness_access imports.%import_ref.13, element0 [template = constants.%Op.3]
+// CHECK:STDOUT:   %Op.call: init %.2 = call %.loc4_15()
 // CHECK:STDOUT:   return
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
@@ -612,11 +612,11 @@ fn MakeC(a: A) -> C {
 // CHECK:STDOUT:     %B.ref: type = name_ref B, file.%B.decl [template = constants.%B]
 // CHECK:STDOUT:     %return.var: ref %B = var <return slot>
 // CHECK:STDOUT:   }
-// CHECK:STDOUT:   %.1: <witness> = interface_witness (%Make.decl) [template = constants.%.9]
+// CHECK:STDOUT:   %.loc11: <witness> = interface_witness (%Make.decl) [template = constants.%.9]
 // CHECK:STDOUT:
 // CHECK:STDOUT: !members:
 // CHECK:STDOUT:   .Make = %Make.decl
-// CHECK:STDOUT:   witness = %.1
+// CHECK:STDOUT:   witness = %.loc11
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
 // CHECK:STDOUT: class @A {
@@ -777,9 +777,9 @@ fn MakeC(a: A) -> C {
 // CHECK:STDOUT:   %.loc5_20: init type = call %Factory.ref(%B.ref) [template = constants.%.4]
 // CHECK:STDOUT:   %.loc5_23: %.7 = specific_constant imports.%import_ref.7, @Factory(constants.%B) [template = constants.%.8]
 // CHECK:STDOUT:   %Make.ref: %.7 = name_ref Make, %.loc5_23 [template = constants.%.8]
-// CHECK:STDOUT:   %.1: %Make.type.2 = interface_witness_access imports.%import_ref.11, element0 [template = constants.%Make.3]
+// CHECK:STDOUT:   %.loc5_11: %Make.type.2 = interface_witness_access imports.%import_ref.11, element0 [template = constants.%Make.3]
 // CHECK:STDOUT:   %.loc4: ref %B = splice_block %return {}
-// CHECK:STDOUT:   %Make.call: init %B = call %.1() to %.loc4
+// CHECK:STDOUT:   %Make.call: init %B = call %.loc5_11() to %.loc4
 // CHECK:STDOUT:   return %Make.call to %return
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
diff --git a/toolchain/check/testdata/impl/no_prelude/no_definition_in_impl_file.carbon b/toolchain/check/testdata/impl/no_prelude/no_definition_in_impl_file.carbon
index 997a48e12708..ffe2e09b8b35 100644
--- a/toolchain/check/testdata/impl/no_prelude/no_definition_in_impl_file.carbon
+++ b/toolchain/check/testdata/impl/no_prelude/no_definition_in_impl_file.carbon
@@ -148,10 +148,10 @@ impl () as D;
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
 // CHECK:STDOUT: impl @impl: %.1 as %.2 {
-// CHECK:STDOUT:   %.1: <witness> = interface_witness () [template = constants.%.3]
+// CHECK:STDOUT:   %.loc6: <witness> = interface_witness () [template = constants.%.3]
 // CHECK:STDOUT:
 // CHECK:STDOUT: !members:
-// CHECK:STDOUT:   witness = %.1
+// CHECK:STDOUT:   witness = %.loc6
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
 // CHECK:STDOUT: --- use_decl_in_api.carbon
diff --git a/toolchain/check/testdata/impl/no_prelude/self_in_class.carbon b/toolchain/check/testdata/impl/no_prelude/self_in_class.carbon
index 02df8aa1edd8..a461abbbdd40 100644
--- a/toolchain/check/testdata/impl/no_prelude/self_in_class.carbon
+++ b/toolchain/check/testdata/impl/no_prelude/self_in_class.carbon
@@ -74,11 +74,11 @@ class A {
 // CHECK:STDOUT:     %Self.ref: type = name_ref Self, constants.%C [template = constants.%C]
 // CHECK:STDOUT:     %return.var: ref %C = var <return slot>
 // CHECK:STDOUT:   }
-// CHECK:STDOUT:   %.1: <witness> = interface_witness (%Make.decl) [template = constants.%.6]
+// CHECK:STDOUT:   %.loc18: <witness> = interface_witness (%Make.decl) [template = constants.%.6]
 // CHECK:STDOUT:
 // CHECK:STDOUT: !members:
 // CHECK:STDOUT:   .Make = %Make.decl
-// CHECK:STDOUT:   witness = %.1
+// CHECK:STDOUT:   witness = %.loc18
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
 // CHECK:STDOUT: class @C {
diff --git a/toolchain/check/testdata/impl/no_prelude/self_in_signature.carbon b/toolchain/check/testdata/impl/no_prelude/self_in_signature.carbon
index 5e27aad06c9d..6d9d7f106e58 100644
--- a/toolchain/check/testdata/impl/no_prelude/self_in_signature.carbon
+++ b/toolchain/check/testdata/impl/no_prelude/self_in_signature.carbon
@@ -174,11 +174,11 @@ impl D as SelfNested {
 // CHECK:STDOUT:     %C.ref.loc20_26: type = name_ref C, file.%C.decl [template = constants.%C]
 // CHECK:STDOUT:     %return.var: ref %C = var <return slot>
 // CHECK:STDOUT:   }
-// CHECK:STDOUT:   %.1: <witness> = interface_witness (%F.decl) [template = constants.%.6]
+// CHECK:STDOUT:   %.loc19: <witness> = interface_witness (%F.decl) [template = constants.%.6]
 // CHECK:STDOUT:
 // CHECK:STDOUT: !members:
 // CHECK:STDOUT:   .F = %F.decl
-// CHECK:STDOUT:   witness = %.1
+// CHECK:STDOUT:   witness = %.loc19
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
 // CHECK:STDOUT: impl @impl.2: %D as %.1 {
@@ -192,11 +192,11 @@ impl D as SelfNested {
 // CHECK:STDOUT:     %Self.ref.loc24_32: type = name_ref Self, constants.%D [template = constants.%D]
 // CHECK:STDOUT:     %return.var: ref %D = var <return slot>
 // CHECK:STDOUT:   }
-// CHECK:STDOUT:   %.1: <witness> = interface_witness (%F.decl) [template = constants.%.8]
+// CHECK:STDOUT:   %.loc23: <witness> = interface_witness (%F.decl) [template = constants.%.8]
 // CHECK:STDOUT:
 // CHECK:STDOUT: !members:
 // CHECK:STDOUT:   .F = %F.decl
-// CHECK:STDOUT:   witness = %.1
+// CHECK:STDOUT:   witness = %.loc23
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
 // CHECK:STDOUT: impl @impl.3: %C as %.9 {
@@ -212,11 +212,11 @@ impl D as SelfNested {
 // CHECK:STDOUT:     %x.loc32_8.1: %.18 = param x
 // CHECK:STDOUT:     %x.loc32_8.2: %.18 = bind_name x, %x.loc32_8.1
 // CHECK:STDOUT:   }
-// CHECK:STDOUT:   %.1: <witness> = interface_witness (%F.decl) [template = constants.%.19]
+// CHECK:STDOUT:   %.loc31: <witness> = interface_witness (%F.decl) [template = constants.%.19]
 // CHECK:STDOUT:
 // CHECK:STDOUT: !members:
 // CHECK:STDOUT:   .F = %F.decl
-// CHECK:STDOUT:   witness = %.1
+// CHECK:STDOUT:   witness = %.loc31
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
 // CHECK:STDOUT: impl @impl.4: %D as %.9 {
@@ -232,11 +232,11 @@ impl D as SelfNested {
 // CHECK:STDOUT:     %x.loc36_8.1: %.22 = param x
 // CHECK:STDOUT:     %x.loc36_8.2: %.22 = bind_name x, %x.loc36_8.1
 // CHECK:STDOUT:   }
-// CHECK:STDOUT:   %.1: <witness> = interface_witness (%F.decl) [template = constants.%.23]
+// CHECK:STDOUT:   %.loc35: <witness> = interface_witness (%F.decl) [template = constants.%.23]
 // CHECK:STDOUT:
 // CHECK:STDOUT: !members:
 // CHECK:STDOUT:   .F = %F.decl
-// CHECK:STDOUT:   witness = %.1
+// CHECK:STDOUT:   witness = %.loc35
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
 // CHECK:STDOUT: class @C {
diff --git a/toolchain/check/testdata/impl/redeclaration.carbon b/toolchain/check/testdata/impl/redeclaration.carbon
index dd48c35c3c2e..ac9b1db78a76 100644
--- a/toolchain/check/testdata/impl/redeclaration.carbon
+++ b/toolchain/check/testdata/impl/redeclaration.carbon
@@ -77,10 +77,10 @@ impl i32 as I {}
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
 // CHECK:STDOUT: impl @impl: i32 as %.1 {
-// CHECK:STDOUT:   %.1: <witness> = interface_witness () [template = constants.%.4]
+// CHECK:STDOUT:   %.loc19: <witness> = interface_witness () [template = constants.%.4]
 // CHECK:STDOUT:
 // CHECK:STDOUT: !members:
-// CHECK:STDOUT:   witness = %.1
+// CHECK:STDOUT:   witness = %.loc19
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
 // CHECK:STDOUT: class @X {
diff --git a/toolchain/check/testdata/interface/no_prelude/default_fn.carbon b/toolchain/check/testdata/interface/no_prelude/default_fn.carbon
index 8a0237651b45..37af99c98238 100644
--- a/toolchain/check/testdata/interface/no_prelude/default_fn.carbon
+++ b/toolchain/check/testdata/interface/no_prelude/default_fn.carbon
@@ -62,11 +62,11 @@ class C {
 // CHECK:STDOUT:
 // CHECK:STDOUT: impl @impl: %C as %.1 {
 // CHECK:STDOUT:   %F.decl: %F.type.2 = fn_decl @F.2 [template = constants.%F.2] {}
-// CHECK:STDOUT:   %.1: <witness> = interface_witness (%F.decl) [template = constants.%.5]
+// CHECK:STDOUT:   %.loc21: <witness> = interface_witness (%F.decl) [template = constants.%.5]
 // CHECK:STDOUT:
 // CHECK:STDOUT: !members:
 // CHECK:STDOUT:   .F = %F.decl
-// CHECK:STDOUT:   witness = %.1
+// CHECK:STDOUT:   witness = %.loc21
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
 // CHECK:STDOUT: class @C {
@@ -96,8 +96,8 @@ class C {
 // CHECK:STDOUT:     %c.ref: ref %C = name_ref c, %c
 // CHECK:STDOUT:     %I.ref: type = name_ref I, @C.%I.decl [template = constants.%.1]
 // CHECK:STDOUT:     %F.ref: %.3 = name_ref F, @I.%.loc14 [template = constants.%.4]
-// CHECK:STDOUT:     %.1: %F.type.1 = interface_witness_access @impl.%.1, element0 [template = constants.%F.2]
-// CHECK:STDOUT:     %F.call: init %.2 = call %.1()
+// CHECK:STDOUT:     %.loc17: %F.type.1 = interface_witness_access @impl.%.loc21, element0 [template = constants.%F.2]
+// CHECK:STDOUT:     %F.call: init %.2 = call %.loc17()
 // CHECK:STDOUT:     return
 // CHECK:STDOUT:   }
 // CHECK:STDOUT: }
diff --git a/toolchain/check/testdata/interface/no_prelude/generic.carbon b/toolchain/check/testdata/interface/no_prelude/generic.carbon
index 2f39413b559d..7523927de6e8 100644
--- a/toolchain/check/testdata/interface/no_prelude/generic.carbon
+++ b/toolchain/check/testdata/interface/no_prelude/generic.carbon
@@ -197,10 +197,10 @@ fn G(T:! Generic(B)) {
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
 // CHECK:STDOUT: impl @impl.1: %C as %.7 {
-// CHECK:STDOUT:   %.1: <witness> = interface_witness () [template = constants.%.8]
+// CHECK:STDOUT:   %.loc14: <witness> = interface_witness () [template = constants.%.8]
 // CHECK:STDOUT:
 // CHECK:STDOUT: !members:
-// CHECK:STDOUT:   witness = %.1
+// CHECK:STDOUT:   witness = %.loc14
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
 // CHECK:STDOUT: impl @impl.2: %C as %.9 {
@@ -208,11 +208,11 @@ fn G(T:! Generic(B)) {
 // CHECK:STDOUT:     %X.ref: type = name_ref X, file.%X.decl [template = constants.%X]
 // CHECK:STDOUT:     %return.var: ref %X = var <return slot>
 // CHECK:STDOUT:   }
-// CHECK:STDOUT:   %.1: <witness> = interface_witness (%F.decl) [template = constants.%.12]
+// CHECK:STDOUT:   %.loc15: <witness> = interface_witness (%F.decl) [template = constants.%.12]
 // CHECK:STDOUT:
 // CHECK:STDOUT: !members:
 // CHECK:STDOUT:   .F = %F.decl
-// CHECK:STDOUT:   witness = %.1
+// CHECK:STDOUT:   witness = %.loc15
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
 // CHECK:STDOUT: class @X {
diff --git a/toolchain/check/testdata/interface/no_prelude/generic_import.carbon b/toolchain/check/testdata/interface/no_prelude/generic_import.carbon
index 5c5139a5d979..6f3454a9fad6 100644
--- a/toolchain/check/testdata/interface/no_prelude/generic_import.carbon
+++ b/toolchain/check/testdata/interface/no_prelude/generic_import.carbon
@@ -162,11 +162,11 @@ impl C as AddWith(C) {
 // CHECK:STDOUT:
 // CHECK:STDOUT: impl @impl: %C as %.6 {
 // CHECK:STDOUT:   %F.decl: %F.type.2 = fn_decl @F.2 [template = constants.%F.2] {}
-// CHECK:STDOUT:   %.1: <witness> = interface_witness (%F.decl) [template = constants.%.9]
+// CHECK:STDOUT:   %.loc7: <witness> = interface_witness (%F.decl) [template = constants.%.9]
 // CHECK:STDOUT:
 // CHECK:STDOUT: !members:
 // CHECK:STDOUT:   .F = %F.decl
-// CHECK:STDOUT:   witness = %.1
+// CHECK:STDOUT:   witness = %.loc7
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
 // CHECK:STDOUT: class @C {
diff --git a/toolchain/check/testdata/interface/no_prelude/generic_vs_params.carbon b/toolchain/check/testdata/interface/no_prelude/generic_vs_params.carbon
index e850f99ca2ba..48b8de914e92 100644
--- a/toolchain/check/testdata/interface/no_prelude/generic_vs_params.carbon
+++ b/toolchain/check/testdata/interface/no_prelude/generic_vs_params.carbon
@@ -206,38 +206,38 @@ interface A(T: type) {}
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
 // CHECK:STDOUT: impl @impl.1: %X as %.1 {
-// CHECK:STDOUT:   %.1: <witness> = interface_witness () [template = constants.%.9]
+// CHECK:STDOUT:   %.loc14: <witness> = interface_witness () [template = constants.%.9]
 // CHECK:STDOUT:
 // CHECK:STDOUT: !members:
-// CHECK:STDOUT:   witness = %.1
+// CHECK:STDOUT:   witness = %.loc14
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
 // CHECK:STDOUT: impl @impl.2: %X as %.3 {
-// CHECK:STDOUT:   %.1: <witness> = interface_witness () [template = constants.%.9]
+// CHECK:STDOUT:   %.loc15: <witness> = interface_witness () [template = constants.%.9]
 // CHECK:STDOUT:
 // CHECK:STDOUT: !members:
-// CHECK:STDOUT:   witness = %.1
+// CHECK:STDOUT:   witness = %.loc15
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
 // CHECK:STDOUT: impl @impl.3: %X as %.10 {
-// CHECK:STDOUT:   %.1: <witness> = interface_witness () [template = constants.%.9]
+// CHECK:STDOUT:   %.loc16: <witness> = interface_witness () [template = constants.%.9]
 // CHECK:STDOUT:
 // CHECK:STDOUT: !members:
-// CHECK:STDOUT:   witness = %.1
+// CHECK:STDOUT:   witness = %.loc16
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
 // CHECK:STDOUT: impl @impl.4: %X as %.5 {
-// CHECK:STDOUT:   %.1: <witness> = interface_witness () [template = constants.%.9]
+// CHECK:STDOUT:   %.loc17: <witness> = interface_witness () [template = constants.%.9]
 // CHECK:STDOUT:
 // CHECK:STDOUT: !members:
-// CHECK:STDOUT:   witness = %.1
+// CHECK:STDOUT:   witness = %.loc17
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
 // CHECK:STDOUT: impl @impl.5: %X as %.12 {
-// CHECK:STDOUT:   %.1: <witness> = interface_witness () [template = constants.%.9]
+// CHECK:STDOUT:   %.loc18: <witness> = interface_witness () [template = constants.%.9]
 // CHECK:STDOUT:
 // CHECK:STDOUT: !members:
-// CHECK:STDOUT:   witness = %.1
+// CHECK:STDOUT:   witness = %.loc18
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
 // CHECK:STDOUT: generic class @C(file.%T.loc8_9.2: type) {
diff --git a/toolchain/check/testdata/operators/overloaded/add.carbon b/toolchain/check/testdata/operators/overloaded/add.carbon
index ac5ba53a0719..fcbc275ec5ec 100644
--- a/toolchain/check/testdata/operators/overloaded/add.carbon
+++ b/toolchain/check/testdata/operators/overloaded/add.carbon
@@ -154,11 +154,11 @@ fn TestAssign(a: C*, b: C) {
 // CHECK:STDOUT:     %C.ref.loc18_31: type = name_ref C, file.%C.decl [template = constants.%C]
 // CHECK:STDOUT:     %return.var: ref %C = var <return slot>
 // CHECK:STDOUT:   }
-// CHECK:STDOUT:   %.1: <witness> = interface_witness (%Op.decl) [template = constants.%.4]
+// CHECK:STDOUT:   %.loc17: <witness> = interface_witness (%Op.decl) [template = constants.%.4]
 // CHECK:STDOUT:
 // CHECK:STDOUT: !members:
 // CHECK:STDOUT:   .Op = %Op.decl
-// CHECK:STDOUT:   witness = %.1
+// CHECK:STDOUT:   witness = %.loc17
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
 // CHECK:STDOUT: impl @impl.2: %C as %.6 {
@@ -172,11 +172,11 @@ fn TestAssign(a: C*, b: C) {
 // CHECK:STDOUT:     %other.loc23_24.1: %C = param other
 // CHECK:STDOUT:     %other.loc23_24.2: %C = bind_name other, %other.loc23_24.1
 // CHECK:STDOUT:   }
-// CHECK:STDOUT:   %.1: <witness> = interface_witness (%Op.decl) [template = constants.%.9]
+// CHECK:STDOUT:   %.loc22: <witness> = interface_witness (%Op.decl) [template = constants.%.9]
 // CHECK:STDOUT:
 // CHECK:STDOUT: !members:
 // CHECK:STDOUT:   .Op = %Op.decl
-// CHECK:STDOUT:   witness = %.1
+// CHECK:STDOUT:   witness = %.loc22
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
 // CHECK:STDOUT: class @C {
@@ -214,10 +214,10 @@ fn TestAssign(a: C*, b: C) {
 // CHECK:STDOUT: !entry:
 // CHECK:STDOUT:   %a.ref: %C = name_ref a, %a
 // CHECK:STDOUT:   %b.ref: %C = name_ref b, %b
-// CHECK:STDOUT:   %.1: %Op.type.2 = interface_witness_access @impl.1.%.1, element0 [template = constants.%Op.1]
-// CHECK:STDOUT:   %.loc27: <bound method> = bound_method %a.ref, %.1
+// CHECK:STDOUT:   %.loc27_12.1: %Op.type.2 = interface_witness_access @impl.1.%.loc17, element0 [template = constants.%Op.1]
+// CHECK:STDOUT:   %.loc27_12.2: <bound method> = bound_method %a.ref, %.loc27_12.1
 // CHECK:STDOUT:   %.loc26: ref %C = splice_block %return {}
-// CHECK:STDOUT:   %Op.call: init %C = call %.loc27(%a.ref, %b.ref) to %.loc26
+// CHECK:STDOUT:   %Op.call: init %C = call %.loc27_12.2(%a.ref, %b.ref) to %.loc26
 // CHECK:STDOUT:   return %Op.call to %return
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
@@ -226,10 +226,10 @@ fn TestAssign(a: C*, b: C) {
 // CHECK:STDOUT:   %a.ref: %.7 = name_ref a, %a
 // CHECK:STDOUT:   %.loc31_3.1: ref %C = deref %a.ref
 // CHECK:STDOUT:   %b.ref: %C = name_ref b, %b
-// CHECK:STDOUT:   %.1: %Op.type.4 = interface_witness_access @impl.2.%.1, element0 [template = constants.%Op.3]
-// CHECK:STDOUT:   %.loc31_6: <bound method> = bound_method %.loc31_3.1, %.1
+// CHECK:STDOUT:   %.loc31_6.1: %Op.type.4 = interface_witness_access @impl.2.%.loc22, element0 [template = constants.%Op.3]
+// CHECK:STDOUT:   %.loc31_6.2: <bound method> = bound_method %.loc31_3.1, %.loc31_6.1
 // CHECK:STDOUT:   %.loc31_3.2: %.7 = addr_of %.loc31_3.1
-// CHECK:STDOUT:   %Op.call: init %.3 = call %.loc31_6(%.loc31_3.2, %b.ref)
+// CHECK:STDOUT:   %Op.call: init %.3 = call %.loc31_6.2(%.loc31_3.2, %b.ref)
 // CHECK:STDOUT:   return
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
diff --git a/toolchain/check/testdata/operators/overloaded/bit_and.carbon b/toolchain/check/testdata/operators/overloaded/bit_and.carbon
index 25c214eb1eb6..d5bf31dce7b7 100644
--- a/toolchain/check/testdata/operators/overloaded/bit_and.carbon
+++ b/toolchain/check/testdata/operators/overloaded/bit_and.carbon
@@ -154,11 +154,11 @@ fn TestAssign(a: C*, b: C) {
 // CHECK:STDOUT:     %C.ref.loc18_31: type = name_ref C, file.%C.decl [template = constants.%C]
 // CHECK:STDOUT:     %return.var: ref %C = var <return slot>
 // CHECK:STDOUT:   }
-// CHECK:STDOUT:   %.1: <witness> = interface_witness (%Op.decl) [template = constants.%.4]
+// CHECK:STDOUT:   %.loc17: <witness> = interface_witness (%Op.decl) [template = constants.%.4]
 // CHECK:STDOUT:
 // CHECK:STDOUT: !members:
 // CHECK:STDOUT:   .Op = %Op.decl
-// CHECK:STDOUT:   witness = %.1
+// CHECK:STDOUT:   witness = %.loc17
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
 // CHECK:STDOUT: impl @impl.2: %C as %.6 {
@@ -172,11 +172,11 @@ fn TestAssign(a: C*, b: C) {
 // CHECK:STDOUT:     %other.loc23_24.1: %C = param other
 // CHECK:STDOUT:     %other.loc23_24.2: %C = bind_name other, %other.loc23_24.1
 // CHECK:STDOUT:   }
-// CHECK:STDOUT:   %.1: <witness> = interface_witness (%Op.decl) [template = constants.%.9]
+// CHECK:STDOUT:   %.loc22: <witness> = interface_witness (%Op.decl) [template = constants.%.9]
 // CHECK:STDOUT:
 // CHECK:STDOUT: !members:
 // CHECK:STDOUT:   .Op = %Op.decl
-// CHECK:STDOUT:   witness = %.1
+// CHECK:STDOUT:   witness = %.loc22
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
 // CHECK:STDOUT: class @C {
@@ -214,10 +214,10 @@ fn TestAssign(a: C*, b: C) {
 // CHECK:STDOUT: !entry:
 // CHECK:STDOUT:   %a.ref: %C = name_ref a, %a
 // CHECK:STDOUT:   %b.ref: %C = name_ref b, %b
-// CHECK:STDOUT:   %.1: %Op.type.2 = interface_witness_access @impl.1.%.1, element0 [template = constants.%Op.1]
-// CHECK:STDOUT:   %.loc27: <bound method> = bound_method %a.ref, %.1
+// CHECK:STDOUT:   %.loc27_12.1: %Op.type.2 = interface_witness_access @impl.1.%.loc17, element0 [template = constants.%Op.1]
+// CHECK:STDOUT:   %.loc27_12.2: <bound method> = bound_method %a.ref, %.loc27_12.1
 // CHECK:STDOUT:   %.loc26: ref %C = splice_block %return {}
-// CHECK:STDOUT:   %Op.call: init %C = call %.loc27(%a.ref, %b.ref) to %.loc26
+// CHECK:STDOUT:   %Op.call: init %C = call %.loc27_12.2(%a.ref, %b.ref) to %.loc26
 // CHECK:STDOUT:   return %Op.call to %return
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
@@ -226,10 +226,10 @@ fn TestAssign(a: C*, b: C) {
 // CHECK:STDOUT:   %a.ref: %.7 = name_ref a, %a
 // CHECK:STDOUT:   %.loc31_3.1: ref %C = deref %a.ref
 // CHECK:STDOUT:   %b.ref: %C = name_ref b, %b
-// CHECK:STDOUT:   %.1: %Op.type.4 = interface_witness_access @impl.2.%.1, element0 [template = constants.%Op.3]
-// CHECK:STDOUT:   %.loc31_6: <bound method> = bound_method %.loc31_3.1, %.1
+// CHECK:STDOUT:   %.loc31_6.1: %Op.type.4 = interface_witness_access @impl.2.%.loc22, element0 [template = constants.%Op.3]
+// CHECK:STDOUT:   %.loc31_6.2: <bound method> = bound_method %.loc31_3.1, %.loc31_6.1
 // CHECK:STDOUT:   %.loc31_3.2: %.7 = addr_of %.loc31_3.1
-// CHECK:STDOUT:   %Op.call: init %.3 = call %.loc31_6(%.loc31_3.2, %b.ref)
+// CHECK:STDOUT:   %Op.call: init %.3 = call %.loc31_6.2(%.loc31_3.2, %b.ref)
 // CHECK:STDOUT:   return
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
diff --git a/toolchain/check/testdata/operators/overloaded/bit_complement.carbon b/toolchain/check/testdata/operators/overloaded/bit_complement.carbon
index 4d3d60bc1425..0f32dddeaeb9 100644
--- a/toolchain/check/testdata/operators/overloaded/bit_complement.carbon
+++ b/toolchain/check/testdata/operators/overloaded/bit_complement.carbon
@@ -100,11 +100,11 @@ fn TestOp(a: C) -> C {
 // CHECK:STDOUT:     %C.ref.loc18_23: type = name_ref C, file.%C.decl [template = constants.%C]
 // CHECK:STDOUT:     %return.var: ref %C = var <return slot>
 // CHECK:STDOUT:   }
-// CHECK:STDOUT:   %.1: <witness> = interface_witness (%Op.decl) [template = constants.%.4]
+// CHECK:STDOUT:   %.loc17: <witness> = interface_witness (%Op.decl) [template = constants.%.4]
 // CHECK:STDOUT:
 // CHECK:STDOUT: !members:
 // CHECK:STDOUT:   .Op = %Op.decl
-// CHECK:STDOUT:   witness = %.1
+// CHECK:STDOUT:   witness = %.loc17
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
 // CHECK:STDOUT: class @C {
@@ -129,10 +129,10 @@ fn TestOp(a: C) -> C {
 // CHECK:STDOUT: fn @TestOp(%a: %C) -> %return: %C {
 // CHECK:STDOUT: !entry:
 // CHECK:STDOUT:   %a.ref: %C = name_ref a, %a
-// CHECK:STDOUT:   %.1: %Op.type.2 = interface_witness_access @impl.%.1, element0 [template = constants.%Op.1]
-// CHECK:STDOUT:   %.loc24: <bound method> = bound_method %a.ref, %.1
+// CHECK:STDOUT:   %.loc24_10.1: %Op.type.2 = interface_witness_access @impl.%.loc17, element0 [template = constants.%Op.1]
+// CHECK:STDOUT:   %.loc24_10.2: <bound method> = bound_method %a.ref, %.loc24_10.1
 // CHECK:STDOUT:   %.loc23: ref %C = splice_block %return {}
-// CHECK:STDOUT:   %Op.call: init %C = call %.loc24(%a.ref) to %.loc23
+// CHECK:STDOUT:   %Op.call: init %C = call %.loc24_10.2(%a.ref) to %.loc23
 // CHECK:STDOUT:   return %Op.call to %return
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
diff --git a/toolchain/check/testdata/operators/overloaded/bit_or.carbon b/toolchain/check/testdata/operators/overloaded/bit_or.carbon
index c087104f72ee..0c77b6d7e5ed 100644
--- a/toolchain/check/testdata/operators/overloaded/bit_or.carbon
+++ b/toolchain/check/testdata/operators/overloaded/bit_or.carbon
@@ -154,11 +154,11 @@ fn TestAssign(a: C*, b: C) {
 // CHECK:STDOUT:     %C.ref.loc18_31: type = name_ref C, file.%C.decl [template = constants.%C]
 // CHECK:STDOUT:     %return.var: ref %C = var <return slot>
 // CHECK:STDOUT:   }
-// CHECK:STDOUT:   %.1: <witness> = interface_witness (%Op.decl) [template = constants.%.4]
+// CHECK:STDOUT:   %.loc17: <witness> = interface_witness (%Op.decl) [template = constants.%.4]
 // CHECK:STDOUT:
 // CHECK:STDOUT: !members:
 // CHECK:STDOUT:   .Op = %Op.decl
-// CHECK:STDOUT:   witness = %.1
+// CHECK:STDOUT:   witness = %.loc17
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
 // CHECK:STDOUT: impl @impl.2: %C as %.6 {
@@ -172,11 +172,11 @@ fn TestAssign(a: C*, b: C) {
 // CHECK:STDOUT:     %other.loc23_24.1: %C = param other
 // CHECK:STDOUT:     %other.loc23_24.2: %C = bind_name other, %other.loc23_24.1
 // CHECK:STDOUT:   }
-// CHECK:STDOUT:   %.1: <witness> = interface_witness (%Op.decl) [template = constants.%.9]
+// CHECK:STDOUT:   %.loc22: <witness> = interface_witness (%Op.decl) [template = constants.%.9]
 // CHECK:STDOUT:
 // CHECK:STDOUT: !members:
 // CHECK:STDOUT:   .Op = %Op.decl
-// CHECK:STDOUT:   witness = %.1
+// CHECK:STDOUT:   witness = %.loc22
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
 // CHECK:STDOUT: class @C {
@@ -214,10 +214,10 @@ fn TestAssign(a: C*, b: C) {
 // CHECK:STDOUT: !entry:
 // CHECK:STDOUT:   %a.ref: %C = name_ref a, %a
 // CHECK:STDOUT:   %b.ref: %C = name_ref b, %b
-// CHECK:STDOUT:   %.1: %Op.type.2 = interface_witness_access @impl.1.%.1, element0 [template = constants.%Op.1]
-// CHECK:STDOUT:   %.loc27: <bound method> = bound_method %a.ref, %.1
+// CHECK:STDOUT:   %.loc27_12.1: %Op.type.2 = interface_witness_access @impl.1.%.loc17, element0 [template = constants.%Op.1]
+// CHECK:STDOUT:   %.loc27_12.2: <bound method> = bound_method %a.ref, %.loc27_12.1
 // CHECK:STDOUT:   %.loc26: ref %C = splice_block %return {}
-// CHECK:STDOUT:   %Op.call: init %C = call %.loc27(%a.ref, %b.ref) to %.loc26
+// CHECK:STDOUT:   %Op.call: init %C = call %.loc27_12.2(%a.ref, %b.ref) to %.loc26
 // CHECK:STDOUT:   return %Op.call to %return
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
@@ -226,10 +226,10 @@ fn TestAssign(a: C*, b: C) {
 // CHECK:STDOUT:   %a.ref: %.7 = name_ref a, %a
 // CHECK:STDOUT:   %.loc31_3.1: ref %C = deref %a.ref
 // CHECK:STDOUT:   %b.ref: %C = name_ref b, %b
-// CHECK:STDOUT:   %.1: %Op.type.4 = interface_witness_access @impl.2.%.1, element0 [template = constants.%Op.3]
-// CHECK:STDOUT:   %.loc31_6: <bound method> = bound_method %.loc31_3.1, %.1
+// CHECK:STDOUT:   %.loc31_6.1: %Op.type.4 = interface_witness_access @impl.2.%.loc22, element0 [template = constants.%Op.3]
+// CHECK:STDOUT:   %.loc31_6.2: <bound method> = bound_method %.loc31_3.1, %.loc31_6.1
 // CHECK:STDOUT:   %.loc31_3.2: %.7 = addr_of %.loc31_3.1
-// CHECK:STDOUT:   %Op.call: init %.3 = call %.loc31_6(%.loc31_3.2, %b.ref)
+// CHECK:STDOUT:   %Op.call: init %.3 = call %.loc31_6.2(%.loc31_3.2, %b.ref)
 // CHECK:STDOUT:   return
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
diff --git a/toolchain/check/testdata/operators/overloaded/bit_xor.carbon b/toolchain/check/testdata/operators/overloaded/bit_xor.carbon
index a93d9dd935b5..aed75065e854 100644
--- a/toolchain/check/testdata/operators/overloaded/bit_xor.carbon
+++ b/toolchain/check/testdata/operators/overloaded/bit_xor.carbon
@@ -154,11 +154,11 @@ fn TestAssign(a: C*, b: C) {
 // CHECK:STDOUT:     %C.ref.loc18_31: type = name_ref C, file.%C.decl [template = constants.%C]
 // CHECK:STDOUT:     %return.var: ref %C = var <return slot>
 // CHECK:STDOUT:   }
-// CHECK:STDOUT:   %.1: <witness> = interface_witness (%Op.decl) [template = constants.%.4]
+// CHECK:STDOUT:   %.loc17: <witness> = interface_witness (%Op.decl) [template = constants.%.4]
 // CHECK:STDOUT:
 // CHECK:STDOUT: !members:
 // CHECK:STDOUT:   .Op = %Op.decl
-// CHECK:STDOUT:   witness = %.1
+// CHECK:STDOUT:   witness = %.loc17
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
 // CHECK:STDOUT: impl @impl.2: %C as %.6 {
@@ -172,11 +172,11 @@ fn TestAssign(a: C*, b: C) {
 // CHECK:STDOUT:     %other.loc23_24.1: %C = param other
 // CHECK:STDOUT:     %other.loc23_24.2: %C = bind_name other, %other.loc23_24.1
 // CHECK:STDOUT:   }
-// CHECK:STDOUT:   %.1: <witness> = interface_witness (%Op.decl) [template = constants.%.9]
+// CHECK:STDOUT:   %.loc22: <witness> = interface_witness (%Op.decl) [template = constants.%.9]
 // CHECK:STDOUT:
 // CHECK:STDOUT: !members:
 // CHECK:STDOUT:   .Op = %Op.decl
-// CHECK:STDOUT:   witness = %.1
+// CHECK:STDOUT:   witness = %.loc22
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
 // CHECK:STDOUT: class @C {
@@ -214,10 +214,10 @@ fn TestAssign(a: C*, b: C) {
 // CHECK:STDOUT: !entry:
 // CHECK:STDOUT:   %a.ref: %C = name_ref a, %a
 // CHECK:STDOUT:   %b.ref: %C = name_ref b, %b
-// CHECK:STDOUT:   %.1: %Op.type.2 = interface_witness_access @impl.1.%.1, element0 [template = constants.%Op.1]
-// CHECK:STDOUT:   %.loc27: <bound method> = bound_method %a.ref, %.1
+// CHECK:STDOUT:   %.loc27_12.1: %Op.type.2 = interface_witness_access @impl.1.%.loc17, element0 [template = constants.%Op.1]
+// CHECK:STDOUT:   %.loc27_12.2: <bound method> = bound_method %a.ref, %.loc27_12.1
 // CHECK:STDOUT:   %.loc26: ref %C = splice_block %return {}
-// CHECK:STDOUT:   %Op.call: init %C = call %.loc27(%a.ref, %b.ref) to %.loc26
+// CHECK:STDOUT:   %Op.call: init %C = call %.loc27_12.2(%a.ref, %b.ref) to %.loc26
 // CHECK:STDOUT:   return %Op.call to %return
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
@@ -226,10 +226,10 @@ fn TestAssign(a: C*, b: C) {
 // CHECK:STDOUT:   %a.ref: %.7 = name_ref a, %a
 // CHECK:STDOUT:   %.loc31_3.1: ref %C = deref %a.ref
 // CHECK:STDOUT:   %b.ref: %C = name_ref b, %b
-// CHECK:STDOUT:   %.1: %Op.type.4 = interface_witness_access @impl.2.%.1, element0 [template = constants.%Op.3]
-// CHECK:STDOUT:   %.loc31_6: <bound method> = bound_method %.loc31_3.1, %.1
+// CHECK:STDOUT:   %.loc31_6.1: %Op.type.4 = interface_witness_access @impl.2.%.loc22, element0 [template = constants.%Op.3]
+// CHECK:STDOUT:   %.loc31_6.2: <bound method> = bound_method %.loc31_3.1, %.loc31_6.1
 // CHECK:STDOUT:   %.loc31_3.2: %.7 = addr_of %.loc31_3.1
-// CHECK:STDOUT:   %Op.call: init %.3 = call %.loc31_6(%.loc31_3.2, %b.ref)
+// CHECK:STDOUT:   %Op.call: init %.3 = call %.loc31_6.2(%.loc31_3.2, %b.ref)
 // CHECK:STDOUT:   return
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
diff --git a/toolchain/check/testdata/operators/overloaded/dec.carbon b/toolchain/check/testdata/operators/overloaded/dec.carbon
index 333bb490e5b3..27cbc0bf2103 100644
--- a/toolchain/check/testdata/operators/overloaded/dec.carbon
+++ b/toolchain/check/testdata/operators/overloaded/dec.carbon
@@ -95,11 +95,11 @@ fn TestOp() {
 // CHECK:STDOUT:     %self.loc18_14.3: %.3 = bind_name self, %self.loc18_14.1
 // CHECK:STDOUT:     %.loc18_9: %.3 = addr_pattern %self.loc18_14.3
 // CHECK:STDOUT:   }
-// CHECK:STDOUT:   %.1: <witness> = interface_witness (%Op.decl) [template = constants.%.6]
+// CHECK:STDOUT:   %.loc17: <witness> = interface_witness (%Op.decl) [template = constants.%.6]
 // CHECK:STDOUT:
 // CHECK:STDOUT: !members:
 // CHECK:STDOUT:   .Op = %Op.decl
-// CHECK:STDOUT:   witness = %.1
+// CHECK:STDOUT:   witness = %.loc17
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
 // CHECK:STDOUT: class @C {
@@ -126,10 +126,10 @@ fn TestOp() {
 // CHECK:STDOUT:   %.loc22_16: init %C = converted %.loc22_15.1, %.loc22_15.2 [template = constants.%struct]
 // CHECK:STDOUT:   assign %c.var, %.loc22_16
 // CHECK:STDOUT:   %c.ref: ref %C = name_ref c, %c
-// CHECK:STDOUT:   %.1: %Op.type.2 = interface_witness_access @impl.%.1, element0 [template = constants.%Op.1]
-// CHECK:STDOUT:   %.loc23_3: <bound method> = bound_method %c.ref, %.1
+// CHECK:STDOUT:   %.loc23_3.1: %Op.type.2 = interface_witness_access @impl.%.loc17, element0 [template = constants.%Op.1]
+// CHECK:STDOUT:   %.loc23_3.2: <bound method> = bound_method %c.ref, %.loc23_3.1
 // CHECK:STDOUT:   %.loc23_5: %.3 = addr_of %c.ref
-// CHECK:STDOUT:   %Op.call: init %.4 = call %.loc23_3(%.loc23_5)
+// CHECK:STDOUT:   %Op.call: init %.4 = call %.loc23_3.2(%.loc23_5)
 // CHECK:STDOUT:   return
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
diff --git a/toolchain/check/testdata/operators/overloaded/div.carbon b/toolchain/check/testdata/operators/overloaded/div.carbon
index 9804b5ef27af..a1e49aa3c14d 100644
--- a/toolchain/check/testdata/operators/overloaded/div.carbon
+++ b/toolchain/check/testdata/operators/overloaded/div.carbon
@@ -154,11 +154,11 @@ fn TestAssign(a: C*, b: C) {
 // CHECK:STDOUT:     %C.ref.loc18_31: type = name_ref C, file.%C.decl [template = constants.%C]
 // CHECK:STDOUT:     %return.var: ref %C = var <return slot>
 // CHECK:STDOUT:   }
-// CHECK:STDOUT:   %.1: <witness> = interface_witness (%Op.decl) [template = constants.%.4]
+// CHECK:STDOUT:   %.loc17: <witness> = interface_witness (%Op.decl) [template = constants.%.4]
 // CHECK:STDOUT:
 // CHECK:STDOUT: !members:
 // CHECK:STDOUT:   .Op = %Op.decl
-// CHECK:STDOUT:   witness = %.1
+// CHECK:STDOUT:   witness = %.loc17
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
 // CHECK:STDOUT: impl @impl.2: %C as %.6 {
@@ -172,11 +172,11 @@ fn TestAssign(a: C*, b: C) {
 // CHECK:STDOUT:     %other.loc23_24.1: %C = param other
 // CHECK:STDOUT:     %other.loc23_24.2: %C = bind_name other, %other.loc23_24.1
 // CHECK:STDOUT:   }
-// CHECK:STDOUT:   %.1: <witness> = interface_witness (%Op.decl) [template = constants.%.9]
+// CHECK:STDOUT:   %.loc22: <witness> = interface_witness (%Op.decl) [template = constants.%.9]
 // CHECK:STDOUT:
 // CHECK:STDOUT: !members:
 // CHECK:STDOUT:   .Op = %Op.decl
-// CHECK:STDOUT:   witness = %.1
+// CHECK:STDOUT:   witness = %.loc22
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
 // CHECK:STDOUT: class @C {
@@ -214,10 +214,10 @@ fn TestAssign(a: C*, b: C) {
 // CHECK:STDOUT: !entry:
 // CHECK:STDOUT:   %a.ref: %C = name_ref a, %a
 // CHECK:STDOUT:   %b.ref: %C = name_ref b, %b
-// CHECK:STDOUT:   %.1: %Op.type.2 = interface_witness_access @impl.1.%.1, element0 [template = constants.%Op.1]
-// CHECK:STDOUT:   %.loc27: <bound method> = bound_method %a.ref, %.1
+// CHECK:STDOUT:   %.loc27_12.1: %Op.type.2 = interface_witness_access @impl.1.%.loc17, element0 [template = constants.%Op.1]
+// CHECK:STDOUT:   %.loc27_12.2: <bound method> = bound_method %a.ref, %.loc27_12.1
 // CHECK:STDOUT:   %.loc26: ref %C = splice_block %return {}
-// CHECK:STDOUT:   %Op.call: init %C = call %.loc27(%a.ref, %b.ref) to %.loc26
+// CHECK:STDOUT:   %Op.call: init %C = call %.loc27_12.2(%a.ref, %b.ref) to %.loc26
 // CHECK:STDOUT:   return %Op.call to %return
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
@@ -226,10 +226,10 @@ fn TestAssign(a: C*, b: C) {
 // CHECK:STDOUT:   %a.ref: %.7 = name_ref a, %a
 // CHECK:STDOUT:   %.loc31_3.1: ref %C = deref %a.ref
 // CHECK:STDOUT:   %b.ref: %C = name_ref b, %b
-// CHECK:STDOUT:   %.1: %Op.type.4 = interface_witness_access @impl.2.%.1, element0 [template = constants.%Op.3]
-// CHECK:STDOUT:   %.loc31_6: <bound method> = bound_method %.loc31_3.1, %.1
+// CHECK:STDOUT:   %.loc31_6.1: %Op.type.4 = interface_witness_access @impl.2.%.loc22, element0 [template = constants.%Op.3]
+// CHECK:STDOUT:   %.loc31_6.2: <bound method> = bound_method %.loc31_3.1, %.loc31_6.1
 // CHECK:STDOUT:   %.loc31_3.2: %.7 = addr_of %.loc31_3.1
-// CHECK:STDOUT:   %Op.call: init %.3 = call %.loc31_6(%.loc31_3.2, %b.ref)
+// CHECK:STDOUT:   %Op.call: init %.3 = call %.loc31_6.2(%.loc31_3.2, %b.ref)
 // CHECK:STDOUT:   return
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
diff --git a/toolchain/check/testdata/operators/overloaded/eq.carbon b/toolchain/check/testdata/operators/overloaded/eq.carbon
index 64d37cdc2718..3843b5e9e018 100644
--- a/toolchain/check/testdata/operators/overloaded/eq.carbon
+++ b/toolchain/check/testdata/operators/overloaded/eq.carbon
@@ -205,12 +205,12 @@ fn TestLhsBad(a: D, b: C) -> bool {
 // CHECK:STDOUT:     %.loc8_37.2: type = converted %bool.make_type.loc8, %.loc8_37.1 [template = bool]
 // CHECK:STDOUT:     %return.var.loc8: ref bool = var <return slot>
 // CHECK:STDOUT:   }
-// CHECK:STDOUT:   %.1: <witness> = interface_witness (%Equal.decl, %NotEqual.decl) [template = constants.%.4]
+// CHECK:STDOUT:   %.loc6: <witness> = interface_witness (%Equal.decl, %NotEqual.decl) [template = constants.%.4]
 // CHECK:STDOUT:
 // CHECK:STDOUT: !members:
 // CHECK:STDOUT:   .Equal = %Equal.decl
 // CHECK:STDOUT:   .NotEqual = %NotEqual.decl
-// CHECK:STDOUT:   witness = %.1
+// CHECK:STDOUT:   witness = %.loc6
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
 // CHECK:STDOUT: class @C {
@@ -240,9 +240,9 @@ fn TestLhsBad(a: D, b: C) -> bool {
 // CHECK:STDOUT: !entry:
 // CHECK:STDOUT:   %a.ref: %C = name_ref a, %a
 // CHECK:STDOUT:   %b.ref: %C = name_ref b, %b
-// CHECK:STDOUT:   %.1: %Equal.type.2 = interface_witness_access @impl.%.1, element0 [template = constants.%Equal.1]
-// CHECK:STDOUT:   %.loc12_12: <bound method> = bound_method %a.ref, %.1
-// CHECK:STDOUT:   %Equal.call: init bool = call %.loc12_12(%a.ref, %b.ref)
+// CHECK:STDOUT:   %.loc12_12.1: %Equal.type.2 = interface_witness_access @impl.%.loc6, element0 [template = constants.%Equal.1]
+// CHECK:STDOUT:   %.loc12_12.2: <bound method> = bound_method %a.ref, %.loc12_12.1
+// CHECK:STDOUT:   %Equal.call: init bool = call %.loc12_12.2(%a.ref, %b.ref)
 // CHECK:STDOUT:   %.loc12_16.1: bool = value_of_initializer %Equal.call
 // CHECK:STDOUT:   %.loc12_16.2: bool = converted %Equal.call, %.loc12_16.1
 // CHECK:STDOUT:   return %.loc12_16.2
@@ -252,9 +252,9 @@ fn TestLhsBad(a: D, b: C) -> bool {
 // CHECK:STDOUT: !entry:
 // CHECK:STDOUT:   %a.ref: %C = name_ref a, %a
 // CHECK:STDOUT:   %b.ref: %C = name_ref b, %b
-// CHECK:STDOUT:   %.1: %NotEqual.type.2 = interface_witness_access @impl.%.1, element1 [template = constants.%NotEqual.1]
-// CHECK:STDOUT:   %.loc16_12: <bound method> = bound_method %a.ref, %.1
-// CHECK:STDOUT:   %NotEqual.call: init bool = call %.loc16_12(%a.ref, %b.ref)
+// CHECK:STDOUT:   %.loc16_12.1: %NotEqual.type.2 = interface_witness_access @impl.%.loc6, element1 [template = constants.%NotEqual.1]
+// CHECK:STDOUT:   %.loc16_12.2: <bound method> = bound_method %a.ref, %.loc16_12.1
+// CHECK:STDOUT:   %NotEqual.call: init bool = call %.loc16_12.2(%a.ref, %b.ref)
 // CHECK:STDOUT:   %.loc16_16.1: bool = value_of_initializer %NotEqual.call
 // CHECK:STDOUT:   %.loc16_16.2: bool = converted %NotEqual.call, %.loc16_16.1
 // CHECK:STDOUT:   return %.loc16_16.2
@@ -537,12 +537,12 @@ fn TestLhsBad(a: D, b: C) -> bool {
 // CHECK:STDOUT:     %.loc9_37.2: type = converted %bool.make_type.loc9, %.loc9_37.1 [template = bool]
 // CHECK:STDOUT:     %return.var.loc9: ref bool = var <return slot>
 // CHECK:STDOUT:   }
-// CHECK:STDOUT:   %.1: <witness> = interface_witness (%Equal.decl, %NotEqual.decl) [template = constants.%.4]
+// CHECK:STDOUT:   %.loc7: <witness> = interface_witness (%Equal.decl, %NotEqual.decl) [template = constants.%.4]
 // CHECK:STDOUT:
 // CHECK:STDOUT: !members:
 // CHECK:STDOUT:   .Equal = %Equal.decl
 // CHECK:STDOUT:   .NotEqual = %NotEqual.decl
-// CHECK:STDOUT:   witness = %.1
+// CHECK:STDOUT:   witness = %.loc7
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
 // CHECK:STDOUT: class @C {
@@ -577,9 +577,9 @@ fn TestLhsBad(a: D, b: C) -> bool {
 // CHECK:STDOUT: !entry:
 // CHECK:STDOUT:   %a.ref: %C = name_ref a, %a
 // CHECK:STDOUT:   %b.ref: %D = name_ref b, %b
-// CHECK:STDOUT:   %.1: %Equal.type.2 = interface_witness_access @impl.%.1, element0 [template = constants.%Equal.1]
-// CHECK:STDOUT:   %.loc20_12: <bound method> = bound_method %a.ref, %.1
-// CHECK:STDOUT:   %Equal.call: init bool = call %.loc20_12(<invalid>) [template = <error>]
+// CHECK:STDOUT:   %.loc20_12.1: %Equal.type.2 = interface_witness_access @impl.%.loc7, element0 [template = constants.%Equal.1]
+// CHECK:STDOUT:   %.loc20_12.2: <bound method> = bound_method %a.ref, %.loc20_12.1
+// CHECK:STDOUT:   %Equal.call: init bool = call %.loc20_12.2(<invalid>) [template = <error>]
 // CHECK:STDOUT:   %.loc20_16.1: bool = value_of_initializer %Equal.call [template = <error>]
 // CHECK:STDOUT:   %.loc20_16.2: bool = converted %Equal.call, %.loc20_16.1 [template = <error>]
 // CHECK:STDOUT:   return %.loc20_16.2
diff --git a/toolchain/check/testdata/operators/overloaded/fail_assign_non_ref.carbon b/toolchain/check/testdata/operators/overloaded/fail_assign_non_ref.carbon
index d1d87a0c20a3..ae6abd7909f1 100644
--- a/toolchain/check/testdata/operators/overloaded/fail_assign_non_ref.carbon
+++ b/toolchain/check/testdata/operators/overloaded/fail_assign_non_ref.carbon
@@ -154,11 +154,11 @@ fn TestAddAssignNonRef(a: C, b: C) {
 // CHECK:STDOUT:     %self.loc16_14.3: %.3 = bind_name self, %self.loc16_14.1
 // CHECK:STDOUT:     %.loc16_9: %.3 = addr_pattern %self.loc16_14.3
 // CHECK:STDOUT:   }
-// CHECK:STDOUT:   %.1: <witness> = interface_witness (%Op.decl) [template = constants.%.6]
+// CHECK:STDOUT:   %.loc15: <witness> = interface_witness (%Op.decl) [template = constants.%.6]
 // CHECK:STDOUT:
 // CHECK:STDOUT: !members:
 // CHECK:STDOUT:   .Op = %Op.decl
-// CHECK:STDOUT:   witness = %.1
+// CHECK:STDOUT:   witness = %.loc15
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
 // CHECK:STDOUT: impl @impl.2: %C as %.7 {
@@ -172,11 +172,11 @@ fn TestAddAssignNonRef(a: C, b: C) {
 // CHECK:STDOUT:     %other.loc19_24.1: %C = param other
 // CHECK:STDOUT:     %other.loc19_24.2: %C = bind_name other, %other.loc19_24.1
 // CHECK:STDOUT:   }
-// CHECK:STDOUT:   %.1: <witness> = interface_witness (%Op.decl) [template = constants.%.9]
+// CHECK:STDOUT:   %.loc18: <witness> = interface_witness (%Op.decl) [template = constants.%.9]
 // CHECK:STDOUT:
 // CHECK:STDOUT: !members:
 // CHECK:STDOUT:   .Op = %Op.decl
-// CHECK:STDOUT:   witness = %.1
+// CHECK:STDOUT:   witness = %.loc18
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
 // CHECK:STDOUT: class @C {
@@ -205,9 +205,9 @@ fn TestAddAssignNonRef(a: C, b: C) {
 // CHECK:STDOUT: fn @TestIncNonRef(%a: %C) {
 // CHECK:STDOUT: !entry:
 // CHECK:STDOUT:   %a.ref: %C = name_ref a, %a
-// CHECK:STDOUT:   %.1: %Op.type.2 = interface_witness_access @impl.1.%.1, element0 [template = constants.%Op.1]
-// CHECK:STDOUT:   %.loc30: <bound method> = bound_method %a.ref, %.1
-// CHECK:STDOUT:   %Op.call: init %.4 = call %.loc30(<invalid>) [template = <error>]
+// CHECK:STDOUT:   %.loc30_3.1: %Op.type.2 = interface_witness_access @impl.1.%.loc15, element0 [template = constants.%Op.1]
+// CHECK:STDOUT:   %.loc30_3.2: <bound method> = bound_method %a.ref, %.loc30_3.1
+// CHECK:STDOUT:   %Op.call: init %.4 = call %.loc30_3.2(<invalid>) [template = <error>]
 // CHECK:STDOUT:   return
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
@@ -215,9 +215,9 @@ fn TestAddAssignNonRef(a: C, b: C) {
 // CHECK:STDOUT: !entry:
 // CHECK:STDOUT:   %a.ref: %C = name_ref a, %a
 // CHECK:STDOUT:   %b.ref: %C = name_ref b, %b
-// CHECK:STDOUT:   %.1: %Op.type.4 = interface_witness_access @impl.2.%.1, element0 [template = constants.%Op.3]
-// CHECK:STDOUT:   %.loc40: <bound method> = bound_method %a.ref, %.1
-// CHECK:STDOUT:   %Op.call: init %.4 = call %.loc40(<invalid>) [template = <error>]
+// CHECK:STDOUT:   %.loc40_5.1: %Op.type.4 = interface_witness_access @impl.2.%.loc18, element0 [template = constants.%Op.3]
+// CHECK:STDOUT:   %.loc40_5.2: <bound method> = bound_method %a.ref, %.loc40_5.1
+// CHECK:STDOUT:   %Op.call: init %.4 = call %.loc40_5.2(<invalid>) [template = <error>]
 // CHECK:STDOUT:   return
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
diff --git a/toolchain/check/testdata/operators/overloaded/fail_no_impl_for_arg.carbon b/toolchain/check/testdata/operators/overloaded/fail_no_impl_for_arg.carbon
index 98639511b49f..2ce85687340f 100644
--- a/toolchain/check/testdata/operators/overloaded/fail_no_impl_for_arg.carbon
+++ b/toolchain/check/testdata/operators/overloaded/fail_no_impl_for_arg.carbon
@@ -164,11 +164,11 @@ fn TestAssign(b: D) {
 // CHECK:STDOUT:     %C.ref.loc17_31: type = name_ref C, file.%C.decl [template = constants.%C]
 // CHECK:STDOUT:     %return.var: ref %C = var <return slot>
 // CHECK:STDOUT:   }
-// CHECK:STDOUT:   %.1: <witness> = interface_witness (%Op.decl) [template = constants.%.4]
+// CHECK:STDOUT:   %.loc16: <witness> = interface_witness (%Op.decl) [template = constants.%.4]
 // CHECK:STDOUT:
 // CHECK:STDOUT: !members:
 // CHECK:STDOUT:   .Op = %Op.decl
-// CHECK:STDOUT:   witness = %.1
+// CHECK:STDOUT:   witness = %.loc16
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
 // CHECK:STDOUT: impl @impl.2: %C as %.5 {
@@ -182,11 +182,11 @@ fn TestAssign(b: D) {
 // CHECK:STDOUT:     %other.loc20_24.1: %C = param other
 // CHECK:STDOUT:     %other.loc20_24.2: %C = bind_name other, %other.loc20_24.1
 // CHECK:STDOUT:   }
-// CHECK:STDOUT:   %.1: <witness> = interface_witness (%Op.decl) [template = constants.%.8]
+// CHECK:STDOUT:   %.loc19: <witness> = interface_witness (%Op.decl) [template = constants.%.8]
 // CHECK:STDOUT:
 // CHECK:STDOUT: !members:
 // CHECK:STDOUT:   .Op = %Op.decl
-// CHECK:STDOUT:   witness = %.1
+// CHECK:STDOUT:   witness = %.loc19
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
 // CHECK:STDOUT: class @C {
@@ -220,10 +220,10 @@ fn TestAssign(b: D) {
 // CHECK:STDOUT: !entry:
 // CHECK:STDOUT:   %a.ref: %C = name_ref a, %a
 // CHECK:STDOUT:   %b.ref: %D = name_ref b, %b
-// CHECK:STDOUT:   %.1: %Op.type.2 = interface_witness_access @impl.1.%.1, element0 [template = constants.%Op.1]
-// CHECK:STDOUT:   %.loc31_12.1: <bound method> = bound_method %a.ref, %.1
-// CHECK:STDOUT:   %.loc31_12.2: ref %C = temporary_storage
-// CHECK:STDOUT:   %Op.call: init %C = call %.loc31_12.1(<invalid>) [template = <error>]
+// CHECK:STDOUT:   %.loc31_12.1: %Op.type.2 = interface_witness_access @impl.1.%.loc16, element0 [template = constants.%Op.1]
+// CHECK:STDOUT:   %.loc31_12.2: <bound method> = bound_method %a.ref, %.loc31_12.1
+// CHECK:STDOUT:   %.loc31_12.3: ref %C = temporary_storage
+// CHECK:STDOUT:   %Op.call: init %C = call %.loc31_12.2(<invalid>) [template = <error>]
 // CHECK:STDOUT:   return %Op.call to %return
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
@@ -238,10 +238,10 @@ fn TestAssign(b: D) {
 // CHECK:STDOUT:   assign %a.var, %.loc35_16
 // CHECK:STDOUT:   %a.ref: ref %C = name_ref a, %a
 // CHECK:STDOUT:   %b.ref: %D = name_ref b, %b
-// CHECK:STDOUT:   %.1: %Op.type.4 = interface_witness_access @impl.2.%.1, element0 [template = constants.%Op.3]
-// CHECK:STDOUT:   %.loc42_5: <bound method> = bound_method %a.ref, %.1
+// CHECK:STDOUT:   %.loc42_5.1: %Op.type.4 = interface_witness_access @impl.2.%.loc19, element0 [template = constants.%Op.3]
+// CHECK:STDOUT:   %.loc42_5.2: <bound method> = bound_method %a.ref, %.loc42_5.1
 // CHECK:STDOUT:   %.loc42_3: %.6 = addr_of %a.ref
-// CHECK:STDOUT:   %Op.call: init %.3 = call %.loc42_5(<invalid>) [template = <error>]
+// CHECK:STDOUT:   %Op.call: init %.3 = call %.loc42_5.2(<invalid>) [template = <error>]
 // CHECK:STDOUT:   return
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
diff --git a/toolchain/check/testdata/operators/overloaded/inc.carbon b/toolchain/check/testdata/operators/overloaded/inc.carbon
index ba75207ede6a..17c79e622b6a 100644
--- a/toolchain/check/testdata/operators/overloaded/inc.carbon
+++ b/toolchain/check/testdata/operators/overloaded/inc.carbon
@@ -95,11 +95,11 @@ fn TestOp() {
 // CHECK:STDOUT:     %self.loc18_14.3: %.3 = bind_name self, %self.loc18_14.1
 // CHECK:STDOUT:     %.loc18_9: %.3 = addr_pattern %self.loc18_14.3
 // CHECK:STDOUT:   }
-// CHECK:STDOUT:   %.1: <witness> = interface_witness (%Op.decl) [template = constants.%.6]
+// CHECK:STDOUT:   %.loc17: <witness> = interface_witness (%Op.decl) [template = constants.%.6]
 // CHECK:STDOUT:
 // CHECK:STDOUT: !members:
 // CHECK:STDOUT:   .Op = %Op.decl
-// CHECK:STDOUT:   witness = %.1
+// CHECK:STDOUT:   witness = %.loc17
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
 // CHECK:STDOUT: class @C {
@@ -126,10 +126,10 @@ fn TestOp() {
 // CHECK:STDOUT:   %.loc22_16: init %C = converted %.loc22_15.1, %.loc22_15.2 [template = constants.%struct]
 // CHECK:STDOUT:   assign %c.var, %.loc22_16
 // CHECK:STDOUT:   %c.ref: ref %C = name_ref c, %c
-// CHECK:STDOUT:   %.1: %Op.type.2 = interface_witness_access @impl.%.1, element0 [template = constants.%Op.1]
-// CHECK:STDOUT:   %.loc23_3: <bound method> = bound_method %c.ref, %.1
+// CHECK:STDOUT:   %.loc23_3.1: %Op.type.2 = interface_witness_access @impl.%.loc17, element0 [template = constants.%Op.1]
+// CHECK:STDOUT:   %.loc23_3.2: <bound method> = bound_method %c.ref, %.loc23_3.1
 // CHECK:STDOUT:   %.loc23_5: %.3 = addr_of %c.ref
-// CHECK:STDOUT:   %Op.call: init %.4 = call %.loc23_3(%.loc23_5)
+// CHECK:STDOUT:   %Op.call: init %.4 = call %.loc23_3.2(%.loc23_5)
 // CHECK:STDOUT:   return
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
diff --git a/toolchain/check/testdata/operators/overloaded/left_shift.carbon b/toolchain/check/testdata/operators/overloaded/left_shift.carbon
index b61f2fa7ad0d..5cdae1089bf2 100644
--- a/toolchain/check/testdata/operators/overloaded/left_shift.carbon
+++ b/toolchain/check/testdata/operators/overloaded/left_shift.carbon
@@ -154,11 +154,11 @@ fn TestAssign(a: C*, b: C) {
 // CHECK:STDOUT:     %C.ref.loc18_31: type = name_ref C, file.%C.decl [template = constants.%C]
 // CHECK:STDOUT:     %return.var: ref %C = var <return slot>
 // CHECK:STDOUT:   }
-// CHECK:STDOUT:   %.1: <witness> = interface_witness (%Op.decl) [template = constants.%.4]
+// CHECK:STDOUT:   %.loc17: <witness> = interface_witness (%Op.decl) [template = constants.%.4]
 // CHECK:STDOUT:
 // CHECK:STDOUT: !members:
 // CHECK:STDOUT:   .Op = %Op.decl
-// CHECK:STDOUT:   witness = %.1
+// CHECK:STDOUT:   witness = %.loc17
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
 // CHECK:STDOUT: impl @impl.2: %C as %.6 {
@@ -172,11 +172,11 @@ fn TestAssign(a: C*, b: C) {
 // CHECK:STDOUT:     %other.loc23_24.1: %C = param other
 // CHECK:STDOUT:     %other.loc23_24.2: %C = bind_name other, %other.loc23_24.1
 // CHECK:STDOUT:   }
-// CHECK:STDOUT:   %.1: <witness> = interface_witness (%Op.decl) [template = constants.%.9]
+// CHECK:STDOUT:   %.loc22: <witness> = interface_witness (%Op.decl) [template = constants.%.9]
 // CHECK:STDOUT:
 // CHECK:STDOUT: !members:
 // CHECK:STDOUT:   .Op = %Op.decl
-// CHECK:STDOUT:   witness = %.1
+// CHECK:STDOUT:   witness = %.loc22
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
 // CHECK:STDOUT: class @C {
@@ -214,10 +214,10 @@ fn TestAssign(a: C*, b: C) {
 // CHECK:STDOUT: !entry:
 // CHECK:STDOUT:   %a.ref: %C = name_ref a, %a
 // CHECK:STDOUT:   %b.ref: %C = name_ref b, %b
-// CHECK:STDOUT:   %.1: %Op.type.2 = interface_witness_access @impl.1.%.1, element0 [template = constants.%Op.1]
-// CHECK:STDOUT:   %.loc27: <bound method> = bound_method %a.ref, %.1
+// CHECK:STDOUT:   %.loc27_12.1: %Op.type.2 = interface_witness_access @impl.1.%.loc17, element0 [template = constants.%Op.1]
+// CHECK:STDOUT:   %.loc27_12.2: <bound method> = bound_method %a.ref, %.loc27_12.1
 // CHECK:STDOUT:   %.loc26: ref %C = splice_block %return {}
-// CHECK:STDOUT:   %Op.call: init %C = call %.loc27(%a.ref, %b.ref) to %.loc26
+// CHECK:STDOUT:   %Op.call: init %C = call %.loc27_12.2(%a.ref, %b.ref) to %.loc26
 // CHECK:STDOUT:   return %Op.call to %return
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
@@ -226,10 +226,10 @@ fn TestAssign(a: C*, b: C) {
 // CHECK:STDOUT:   %a.ref: %.7 = name_ref a, %a
 // CHECK:STDOUT:   %.loc31_3.1: ref %C = deref %a.ref
 // CHECK:STDOUT:   %b.ref: %C = name_ref b, %b
-// CHECK:STDOUT:   %.1: %Op.type.4 = interface_witness_access @impl.2.%.1, element0 [template = constants.%Op.3]
-// CHECK:STDOUT:   %.loc31_6: <bound method> = bound_method %.loc31_3.1, %.1
+// CHECK:STDOUT:   %.loc31_6.1: %Op.type.4 = interface_witness_access @impl.2.%.loc22, element0 [template = constants.%Op.3]
+// CHECK:STDOUT:   %.loc31_6.2: <bound method> = bound_method %.loc31_3.1, %.loc31_6.1
 // CHECK:STDOUT:   %.loc31_3.2: %.7 = addr_of %.loc31_3.1
-// CHECK:STDOUT:   %Op.call: init %.3 = call %.loc31_6(%.loc31_3.2, %b.ref)
+// CHECK:STDOUT:   %Op.call: init %.3 = call %.loc31_6.2(%.loc31_3.2, %b.ref)
 // CHECK:STDOUT:   return
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
diff --git a/toolchain/check/testdata/operators/overloaded/mod.carbon b/toolchain/check/testdata/operators/overloaded/mod.carbon
index 144664dc827d..271554915051 100644
--- a/toolchain/check/testdata/operators/overloaded/mod.carbon
+++ b/toolchain/check/testdata/operators/overloaded/mod.carbon
@@ -154,11 +154,11 @@ fn TestAssign(a: C*, b: C) {
 // CHECK:STDOUT:     %C.ref.loc18_31: type = name_ref C, file.%C.decl [template = constants.%C]
 // CHECK:STDOUT:     %return.var: ref %C = var <return slot>
 // CHECK:STDOUT:   }
-// CHECK:STDOUT:   %.1: <witness> = interface_witness (%Op.decl) [template = constants.%.4]
+// CHECK:STDOUT:   %.loc17: <witness> = interface_witness (%Op.decl) [template = constants.%.4]
 // CHECK:STDOUT:
 // CHECK:STDOUT: !members:
 // CHECK:STDOUT:   .Op = %Op.decl
-// CHECK:STDOUT:   witness = %.1
+// CHECK:STDOUT:   witness = %.loc17
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
 // CHECK:STDOUT: impl @impl.2: %C as %.6 {
@@ -172,11 +172,11 @@ fn TestAssign(a: C*, b: C) {
 // CHECK:STDOUT:     %other.loc23_24.1: %C = param other
 // CHECK:STDOUT:     %other.loc23_24.2: %C = bind_name other, %other.loc23_24.1
 // CHECK:STDOUT:   }
-// CHECK:STDOUT:   %.1: <witness> = interface_witness (%Op.decl) [template = constants.%.9]
+// CHECK:STDOUT:   %.loc22: <witness> = interface_witness (%Op.decl) [template = constants.%.9]
 // CHECK:STDOUT:
 // CHECK:STDOUT: !members:
 // CHECK:STDOUT:   .Op = %Op.decl
-// CHECK:STDOUT:   witness = %.1
+// CHECK:STDOUT:   witness = %.loc22
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
 // CHECK:STDOUT: class @C {
@@ -214,10 +214,10 @@ fn TestAssign(a: C*, b: C) {
 // CHECK:STDOUT: !entry:
 // CHECK:STDOUT:   %a.ref: %C = name_ref a, %a
 // CHECK:STDOUT:   %b.ref: %C = name_ref b, %b
-// CHECK:STDOUT:   %.1: %Op.type.2 = interface_witness_access @impl.1.%.1, element0 [template = constants.%Op.1]
-// CHECK:STDOUT:   %.loc27: <bound method> = bound_method %a.ref, %.1
+// CHECK:STDOUT:   %.loc27_12.1: %Op.type.2 = interface_witness_access @impl.1.%.loc17, element0 [template = constants.%Op.1]
+// CHECK:STDOUT:   %.loc27_12.2: <bound method> = bound_method %a.ref, %.loc27_12.1
 // CHECK:STDOUT:   %.loc26: ref %C = splice_block %return {}
-// CHECK:STDOUT:   %Op.call: init %C = call %.loc27(%a.ref, %b.ref) to %.loc26
+// CHECK:STDOUT:   %Op.call: init %C = call %.loc27_12.2(%a.ref, %b.ref) to %.loc26
 // CHECK:STDOUT:   return %Op.call to %return
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
@@ -226,10 +226,10 @@ fn TestAssign(a: C*, b: C) {
 // CHECK:STDOUT:   %a.ref: %.7 = name_ref a, %a
 // CHECK:STDOUT:   %.loc31_3.1: ref %C = deref %a.ref
 // CHECK:STDOUT:   %b.ref: %C = name_ref b, %b
-// CHECK:STDOUT:   %.1: %Op.type.4 = interface_witness_access @impl.2.%.1, element0 [template = constants.%Op.3]
-// CHECK:STDOUT:   %.loc31_6: <bound method> = bound_method %.loc31_3.1, %.1
+// CHECK:STDOUT:   %.loc31_6.1: %Op.type.4 = interface_witness_access @impl.2.%.loc22, element0 [template = constants.%Op.3]
+// CHECK:STDOUT:   %.loc31_6.2: <bound method> = bound_method %.loc31_3.1, %.loc31_6.1
 // CHECK:STDOUT:   %.loc31_3.2: %.7 = addr_of %.loc31_3.1
-// CHECK:STDOUT:   %Op.call: init %.3 = call %.loc31_6(%.loc31_3.2, %b.ref)
+// CHECK:STDOUT:   %Op.call: init %.3 = call %.loc31_6.2(%.loc31_3.2, %b.ref)
 // CHECK:STDOUT:   return
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
diff --git a/toolchain/check/testdata/operators/overloaded/mul.carbon b/toolchain/check/testdata/operators/overloaded/mul.carbon
index 0d4be385fff0..d77f9e3b862f 100644
--- a/toolchain/check/testdata/operators/overloaded/mul.carbon
+++ b/toolchain/check/testdata/operators/overloaded/mul.carbon
@@ -154,11 +154,11 @@ fn TestAssign(a: C*, b: C) {
 // CHECK:STDOUT:     %C.ref.loc18_31: type = name_ref C, file.%C.decl [template = constants.%C]
 // CHECK:STDOUT:     %return.var: ref %C = var <return slot>
 // CHECK:STDOUT:   }
-// CHECK:STDOUT:   %.1: <witness> = interface_witness (%Op.decl) [template = constants.%.4]
+// CHECK:STDOUT:   %.loc17: <witness> = interface_witness (%Op.decl) [template = constants.%.4]
 // CHECK:STDOUT:
 // CHECK:STDOUT: !members:
 // CHECK:STDOUT:   .Op = %Op.decl
-// CHECK:STDOUT:   witness = %.1
+// CHECK:STDOUT:   witness = %.loc17
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
 // CHECK:STDOUT: impl @impl.2: %C as %.6 {
@@ -172,11 +172,11 @@ fn TestAssign(a: C*, b: C) {
 // CHECK:STDOUT:     %other.loc23_24.1: %C = param other
 // CHECK:STDOUT:     %other.loc23_24.2: %C = bind_name other, %other.loc23_24.1
 // CHECK:STDOUT:   }
-// CHECK:STDOUT:   %.1: <witness> = interface_witness (%Op.decl) [template = constants.%.9]
+// CHECK:STDOUT:   %.loc22: <witness> = interface_witness (%Op.decl) [template = constants.%.9]
 // CHECK:STDOUT:
 // CHECK:STDOUT: !members:
 // CHECK:STDOUT:   .Op = %Op.decl
-// CHECK:STDOUT:   witness = %.1
+// CHECK:STDOUT:   witness = %.loc22
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
 // CHECK:STDOUT: class @C {
@@ -214,10 +214,10 @@ fn TestAssign(a: C*, b: C) {
 // CHECK:STDOUT: !entry:
 // CHECK:STDOUT:   %a.ref: %C = name_ref a, %a
 // CHECK:STDOUT:   %b.ref: %C = name_ref b, %b
-// CHECK:STDOUT:   %.1: %Op.type.2 = interface_witness_access @impl.1.%.1, element0 [template = constants.%Op.1]
-// CHECK:STDOUT:   %.loc27: <bound method> = bound_method %a.ref, %.1
+// CHECK:STDOUT:   %.loc27_12.1: %Op.type.2 = interface_witness_access @impl.1.%.loc17, element0 [template = constants.%Op.1]
+// CHECK:STDOUT:   %.loc27_12.2: <bound method> = bound_method %a.ref, %.loc27_12.1
 // CHECK:STDOUT:   %.loc26: ref %C = splice_block %return {}
-// CHECK:STDOUT:   %Op.call: init %C = call %.loc27(%a.ref, %b.ref) to %.loc26
+// CHECK:STDOUT:   %Op.call: init %C = call %.loc27_12.2(%a.ref, %b.ref) to %.loc26
 // CHECK:STDOUT:   return %Op.call to %return
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
@@ -226,10 +226,10 @@ fn TestAssign(a: C*, b: C) {
 // CHECK:STDOUT:   %a.ref: %.7 = name_ref a, %a
 // CHECK:STDOUT:   %.loc31_3.1: ref %C = deref %a.ref
 // CHECK:STDOUT:   %b.ref: %C = name_ref b, %b
-// CHECK:STDOUT:   %.1: %Op.type.4 = interface_witness_access @impl.2.%.1, element0 [template = constants.%Op.3]
-// CHECK:STDOUT:   %.loc31_6: <bound method> = bound_method %.loc31_3.1, %.1
+// CHECK:STDOUT:   %.loc31_6.1: %Op.type.4 = interface_witness_access @impl.2.%.loc22, element0 [template = constants.%Op.3]
+// CHECK:STDOUT:   %.loc31_6.2: <bound method> = bound_method %.loc31_3.1, %.loc31_6.1
 // CHECK:STDOUT:   %.loc31_3.2: %.7 = addr_of %.loc31_3.1
-// CHECK:STDOUT:   %Op.call: init %.3 = call %.loc31_6(%.loc31_3.2, %b.ref)
+// CHECK:STDOUT:   %Op.call: init %.3 = call %.loc31_6.2(%.loc31_3.2, %b.ref)
 // CHECK:STDOUT:   return
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
diff --git a/toolchain/check/testdata/operators/overloaded/negate.carbon b/toolchain/check/testdata/operators/overloaded/negate.carbon
index d490f46eafb9..e0558cd5af7f 100644
--- a/toolchain/check/testdata/operators/overloaded/negate.carbon
+++ b/toolchain/check/testdata/operators/overloaded/negate.carbon
@@ -100,11 +100,11 @@ fn TestOp(a: C) -> C {
 // CHECK:STDOUT:     %C.ref.loc18_23: type = name_ref C, file.%C.decl [template = constants.%C]
 // CHECK:STDOUT:     %return.var: ref %C = var <return slot>
 // CHECK:STDOUT:   }
-// CHECK:STDOUT:   %.1: <witness> = interface_witness (%Op.decl) [template = constants.%.4]
+// CHECK:STDOUT:   %.loc17: <witness> = interface_witness (%Op.decl) [template = constants.%.4]
 // CHECK:STDOUT:
 // CHECK:STDOUT: !members:
 // CHECK:STDOUT:   .Op = %Op.decl
-// CHECK:STDOUT:   witness = %.1
+// CHECK:STDOUT:   witness = %.loc17
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
 // CHECK:STDOUT: class @C {
@@ -129,10 +129,10 @@ fn TestOp(a: C) -> C {
 // CHECK:STDOUT: fn @TestOp(%a: %C) -> %return: %C {
 // CHECK:STDOUT: !entry:
 // CHECK:STDOUT:   %a.ref: %C = name_ref a, %a
-// CHECK:STDOUT:   %.1: %Op.type.2 = interface_witness_access @impl.%.1, element0 [template = constants.%Op.1]
-// CHECK:STDOUT:   %.loc24: <bound method> = bound_method %a.ref, %.1
+// CHECK:STDOUT:   %.loc24_10.1: %Op.type.2 = interface_witness_access @impl.%.loc17, element0 [template = constants.%Op.1]
+// CHECK:STDOUT:   %.loc24_10.2: <bound method> = bound_method %a.ref, %.loc24_10.1
 // CHECK:STDOUT:   %.loc23: ref %C = splice_block %return {}
-// CHECK:STDOUT:   %Op.call: init %C = call %.loc24(%a.ref) to %.loc23
+// CHECK:STDOUT:   %Op.call: init %C = call %.loc24_10.2(%a.ref) to %.loc23
 // CHECK:STDOUT:   return %Op.call to %return
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
diff --git a/toolchain/check/testdata/operators/overloaded/ordered.carbon b/toolchain/check/testdata/operators/overloaded/ordered.carbon
index a8b19aa306ad..6c4938e0eb44 100644
--- a/toolchain/check/testdata/operators/overloaded/ordered.carbon
+++ b/toolchain/check/testdata/operators/overloaded/ordered.carbon
@@ -274,14 +274,14 @@ fn TestGreaterEqual(a: D, b: D) -> bool {
 // CHECK:STDOUT:     %.loc10_48.2: type = converted %bool.make_type.loc10, %.loc10_48.1 [template = bool]
 // CHECK:STDOUT:     %return.var.loc10: ref bool = var <return slot>
 // CHECK:STDOUT:   }
-// CHECK:STDOUT:   %.1: <witness> = interface_witness (%Less.decl, %LessOrEquivalent.decl, %Greater.decl, %GreaterOrEquivalent.decl) [template = constants.%.4]
+// CHECK:STDOUT:   %.loc6: <witness> = interface_witness (%Less.decl, %LessOrEquivalent.decl, %Greater.decl, %GreaterOrEquivalent.decl) [template = constants.%.4]
 // CHECK:STDOUT:
 // CHECK:STDOUT: !members:
 // CHECK:STDOUT:   .Less = %Less.decl
 // CHECK:STDOUT:   .LessOrEquivalent = %LessOrEquivalent.decl
 // CHECK:STDOUT:   .Greater = %Greater.decl
 // CHECK:STDOUT:   .GreaterOrEquivalent = %GreaterOrEquivalent.decl
-// CHECK:STDOUT:   witness = %.1
+// CHECK:STDOUT:   witness = %.loc6
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
 // CHECK:STDOUT: class @C {
@@ -327,9 +327,9 @@ fn TestGreaterEqual(a: D, b: D) -> bool {
 // CHECK:STDOUT: !entry:
 // CHECK:STDOUT:   %a.ref: %C = name_ref a, %a
 // CHECK:STDOUT:   %b.ref: %C = name_ref b, %b
-// CHECK:STDOUT:   %.1: %Less.type.2 = interface_witness_access @impl.%.1, element0 [template = constants.%Less.1]
-// CHECK:STDOUT:   %.loc14_12: <bound method> = bound_method %a.ref, %.1
-// CHECK:STDOUT:   %Less.call: init bool = call %.loc14_12(%a.ref, %b.ref)
+// CHECK:STDOUT:   %.loc14_12.1: %Less.type.2 = interface_witness_access @impl.%.loc6, element0 [template = constants.%Less.1]
+// CHECK:STDOUT:   %.loc14_12.2: <bound method> = bound_method %a.ref, %.loc14_12.1
+// CHECK:STDOUT:   %Less.call: init bool = call %.loc14_12.2(%a.ref, %b.ref)
 // CHECK:STDOUT:   %.loc14_15.1: bool = value_of_initializer %Less.call
 // CHECK:STDOUT:   %.loc14_15.2: bool = converted %Less.call, %.loc14_15.1
 // CHECK:STDOUT:   return %.loc14_15.2
@@ -339,9 +339,9 @@ fn TestGreaterEqual(a: D, b: D) -> bool {
 // CHECK:STDOUT: !entry:
 // CHECK:STDOUT:   %a.ref: %C = name_ref a, %a
 // CHECK:STDOUT:   %b.ref: %C = name_ref b, %b
-// CHECK:STDOUT:   %.1: %LessOrEquivalent.type.2 = interface_witness_access @impl.%.1, element1 [template = constants.%LessOrEquivalent.1]
-// CHECK:STDOUT:   %.loc18_12: <bound method> = bound_method %a.ref, %.1
-// CHECK:STDOUT:   %LessOrEquivalent.call: init bool = call %.loc18_12(%a.ref, %b.ref)
+// CHECK:STDOUT:   %.loc18_12.1: %LessOrEquivalent.type.2 = interface_witness_access @impl.%.loc6, element1 [template = constants.%LessOrEquivalent.1]
+// CHECK:STDOUT:   %.loc18_12.2: <bound method> = bound_method %a.ref, %.loc18_12.1
+// CHECK:STDOUT:   %LessOrEquivalent.call: init bool = call %.loc18_12.2(%a.ref, %b.ref)
 // CHECK:STDOUT:   %.loc18_16.1: bool = value_of_initializer %LessOrEquivalent.call
 // CHECK:STDOUT:   %.loc18_16.2: bool = converted %LessOrEquivalent.call, %.loc18_16.1
 // CHECK:STDOUT:   return %.loc18_16.2
@@ -351,9 +351,9 @@ fn TestGreaterEqual(a: D, b: D) -> bool {
 // CHECK:STDOUT: !entry:
 // CHECK:STDOUT:   %a.ref: %C = name_ref a, %a
 // CHECK:STDOUT:   %b.ref: %C = name_ref b, %b
-// CHECK:STDOUT:   %.1: %Greater.type.2 = interface_witness_access @impl.%.1, element2 [template = constants.%Greater.1]
-// CHECK:STDOUT:   %.loc22_12: <bound method> = bound_method %a.ref, %.1
-// CHECK:STDOUT:   %Greater.call: init bool = call %.loc22_12(%a.ref, %b.ref)
+// CHECK:STDOUT:   %.loc22_12.1: %Greater.type.2 = interface_witness_access @impl.%.loc6, element2 [template = constants.%Greater.1]
+// CHECK:STDOUT:   %.loc22_12.2: <bound method> = bound_method %a.ref, %.loc22_12.1
+// CHECK:STDOUT:   %Greater.call: init bool = call %.loc22_12.2(%a.ref, %b.ref)
 // CHECK:STDOUT:   %.loc22_15.1: bool = value_of_initializer %Greater.call
 // CHECK:STDOUT:   %.loc22_15.2: bool = converted %Greater.call, %.loc22_15.1
 // CHECK:STDOUT:   return %.loc22_15.2
@@ -363,9 +363,9 @@ fn TestGreaterEqual(a: D, b: D) -> bool {
 // CHECK:STDOUT: !entry:
 // CHECK:STDOUT:   %a.ref: %C = name_ref a, %a
 // CHECK:STDOUT:   %b.ref: %C = name_ref b, %b
-// CHECK:STDOUT:   %.1: %GreaterOrEquivalent.type.2 = interface_witness_access @impl.%.1, element3 [template = constants.%GreaterOrEquivalent.1]
-// CHECK:STDOUT:   %.loc26_12: <bound method> = bound_method %a.ref, %.1
-// CHECK:STDOUT:   %GreaterOrEquivalent.call: init bool = call %.loc26_12(%a.ref, %b.ref)
+// CHECK:STDOUT:   %.loc26_12.1: %GreaterOrEquivalent.type.2 = interface_witness_access @impl.%.loc6, element3 [template = constants.%GreaterOrEquivalent.1]
+// CHECK:STDOUT:   %.loc26_12.2: <bound method> = bound_method %a.ref, %.loc26_12.1
+// CHECK:STDOUT:   %GreaterOrEquivalent.call: init bool = call %.loc26_12.2(%a.ref, %b.ref)
 // CHECK:STDOUT:   %.loc26_16.1: bool = value_of_initializer %GreaterOrEquivalent.call
 // CHECK:STDOUT:   %.loc26_16.2: bool = converted %GreaterOrEquivalent.call, %.loc26_16.1
 // CHECK:STDOUT:   return %.loc26_16.2
diff --git a/toolchain/check/testdata/operators/overloaded/right_shift.carbon b/toolchain/check/testdata/operators/overloaded/right_shift.carbon
index dfa846a3c4c4..2b12c3d124f6 100644
--- a/toolchain/check/testdata/operators/overloaded/right_shift.carbon
+++ b/toolchain/check/testdata/operators/overloaded/right_shift.carbon
@@ -154,11 +154,11 @@ fn TestAssign(a: C*, b: C) {
 // CHECK:STDOUT:     %C.ref.loc18_31: type = name_ref C, file.%C.decl [template = constants.%C]
 // CHECK:STDOUT:     %return.var: ref %C = var <return slot>
 // CHECK:STDOUT:   }
-// CHECK:STDOUT:   %.1: <witness> = interface_witness (%Op.decl) [template = constants.%.4]
+// CHECK:STDOUT:   %.loc17: <witness> = interface_witness (%Op.decl) [template = constants.%.4]
 // CHECK:STDOUT:
 // CHECK:STDOUT: !members:
 // CHECK:STDOUT:   .Op = %Op.decl
-// CHECK:STDOUT:   witness = %.1
+// CHECK:STDOUT:   witness = %.loc17
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
 // CHECK:STDOUT: impl @impl.2: %C as %.6 {
@@ -172,11 +172,11 @@ fn TestAssign(a: C*, b: C) {
 // CHECK:STDOUT:     %other.loc23_24.1: %C = param other
 // CHECK:STDOUT:     %other.loc23_24.2: %C = bind_name other, %other.loc23_24.1
 // CHECK:STDOUT:   }
-// CHECK:STDOUT:   %.1: <witness> = interface_witness (%Op.decl) [template = constants.%.9]
+// CHECK:STDOUT:   %.loc22: <witness> = interface_witness (%Op.decl) [template = constants.%.9]
 // CHECK:STDOUT:
 // CHECK:STDOUT: !members:
 // CHECK:STDOUT:   .Op = %Op.decl
-// CHECK:STDOUT:   witness = %.1
+// CHECK:STDOUT:   witness = %.loc22
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
 // CHECK:STDOUT: class @C {
@@ -214,10 +214,10 @@ fn TestAssign(a: C*, b: C) {
 // CHECK:STDOUT: !entry:
 // CHECK:STDOUT:   %a.ref: %C = name_ref a, %a
 // CHECK:STDOUT:   %b.ref: %C = name_ref b, %b
-// CHECK:STDOUT:   %.1: %Op.type.2 = interface_witness_access @impl.1.%.1, element0 [template = constants.%Op.1]
-// CHECK:STDOUT:   %.loc27: <bound method> = bound_method %a.ref, %.1
+// CHECK:STDOUT:   %.loc27_12.1: %Op.type.2 = interface_witness_access @impl.1.%.loc17, element0 [template = constants.%Op.1]
+// CHECK:STDOUT:   %.loc27_12.2: <bound method> = bound_method %a.ref, %.loc27_12.1
 // CHECK:STDOUT:   %.loc26: ref %C = splice_block %return {}
-// CHECK:STDOUT:   %Op.call: init %C = call %.loc27(%a.ref, %b.ref) to %.loc26
+// CHECK:STDOUT:   %Op.call: init %C = call %.loc27_12.2(%a.ref, %b.ref) to %.loc26
 // CHECK:STDOUT:   return %Op.call to %return
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
@@ -226,10 +226,10 @@ fn TestAssign(a: C*, b: C) {
 // CHECK:STDOUT:   %a.ref: %.7 = name_ref a, %a
 // CHECK:STDOUT:   %.loc31_3.1: ref %C = deref %a.ref
 // CHECK:STDOUT:   %b.ref: %C = name_ref b, %b
-// CHECK:STDOUT:   %.1: %Op.type.4 = interface_witness_access @impl.2.%.1, element0 [template = constants.%Op.3]
-// CHECK:STDOUT:   %.loc31_6: <bound method> = bound_method %.loc31_3.1, %.1
+// CHECK:STDOUT:   %.loc31_6.1: %Op.type.4 = interface_witness_access @impl.2.%.loc22, element0 [template = constants.%Op.3]
+// CHECK:STDOUT:   %.loc31_6.2: <bound method> = bound_method %.loc31_3.1, %.loc31_6.1
 // CHECK:STDOUT:   %.loc31_3.2: %.7 = addr_of %.loc31_3.1
-// CHECK:STDOUT:   %Op.call: init %.3 = call %.loc31_6(%.loc31_3.2, %b.ref)
+// CHECK:STDOUT:   %Op.call: init %.3 = call %.loc31_6.2(%.loc31_3.2, %b.ref)
 // CHECK:STDOUT:   return
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
diff --git a/toolchain/check/testdata/operators/overloaded/sub.carbon b/toolchain/check/testdata/operators/overloaded/sub.carbon
index 2f29d4185734..47f51936b8ed 100644
--- a/toolchain/check/testdata/operators/overloaded/sub.carbon
+++ b/toolchain/check/testdata/operators/overloaded/sub.carbon
@@ -154,11 +154,11 @@ fn TestAssign(a: C*, b: C) {
 // CHECK:STDOUT:     %C.ref.loc18_31: type = name_ref C, file.%C.decl [template = constants.%C]
 // CHECK:STDOUT:     %return.var: ref %C = var <return slot>
 // CHECK:STDOUT:   }
-// CHECK:STDOUT:   %.1: <witness> = interface_witness (%Op.decl) [template = constants.%.4]
+// CHECK:STDOUT:   %.loc17: <witness> = interface_witness (%Op.decl) [template = constants.%.4]
 // CHECK:STDOUT:
 // CHECK:STDOUT: !members:
 // CHECK:STDOUT:   .Op = %Op.decl
-// CHECK:STDOUT:   witness = %.1
+// CHECK:STDOUT:   witness = %.loc17
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
 // CHECK:STDOUT: impl @impl.2: %C as %.6 {
@@ -172,11 +172,11 @@ fn TestAssign(a: C*, b: C) {
 // CHECK:STDOUT:     %other.loc23_24.1: %C = param other
 // CHECK:STDOUT:     %other.loc23_24.2: %C = bind_name other, %other.loc23_24.1
 // CHECK:STDOUT:   }
-// CHECK:STDOUT:   %.1: <witness> = interface_witness (%Op.decl) [template = constants.%.9]
+// CHECK:STDOUT:   %.loc22: <witness> = interface_witness (%Op.decl) [template = constants.%.9]
 // CHECK:STDOUT:
 // CHECK:STDOUT: !members:
 // CHECK:STDOUT:   .Op = %Op.decl
-// CHECK:STDOUT:   witness = %.1
+// CHECK:STDOUT:   witness = %.loc22
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
 // CHECK:STDOUT: class @C {
@@ -214,10 +214,10 @@ fn TestAssign(a: C*, b: C) {
 // CHECK:STDOUT: !entry:
 // CHECK:STDOUT:   %a.ref: %C = name_ref a, %a
 // CHECK:STDOUT:   %b.ref: %C = name_ref b, %b
-// CHECK:STDOUT:   %.1: %Op.type.2 = interface_witness_access @impl.1.%.1, element0 [template = constants.%Op.1]
-// CHECK:STDOUT:   %.loc27: <bound method> = bound_method %a.ref, %.1
+// CHECK:STDOUT:   %.loc27_12.1: %Op.type.2 = interface_witness_access @impl.1.%.loc17, element0 [template = constants.%Op.1]
+// CHECK:STDOUT:   %.loc27_12.2: <bound method> = bound_method %a.ref, %.loc27_12.1
 // CHECK:STDOUT:   %.loc26: ref %C = splice_block %return {}
-// CHECK:STDOUT:   %Op.call: init %C = call %.loc27(%a.ref, %b.ref) to %.loc26
+// CHECK:STDOUT:   %Op.call: init %C = call %.loc27_12.2(%a.ref, %b.ref) to %.loc26
 // CHECK:STDOUT:   return %Op.call to %return
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
@@ -226,10 +226,10 @@ fn TestAssign(a: C*, b: C) {
 // CHECK:STDOUT:   %a.ref: %.7 = name_ref a, %a
 // CHECK:STDOUT:   %.loc31_3.1: ref %C = deref %a.ref
 // CHECK:STDOUT:   %b.ref: %C = name_ref b, %b
-// CHECK:STDOUT:   %.1: %Op.type.4 = interface_witness_access @impl.2.%.1, element0 [template = constants.%Op.3]
-// CHECK:STDOUT:   %.loc31_6: <bound method> = bound_method %.loc31_3.1, %.1
+// CHECK:STDOUT:   %.loc31_6.1: %Op.type.4 = interface_witness_access @impl.2.%.loc22, element0 [template = constants.%Op.3]
+// CHECK:STDOUT:   %.loc31_6.2: <bound method> = bound_method %.loc31_3.1, %.loc31_6.1
 // CHECK:STDOUT:   %.loc31_3.2: %.7 = addr_of %.loc31_3.1
-// CHECK:STDOUT:   %Op.call: init %.3 = call %.loc31_6(%.loc31_3.2, %b.ref)
+// CHECK:STDOUT:   %Op.call: init %.3 = call %.loc31_6.2(%.loc31_3.2, %b.ref)
 // CHECK:STDOUT:   return
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
diff --git a/toolchain/lower/testdata/builtins/overloaded_operator.carbon b/toolchain/lower/testdata/builtins/overloaded_operator.carbon
index 6aef3a0d517a..ff5b82a41599 100644
--- a/toolchain/lower/testdata/builtins/overloaded_operator.carbon
+++ b/toolchain/lower/testdata/builtins/overloaded_operator.carbon
@@ -24,10 +24,10 @@ fn AddThreeIntegers(a: i32, b: i32, c: i32) -> i32 {
 // CHECK:STDOUT: define i32 @AddThreeIntegers(i32 %a, i32 %b, i32 %c) !dbg !4 {
 // CHECK:STDOUT: entry:
 // CHECK:STDOUT:   %int.sadd.loc18_12 = add i32 %a, %b, !dbg !7
-// CHECK:STDOUT:   %.loc18_12.2.temp = alloca i32, align 4, !dbg !7
-// CHECK:STDOUT:   store i32 %int.sadd.loc18_12, ptr %.loc18_12.2.temp, align 4, !dbg !7
-// CHECK:STDOUT:   %.loc18_12.4 = load i32, ptr %.loc18_12.2.temp, align 4, !dbg !7
-// CHECK:STDOUT:   %int.sadd.loc18_16 = add i32 %.loc18_12.4, %c, !dbg !7
+// CHECK:STDOUT:   %.loc18_12.3.temp = alloca i32, align 4, !dbg !7
+// CHECK:STDOUT:   store i32 %int.sadd.loc18_12, ptr %.loc18_12.3.temp, align 4, !dbg !7
+// CHECK:STDOUT:   %.loc18_12.5 = load i32, ptr %.loc18_12.3.temp, align 4, !dbg !7
+// CHECK:STDOUT:   %int.sadd.loc18_16 = add i32 %.loc18_12.5, %c, !dbg !7
 // CHECK:STDOUT:   ret i32 %int.sadd.loc18_16, !dbg !8
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
diff --git a/toolchain/lower/testdata/operators/overloaded.carbon b/toolchain/lower/testdata/operators/overloaded.carbon
index daf3c39bca25..9f31913a829e 100644
--- a/toolchain/lower/testdata/operators/overloaded.carbon
+++ b/toolchain/lower/testdata/operators/overloaded.carbon
@@ -82,9 +82,9 @@ fn Calculate(a: Number, b: Number) -> Number {
 // CHECK:STDOUT:
 // CHECK:STDOUT: define void @Calculate(ptr sret({ i1 }) %return, ptr %a, ptr %b) !dbg !21 {
 // CHECK:STDOUT: entry:
-// CHECK:STDOUT:   %.loc28_10.2.temp = alloca { i1 }, align 8, !dbg !22
-// CHECK:STDOUT:   call void @Op(ptr %.loc28_10.2.temp, ptr %a), !dbg !22
-// CHECK:STDOUT:   call void @Op.1(ptr %return, ptr %.loc28_10.2.temp, ptr %b), !dbg !22
+// CHECK:STDOUT:   %.loc28_10.3.temp = alloca { i1 }, align 8, !dbg !22
+// CHECK:STDOUT:   call void @Op(ptr %.loc28_10.3.temp, ptr %a), !dbg !22
+// CHECK:STDOUT:   call void @Op.1(ptr %return, ptr %.loc28_10.3.temp, ptr %b), !dbg !22
 // CHECK:STDOUT:   ret void, !dbg !23
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
diff --git a/toolchain/sem_ir/typed_insts.h b/toolchain/sem_ir/typed_insts.h
index 002deb09d4e2..99ad99399a9d 100644
--- a/toolchain/sem_ir/typed_insts.h
+++ b/toolchain/sem_ir/typed_insts.h
@@ -710,11 +710,10 @@ struct InterfaceType {
 
 // A witness that a type implements an interface.
 struct InterfaceWitness {
-  static constexpr auto Kind =
-      InstKind::InterfaceWitness.Define<Parse::InvalidNodeId>(
-          {.ir_name = "interface_witness",
-           .constant_kind = InstConstantKind::Conditional,
-           .is_lowered = false});
+  static constexpr auto Kind = InstKind::InterfaceWitness.Define<Parse::NodeId>(
+      {.ir_name = "interface_witness",
+       .constant_kind = InstConstantKind::Conditional,
+       .is_lowered = false});
 
   TypeId type_id;
   InstBlockId elements_id;
@@ -723,7 +722,7 @@ struct InterfaceWitness {
 // Accesses an element of an interface witness by index.
 struct InterfaceWitnessAccess {
   static constexpr auto Kind =
-      InstKind::InterfaceWitnessAccess.Define<Parse::InvalidNodeId>(
+      InstKind::InterfaceWitnessAccess.Define<Parse::NodeId>(
           {.ir_name = "interface_witness_access",
            .is_type = InstIsType::Maybe,
            .constant_kind = InstConstantKind::SymbolicOnly,