diff --git a/toolchain/check/.handle_class.cpp.swn b/toolchain/check/.handle_class.cpp.swn
new file mode 100644
index 000000000000..e69de29bb2d1
diff --git a/toolchain/check/.handle_class.cpp.swo b/toolchain/check/.handle_class.cpp.swo
new file mode 100644
index 000000000000..e69de29bb2d1
diff --git a/toolchain/check/handle_class.cpp b/toolchain/check/handle_class.cpp
index df002374430b..dea13ca8d4c4 100644
--- a/toolchain/check/handle_class.cpp
+++ b/toolchain/check/handle_class.cpp
@@ -512,6 +512,15 @@ auto HandleParseNode(Context& context, Parse::BaseDeclId node_id) -> bool {
     return true;
   }
 
+  if (!context.struct_type_fields_stack().PeekArray().empty()) {
+    // TODO: Add note that includes the first field location as an example.
+    CARBON_DIAGNOSTIC(
+        BaseDeclAfterFieldDecl, Error,
+        "`base` declaration must appear before field declarations");
+    context.emitter().Emit(node_id, BaseDeclAfterFieldDecl);
+    return true;
+  }
+
   auto base_info = CheckBaseType(context, base_type_node_id, base_type_expr_id);
 
   // The `base` value in the class scope has an unbound element type. Instance
diff --git a/toolchain/check/testdata/class/base.carbon b/toolchain/check/testdata/class/base.carbon
index 781e22fbe219..46509a9c9db9 100644
--- a/toolchain/check/testdata/class/base.carbon
+++ b/toolchain/check/testdata/class/base.carbon
@@ -8,6 +8,9 @@
 // TIP: To dump output, run:
 // TIP:   bazel run //toolchain/testing:file_test -- --dump_output --file_tests=toolchain/check/testdata/class/base.carbon
 
+// --- base.carbon
+package Base;
+
 base class Base {
   var b: i32;
 }
@@ -26,6 +29,21 @@ fn Access(d: Derived) -> (i32, i32) {
   return (d.d, d.base.b);
 }
 
+// --- fail_base_after_field.carbon
+package BaseAfterField;
+
+base class Base {
+}
+
+class Derived {
+  var d: i32;
+
+  // CHECK:STDERR: fail_base_after_field.carbon:[[@LINE+3]]:3: error: `base` declaration must appear before field declarations [BaseDeclAfterFieldDecl]
+  // CHECK:STDERR:   extend base: Base;
+  // CHECK:STDERR:   ^~~~~~~~~~~~~~~~~~
+  extend base: Base;
+}
+
 // CHECK:STDOUT: --- base.carbon
 // CHECK:STDOUT:
 // CHECK:STDOUT: constants {
@@ -101,16 +119,16 @@ fn Access(d: Derived) -> (i32, i32) {
 // CHECK:STDOUT:     %return.param_patt: %tuple.type.2 = out_param_pattern %return.patt, runtime_param1
 // CHECK:STDOUT:   } {
 // CHECK:STDOUT:     %Derived.ref: type = name_ref Derived, file.%Derived.decl [template = constants.%Derived]
-// CHECK:STDOUT:     %.loc25_27: Core.IntLiteral = int_value 32 [template = constants.%.1]
-// CHECK:STDOUT:     %int.make_type_signed.loc25_27: init type = call constants.%Int(%.loc25_27) [template = constants.%i32]
-// CHECK:STDOUT:     %.loc25_32: Core.IntLiteral = int_value 32 [template = constants.%.1]
-// CHECK:STDOUT:     %int.make_type_signed.loc25_32: init type = call constants.%Int(%.loc25_32) [template = constants.%i32]
-// CHECK:STDOUT:     %.loc25_35.1: %tuple.type.1 = tuple_literal (%int.make_type_signed.loc25_27, %int.make_type_signed.loc25_32)
-// CHECK:STDOUT:     %.loc25_35.2: type = value_of_initializer %int.make_type_signed.loc25_27 [template = constants.%i32]
-// CHECK:STDOUT:     %.loc25_35.3: type = converted %int.make_type_signed.loc25_27, %.loc25_35.2 [template = constants.%i32]
-// CHECK:STDOUT:     %.loc25_35.4: type = value_of_initializer %int.make_type_signed.loc25_32 [template = constants.%i32]
-// CHECK:STDOUT:     %.loc25_35.5: type = converted %int.make_type_signed.loc25_32, %.loc25_35.4 [template = constants.%i32]
-// CHECK:STDOUT:     %.loc25_35.6: type = converted %.loc25_35.1, constants.%tuple.type.2 [template = constants.%tuple.type.2]
+// CHECK:STDOUT:     %.loc17_27: Core.IntLiteral = int_value 32 [template = constants.%.1]
+// CHECK:STDOUT:     %int.make_type_signed.loc17_27: init type = call constants.%Int(%.loc17_27) [template = constants.%i32]
+// CHECK:STDOUT:     %.loc17_32: Core.IntLiteral = int_value 32 [template = constants.%.1]
+// CHECK:STDOUT:     %int.make_type_signed.loc17_32: init type = call constants.%Int(%.loc17_32) [template = constants.%i32]
+// CHECK:STDOUT:     %.loc17_35.1: %tuple.type.1 = tuple_literal (%int.make_type_signed.loc17_27, %int.make_type_signed.loc17_32)
+// CHECK:STDOUT:     %.loc17_35.2: type = value_of_initializer %int.make_type_signed.loc17_27 [template = constants.%i32]
+// CHECK:STDOUT:     %.loc17_35.3: type = converted %int.make_type_signed.loc17_27, %.loc17_35.2 [template = constants.%i32]
+// CHECK:STDOUT:     %.loc17_35.4: type = value_of_initializer %int.make_type_signed.loc17_32 [template = constants.%i32]
+// CHECK:STDOUT:     %.loc17_35.5: type = converted %int.make_type_signed.loc17_32, %.loc17_35.4 [template = constants.%i32]
+// CHECK:STDOUT:     %.loc17_35.6: type = converted %.loc17_35.1, constants.%tuple.type.2 [template = constants.%tuple.type.2]
 // CHECK:STDOUT:     %d.param: %Derived = value_param runtime_param0
 // CHECK:STDOUT:     %d: %Derived = bind_name d, %d.param
 // CHECK:STDOUT:     %return.param: ref %tuple.type.2 = out_param runtime_param1
@@ -119,83 +137,139 @@ fn Access(d: Derived) -> (i32, i32) {
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
 // CHECK:STDOUT: class @Base {
-// CHECK:STDOUT:   %.loc12_10.1: Core.IntLiteral = int_value 32 [template = constants.%.1]
-// CHECK:STDOUT:   %int.make_type_signed: init type = call constants.%Int(%.loc12_10.1) [template = constants.%i32]
-// CHECK:STDOUT:   %.loc12_10.2: type = value_of_initializer %int.make_type_signed [template = constants.%i32]
-// CHECK:STDOUT:   %.loc12_10.3: type = converted %int.make_type_signed, %.loc12_10.2 [template = constants.%i32]
-// CHECK:STDOUT:   %.loc12_8: %.2 = field_decl b, element0 [template]
-// CHECK:STDOUT:   %.loc13: <witness> = complete_type_witness %.3 [template = constants.%.4]
+// CHECK:STDOUT:   %.loc4_10.1: Core.IntLiteral = int_value 32 [template = constants.%.1]
+// CHECK:STDOUT:   %int.make_type_signed: init type = call constants.%Int(%.loc4_10.1) [template = constants.%i32]
+// CHECK:STDOUT:   %.loc4_10.2: type = value_of_initializer %int.make_type_signed [template = constants.%i32]
+// CHECK:STDOUT:   %.loc4_10.3: type = converted %int.make_type_signed, %.loc4_10.2 [template = constants.%i32]
+// CHECK:STDOUT:   %.loc4_8: %.2 = field_decl b, element0 [template]
+// CHECK:STDOUT:   %.loc5: <witness> = complete_type_witness %.3 [template = constants.%.4]
 // CHECK:STDOUT:
 // CHECK:STDOUT: !members:
 // CHECK:STDOUT:   .Self = constants.%Base
-// CHECK:STDOUT:   .b = %.loc12_8
+// CHECK:STDOUT:   .b = %.loc4_8
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
 // CHECK:STDOUT: class @Derived {
 // CHECK:STDOUT:   %Base.ref: type = name_ref Base, file.%Base.decl [template = constants.%Base]
-// CHECK:STDOUT:   %.loc16: %.6 = base_decl %Base, element0 [template]
-// CHECK:STDOUT:   %.loc18_10.1: Core.IntLiteral = int_value 32 [template = constants.%.1]
-// CHECK:STDOUT:   %int.make_type_signed: init type = call constants.%Int(%.loc18_10.1) [template = constants.%i32]
-// CHECK:STDOUT:   %.loc18_10.2: type = value_of_initializer %int.make_type_signed [template = constants.%i32]
-// CHECK:STDOUT:   %.loc18_10.3: type = converted %int.make_type_signed, %.loc18_10.2 [template = constants.%i32]
-// CHECK:STDOUT:   %.loc18_8: %.7 = field_decl d, element1 [template]
-// CHECK:STDOUT:   %.loc19: <witness> = complete_type_witness %.8 [template = constants.%.9]
+// CHECK:STDOUT:   %.loc8: %.6 = base_decl %Base, element0 [template]
+// CHECK:STDOUT:   %.loc10_10.1: Core.IntLiteral = int_value 32 [template = constants.%.1]
+// CHECK:STDOUT:   %int.make_type_signed: init type = call constants.%Int(%.loc10_10.1) [template = constants.%i32]
+// CHECK:STDOUT:   %.loc10_10.2: type = value_of_initializer %int.make_type_signed [template = constants.%i32]
+// CHECK:STDOUT:   %.loc10_10.3: type = converted %int.make_type_signed, %.loc10_10.2 [template = constants.%i32]
+// CHECK:STDOUT:   %.loc10_8: %.7 = field_decl d, element1 [template]
+// CHECK:STDOUT:   %.loc11: <witness> = complete_type_witness %.8 [template = constants.%.9]
 // CHECK:STDOUT:
 // CHECK:STDOUT: !members:
 // CHECK:STDOUT:   .Self = constants.%Derived
-// CHECK:STDOUT:   .base = %.loc16
-// CHECK:STDOUT:   .d = %.loc18_8
+// CHECK:STDOUT:   .base = %.loc8
+// CHECK:STDOUT:   .d = %.loc10_8
 // CHECK:STDOUT:   extend %Base.ref
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
 // CHECK:STDOUT: fn @Make() -> %return: %Derived {
 // CHECK:STDOUT: !entry:
-// CHECK:STDOUT:   %.loc22_25: Core.IntLiteral = int_value 4 [template = constants.%.13]
-// CHECK:STDOUT:   %.loc22_26.1: %.14 = struct_literal (%.loc22_25)
-// CHECK:STDOUT:   %.loc22_34: Core.IntLiteral = int_value 7 [template = constants.%.15]
-// CHECK:STDOUT:   %.loc22_35.1: %.16 = struct_literal (%.loc22_26.1, %.loc22_34)
-// CHECK:STDOUT:   %.loc22_26.2: %Convert.type.2 = interface_witness_access constants.%.36, element0 [template = constants.%Convert.14]
-// CHECK:STDOUT:   %.loc22_26.3: <bound method> = bound_method %.loc22_25, %.loc22_26.2 [template = constants.%.37]
-// CHECK:STDOUT:   %.loc22_26.4: <specific function> = specific_function %.loc22_26.3, @Convert.2(constants.%.1) [template = constants.%.38]
-// CHECK:STDOUT:   %int.convert_checked.loc22_26: init %i32 = call %.loc22_26.4(%.loc22_25) [template = constants.%.39]
-// CHECK:STDOUT:   %.loc22_26.5: init %i32 = converted %.loc22_25, %int.convert_checked.loc22_26 [template = constants.%.39]
-// CHECK:STDOUT:   %.loc22_35.2: ref %Base = class_element_access %return, element0
-// CHECK:STDOUT:   %.loc22_26.6: ref %i32 = class_element_access %.loc22_35.2, element0
-// CHECK:STDOUT:   %.loc22_26.7: init %i32 = initialize_from %.loc22_26.5 to %.loc22_26.6 [template = constants.%.39]
-// CHECK:STDOUT:   %.loc22_26.8: init %Base = class_init (%.loc22_26.7), %.loc22_35.2 [template = constants.%struct.1]
-// CHECK:STDOUT:   %.loc22_35.3: init %Base = converted %.loc22_26.1, %.loc22_26.8 [template = constants.%struct.1]
-// CHECK:STDOUT:   %.loc22_35.4: %Convert.type.2 = interface_witness_access constants.%.36, element0 [template = constants.%Convert.14]
-// CHECK:STDOUT:   %.loc22_35.5: <bound method> = bound_method %.loc22_34, %.loc22_35.4 [template = constants.%.40]
-// CHECK:STDOUT:   %.loc22_35.6: <specific function> = specific_function %.loc22_35.5, @Convert.2(constants.%.1) [template = constants.%.41]
-// CHECK:STDOUT:   %int.convert_checked.loc22_35: init %i32 = call %.loc22_35.6(%.loc22_34) [template = constants.%.42]
-// CHECK:STDOUT:   %.loc22_35.7: init %i32 = converted %.loc22_34, %int.convert_checked.loc22_35 [template = constants.%.42]
-// CHECK:STDOUT:   %.loc22_35.8: ref %i32 = class_element_access %return, element1
-// CHECK:STDOUT:   %.loc22_35.9: init %i32 = initialize_from %.loc22_35.7 to %.loc22_35.8 [template = constants.%.42]
-// CHECK:STDOUT:   %.loc22_35.10: init %Derived = class_init (%.loc22_35.3, %.loc22_35.9), %return [template = constants.%struct.2]
-// CHECK:STDOUT:   %.loc22_36: init %Derived = converted %.loc22_35.1, %.loc22_35.10 [template = constants.%struct.2]
-// CHECK:STDOUT:   return %.loc22_36 to %return
+// CHECK:STDOUT:   %.loc14_25: Core.IntLiteral = int_value 4 [template = constants.%.13]
+// CHECK:STDOUT:   %.loc14_26.1: %.14 = struct_literal (%.loc14_25)
+// CHECK:STDOUT:   %.loc14_34: Core.IntLiteral = int_value 7 [template = constants.%.15]
+// CHECK:STDOUT:   %.loc14_35.1: %.16 = struct_literal (%.loc14_26.1, %.loc14_34)
+// CHECK:STDOUT:   %.loc14_26.2: %Convert.type.2 = interface_witness_access constants.%.36, element0 [template = constants.%Convert.14]
+// CHECK:STDOUT:   %.loc14_26.3: <bound method> = bound_method %.loc14_25, %.loc14_26.2 [template = constants.%.37]
+// CHECK:STDOUT:   %.loc14_26.4: <specific function> = specific_function %.loc14_26.3, @Convert.2(constants.%.1) [template = constants.%.38]
+// CHECK:STDOUT:   %int.convert_checked.loc14_26: init %i32 = call %.loc14_26.4(%.loc14_25) [template = constants.%.39]
+// CHECK:STDOUT:   %.loc14_26.5: init %i32 = converted %.loc14_25, %int.convert_checked.loc14_26 [template = constants.%.39]
+// CHECK:STDOUT:   %.loc14_35.2: ref %Base = class_element_access %return, element0
+// CHECK:STDOUT:   %.loc14_26.6: ref %i32 = class_element_access %.loc14_35.2, element0
+// CHECK:STDOUT:   %.loc14_26.7: init %i32 = initialize_from %.loc14_26.5 to %.loc14_26.6 [template = constants.%.39]
+// CHECK:STDOUT:   %.loc14_26.8: init %Base = class_init (%.loc14_26.7), %.loc14_35.2 [template = constants.%struct.1]
+// CHECK:STDOUT:   %.loc14_35.3: init %Base = converted %.loc14_26.1, %.loc14_26.8 [template = constants.%struct.1]
+// CHECK:STDOUT:   %.loc14_35.4: %Convert.type.2 = interface_witness_access constants.%.36, element0 [template = constants.%Convert.14]
+// CHECK:STDOUT:   %.loc14_35.5: <bound method> = bound_method %.loc14_34, %.loc14_35.4 [template = constants.%.40]
+// CHECK:STDOUT:   %.loc14_35.6: <specific function> = specific_function %.loc14_35.5, @Convert.2(constants.%.1) [template = constants.%.41]
+// CHECK:STDOUT:   %int.convert_checked.loc14_35: init %i32 = call %.loc14_35.6(%.loc14_34) [template = constants.%.42]
+// CHECK:STDOUT:   %.loc14_35.7: init %i32 = converted %.loc14_34, %int.convert_checked.loc14_35 [template = constants.%.42]
+// CHECK:STDOUT:   %.loc14_35.8: ref %i32 = class_element_access %return, element1
+// CHECK:STDOUT:   %.loc14_35.9: init %i32 = initialize_from %.loc14_35.7 to %.loc14_35.8 [template = constants.%.42]
+// CHECK:STDOUT:   %.loc14_35.10: init %Derived = class_init (%.loc14_35.3, %.loc14_35.9), %return [template = constants.%struct.2]
+// CHECK:STDOUT:   %.loc14_36: init %Derived = converted %.loc14_35.1, %.loc14_35.10 [template = constants.%struct.2]
+// CHECK:STDOUT:   return %.loc14_36 to %return
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
 // CHECK:STDOUT: fn @Access(%d.param_patt: %Derived) -> %return: %tuple.type.2 {
 // CHECK:STDOUT: !entry:
-// CHECK:STDOUT:   %d.ref.loc26_11: %Derived = name_ref d, %d
-// CHECK:STDOUT:   %d.ref.loc26_12: %.7 = name_ref d, @Derived.%.loc18_8 [template = @Derived.%.loc18_8]
-// CHECK:STDOUT:   %.loc26_12.1: ref %i32 = class_element_access %d.ref.loc26_11, element1
-// CHECK:STDOUT:   %.loc26_12.2: %i32 = bind_value %.loc26_12.1
-// CHECK:STDOUT:   %d.ref.loc26_16: %Derived = name_ref d, %d
-// CHECK:STDOUT:   %base.ref: %.6 = name_ref base, @Derived.%.loc16 [template = @Derived.%.loc16]
-// CHECK:STDOUT:   %.loc26_17.1: ref %Base = class_element_access %d.ref.loc26_16, element0
-// CHECK:STDOUT:   %.loc26_17.2: %Base = bind_value %.loc26_17.1
-// CHECK:STDOUT:   %b.ref: %.2 = name_ref b, @Base.%.loc12_8 [template = @Base.%.loc12_8]
-// CHECK:STDOUT:   %.loc26_22.1: ref %i32 = class_element_access %.loc26_17.2, element0
-// CHECK:STDOUT:   %.loc26_22.2: %i32 = bind_value %.loc26_22.1
-// CHECK:STDOUT:   %.loc26_24.1: %tuple.type.2 = tuple_literal (%.loc26_12.2, %.loc26_22.2)
-// CHECK:STDOUT:   %.loc26_24.2: ref %i32 = tuple_access %return, element0
-// CHECK:STDOUT:   %.loc26_24.3: init %i32 = initialize_from %.loc26_12.2 to %.loc26_24.2
-// CHECK:STDOUT:   %.loc26_24.4: ref %i32 = tuple_access %return, element1
-// CHECK:STDOUT:   %.loc26_24.5: init %i32 = initialize_from %.loc26_22.2 to %.loc26_24.4
-// CHECK:STDOUT:   %.loc26_24.6: init %tuple.type.2 = tuple_init (%.loc26_24.3, %.loc26_24.5) to %return
-// CHECK:STDOUT:   %.loc26_25: init %tuple.type.2 = converted %.loc26_24.1, %.loc26_24.6
-// CHECK:STDOUT:   return %.loc26_25 to %return
+// CHECK:STDOUT:   %d.ref.loc18_11: %Derived = name_ref d, %d
+// CHECK:STDOUT:   %d.ref.loc18_12: %.7 = name_ref d, @Derived.%.loc10_8 [template = @Derived.%.loc10_8]
+// CHECK:STDOUT:   %.loc18_12.1: ref %i32 = class_element_access %d.ref.loc18_11, element1
+// CHECK:STDOUT:   %.loc18_12.2: %i32 = bind_value %.loc18_12.1
+// CHECK:STDOUT:   %d.ref.loc18_16: %Derived = name_ref d, %d
+// CHECK:STDOUT:   %base.ref: %.6 = name_ref base, @Derived.%.loc8 [template = @Derived.%.loc8]
+// CHECK:STDOUT:   %.loc18_17.1: ref %Base = class_element_access %d.ref.loc18_16, element0
+// CHECK:STDOUT:   %.loc18_17.2: %Base = bind_value %.loc18_17.1
+// CHECK:STDOUT:   %b.ref: %.2 = name_ref b, @Base.%.loc4_8 [template = @Base.%.loc4_8]
+// CHECK:STDOUT:   %.loc18_22.1: ref %i32 = class_element_access %.loc18_17.2, element0
+// CHECK:STDOUT:   %.loc18_22.2: %i32 = bind_value %.loc18_22.1
+// CHECK:STDOUT:   %.loc18_24.1: %tuple.type.2 = tuple_literal (%.loc18_12.2, %.loc18_22.2)
+// CHECK:STDOUT:   %.loc18_24.2: ref %i32 = tuple_access %return, element0
+// CHECK:STDOUT:   %.loc18_24.3: init %i32 = initialize_from %.loc18_12.2 to %.loc18_24.2
+// CHECK:STDOUT:   %.loc18_24.4: ref %i32 = tuple_access %return, element1
+// CHECK:STDOUT:   %.loc18_24.5: init %i32 = initialize_from %.loc18_22.2 to %.loc18_24.4
+// CHECK:STDOUT:   %.loc18_24.6: init %tuple.type.2 = tuple_init (%.loc18_24.3, %.loc18_24.5) to %return
+// CHECK:STDOUT:   %.loc18_25: init %tuple.type.2 = converted %.loc18_24.1, %.loc18_24.6
+// CHECK:STDOUT:   return %.loc18_25 to %return
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: --- fail_base_after_field.carbon
+// CHECK:STDOUT:
+// CHECK:STDOUT: constants {
+// CHECK:STDOUT:   %Base: type = class_type @Base [template]
+// CHECK:STDOUT:   %.1: type = struct_type {} [template]
+// CHECK:STDOUT:   %.2: <witness> = complete_type_witness %.1 [template]
+// CHECK:STDOUT:   %Derived: type = class_type @Derived [template]
+// CHECK:STDOUT:   %.3: Core.IntLiteral = int_value 32 [template]
+// CHECK:STDOUT:   %Int.type: type = fn_type @Int [template]
+// CHECK:STDOUT:   %Int: %Int.type = struct_value () [template]
+// CHECK:STDOUT:   %i32: type = int_type signed, %.3 [template]
+// CHECK:STDOUT:   %.4: type = unbound_element_type %Derived, %i32 [template]
+// CHECK:STDOUT:   %.5: type = struct_type {.d: %i32} [template]
+// CHECK:STDOUT:   %.6: <witness> = complete_type_witness %.5 [template]
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: imports {
+// CHECK:STDOUT:   %Core: <namespace> = namespace file.%Core.import, [template] {
+// CHECK:STDOUT:     .Int = %import_ref
+// CHECK:STDOUT:     import Core//prelude
+// CHECK:STDOUT:     import Core//prelude/...
+// CHECK:STDOUT:   }
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: file {
+// CHECK:STDOUT:   package: <namespace> = namespace [template] {
+// CHECK:STDOUT:     .Core = imports.%Core
+// CHECK:STDOUT:     .Base = %Base.decl
+// CHECK:STDOUT:     .Derived = %Derived.decl
+// CHECK:STDOUT:   }
+// CHECK:STDOUT:   %Core.import = import Core
+// CHECK:STDOUT:   %Base.decl: type = class_decl @Base [template = constants.%Base] {} {}
+// CHECK:STDOUT:   %Derived.decl: type = class_decl @Derived [template = constants.%Derived] {} {}
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: class @Base {
+// CHECK:STDOUT:   %.loc4: <witness> = complete_type_witness %.1 [template = constants.%.2]
+// CHECK:STDOUT:
+// CHECK:STDOUT: !members:
+// CHECK:STDOUT:   .Self = constants.%Base
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: class @Derived {
+// CHECK:STDOUT:   %.loc7_10.1: Core.IntLiteral = int_value 32 [template = constants.%.3]
+// CHECK:STDOUT:   %int.make_type_signed: init type = call constants.%Int(%.loc7_10.1) [template = constants.%i32]
+// CHECK:STDOUT:   %.loc7_10.2: type = value_of_initializer %int.make_type_signed [template = constants.%i32]
+// CHECK:STDOUT:   %.loc7_10.3: type = converted %int.make_type_signed, %.loc7_10.2 [template = constants.%i32]
+// CHECK:STDOUT:   %.loc7_8: %.4 = field_decl d, element0 [template]
+// CHECK:STDOUT:   %Base.ref: type = name_ref Base, file.%Base.decl [template = constants.%Base]
+// CHECK:STDOUT:   %.loc13: <witness> = complete_type_witness %.5 [template = constants.%.6]
+// CHECK:STDOUT:
+// CHECK:STDOUT: !members:
+// CHECK:STDOUT:   .Self = constants.%Derived
+// CHECK:STDOUT:   .d = %.loc7_8
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
diff --git a/toolchain/diagnostics/diagnostic_kind.def b/toolchain/diagnostics/diagnostic_kind.def
index 904a6635a1d9..697c7b73d613 100644
--- a/toolchain/diagnostics/diagnostic_kind.def
+++ b/toolchain/diagnostics/diagnostic_kind.def
@@ -218,6 +218,7 @@ CARBON_DIAGNOSTIC_KIND(AdaptWithVirtualHere)
 CARBON_DIAGNOSTIC_KIND(BaseDeclRepeated)
 CARBON_DIAGNOSTIC_KIND(BaseIsFinal)
 CARBON_DIAGNOSTIC_KIND(BaseMissingExtend)
+CARBON_DIAGNOSTIC_KIND(BaseDeclAfterFieldDecl)
 CARBON_DIAGNOSTIC_KIND(ClassAbstractHere)
 CARBON_DIAGNOSTIC_KIND(ClassForwardDeclaredHere)
 CARBON_DIAGNOSTIC_KIND(ClassSpecificDeclOutsideClass)