diff --git a/toolchain/check/testdata/array/basics.carbon b/toolchain/check/testdata/array/basics.carbon
index 3e532c10ce4c..7c4fb050f8c4 100644
--- a/toolchain/check/testdata/array/basics.carbon
+++ b/toolchain/check/testdata/array/basics.carbon
@@ -2,7 +2,7 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// EXTRA-ARGS: --no-prelude-import
+// INCLUDE-FILE: toolchain/testing/testdata/min_prelude/convert.carbon
 //
 // AUTOUPDATE
 // TIP: To test this file alone, run:
@@ -96,7 +96,10 @@ var p: Incomplete* = &a[0];
 
 library "[[@TEST_NAME]]";
 
-// CHECK:STDERR: fail_invalid_element.carbon:[[@LINE+4]]:14: error: `Core.ImplicitAs` implicitly referenced here, but package `Core` not found [CoreNotFound]
+// CHECK:STDERR: fail_invalid_element.carbon:[[@LINE+7]]:14: error: cannot implicitly convert non-type value of type `Core.IntLiteral` to `type` [ConversionFailureNonTypeToFacet]
+// CHECK:STDERR: var a: array(1, 1);
+// CHECK:STDERR:              ^
+// CHECK:STDERR: fail_invalid_element.carbon:[[@LINE+4]]:14: note: type `Core.IntLiteral` does not implement interface `Core.ImplicitAs(type)` [MissingImplInMemberAccessNote]
 // CHECK:STDERR: var a: array(1, 1);
 // CHECK:STDERR:              ^
 // CHECK:STDERR:
@@ -120,6 +123,9 @@ var a: array(1, 1);
 // CHECK:STDOUT:   %array: %array_type = tuple_value (%empty_tuple, %empty_tuple, %empty_tuple) [concrete]
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
+// CHECK:STDOUT: imports {
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
 // CHECK:STDOUT: file {
 // CHECK:STDOUT:   name_binding_decl {
 // CHECK:STDOUT:     %b.patt: %pattern_type.035 = binding_pattern b [concrete]
@@ -175,6 +181,9 @@ var a: array(1, 1);
 // CHECK:STDOUT:   %int_1: Core.IntLiteral = int_value 1 [concrete]
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
+// CHECK:STDOUT: imports {
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
 // CHECK:STDOUT: fn @G() {
 // CHECK:STDOUT: !entry:
 // CHECK:STDOUT:   name_binding_decl {
@@ -222,6 +231,9 @@ var a: array(1, 1);
 // CHECK:STDOUT:   %pattern_type.8c1: type = pattern_type %tuple.type [concrete]
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
+// CHECK:STDOUT: imports {
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
 // CHECK:STDOUT: fn @G() {
 // CHECK:STDOUT: !entry:
 // CHECK:STDOUT:   name_binding_decl {
@@ -270,6 +282,9 @@ var a: array(1, 1);
 // CHECK:STDOUT:   %array: %array_type = tuple_value (%empty_tuple) [concrete]
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
+// CHECK:STDOUT: imports {
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
 // CHECK:STDOUT: fn @Run() {
 // CHECK:STDOUT: !entry:
 // CHECK:STDOUT:   name_binding_decl {
@@ -329,6 +344,9 @@ var a: array(1, 1);
 // CHECK:STDOUT:   %array: %array_type = tuple_value (%empty_tuple, %empty_tuple, %empty_tuple, %empty_tuple, %empty_tuple, %empty_tuple, %empty_tuple, %empty_tuple, %empty_tuple) [concrete]
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
+// CHECK:STDOUT: imports {
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
 // CHECK:STDOUT: file {
 // CHECK:STDOUT:   name_binding_decl {
 // CHECK:STDOUT:     %b.patt: %pattern_type.3db = binding_pattern b [concrete]
diff --git a/toolchain/check/testdata/array/element_mismatches.carbon b/toolchain/check/testdata/array/element_mismatches.carbon
index d167c7bd304e..152a88c8366a 100644
--- a/toolchain/check/testdata/array/element_mismatches.carbon
+++ b/toolchain/check/testdata/array/element_mismatches.carbon
@@ -2,7 +2,7 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// EXTRA-ARGS: --no-prelude-import
+// INCLUDE-FILE: toolchain/testing/testdata/min_prelude/convert.carbon
 //
 // AUTOUPDATE
 // TIP: To test this file alone, run:
@@ -16,7 +16,10 @@ library "[[@TEST_NAME]]";
 
 class C {}
 
-// CHECK:STDERR: fail_arg_wrong_type.carbon:[[@LINE+4]]:22: error: `Core.ImplicitAs` implicitly referenced here, but package `Core` not found [CoreNotFound]
+// CHECK:STDERR: fail_arg_wrong_type.carbon:[[@LINE+7]]:22: error: cannot implicitly convert expression of type `String` to `C` [ConversionFailure]
+// CHECK:STDERR: var a: array(C, 3) = ({}, "Hello", "World");
+// CHECK:STDERR:                      ^~~~~~~~~~~~~~~~~~~~~~
+// CHECK:STDERR: fail_arg_wrong_type.carbon:[[@LINE+4]]:22: note: type `String` does not implement interface `Core.ImplicitAs(C)` [MissingImplInMemberAccessNote]
 // CHECK:STDERR: var a: array(C, 3) = ({}, "Hello", "World");
 // CHECK:STDERR:                      ^~~~~~~~~~~~~~~~~~~~~~
 // CHECK:STDERR:
diff --git a/toolchain/check/testdata/function/generic/fail_type_param_mismatch.carbon b/toolchain/check/testdata/function/generic/fail_type_param_mismatch.carbon
index 9acf5aa65ac3..e9ec370838f0 100644
--- a/toolchain/check/testdata/function/generic/fail_type_param_mismatch.carbon
+++ b/toolchain/check/testdata/function/generic/fail_type_param_mismatch.carbon
@@ -4,7 +4,7 @@
 //
 // TODO: Add ranges and switch to "--dump-sem-ir-ranges=only".
 // EXTRA-ARGS: --dump-sem-ir-ranges=if-present
-// INCLUDE-FILE: toolchain/testing/testdata/min_prelude/destroy.carbon
+// INCLUDE-FILE: toolchain/testing/testdata/min_prelude/convert.carbon
 //
 // AUTOUPDATE
 // TIP: To test this file alone, run:
@@ -14,7 +14,10 @@
 
 fn F(T:! type, U:! type) {
   var p: T*;
-  // CHECK:STDERR: fail_type_param_mismatch.carbon:[[@LINE+4]]:14: error: name `Core.ImplicitAs` implicitly referenced here, but not found [CoreNameNotFound]
+  // CHECK:STDERR: fail_type_param_mismatch.carbon:[[@LINE+7]]:14: error: cannot implicitly convert expression of type `T` to `U` [ConversionFailure]
+  // CHECK:STDERR:   let n: U = *p;
+  // CHECK:STDERR:              ^~
+  // CHECK:STDERR: fail_type_param_mismatch.carbon:[[@LINE+4]]:14: note: type `T` does not implement interface `Core.ImplicitAs(U)` [MissingImplInMemberAccessNote]
   // CHECK:STDERR:   let n: U = *p;
   // CHECK:STDERR:              ^~
   // CHECK:STDERR:
@@ -34,13 +37,29 @@ fn F(T:! type, U:! type) {
 // CHECK:STDOUT:   %pattern_type.afe: type = pattern_type %ptr [symbolic]
 // CHECK:STDOUT:   %require_complete.b54: <witness> = require_complete_type %U [symbolic]
 // CHECK:STDOUT:   %pattern_type.a32: type = pattern_type %U [symbolic]
+// CHECK:STDOUT:   %ImplicitAs.type.cc7: type = generic_interface_type @ImplicitAs [concrete]
+// CHECK:STDOUT:   %ImplicitAs.generic: %ImplicitAs.type.cc7 = struct_value () [concrete]
+// CHECK:STDOUT:   %Dest: type = bind_symbolic_name Dest, 0 [symbolic]
+// CHECK:STDOUT:   %Convert.type.275: type = fn_type @Convert, @ImplicitAs(%Dest) [symbolic]
+// CHECK:STDOUT:   %Convert.42e: %Convert.type.275 = struct_value () [symbolic]
+// CHECK:STDOUT:   %ImplicitAs.assoc_type.ca0: type = assoc_entity_type @ImplicitAs, @ImplicitAs(%Dest) [symbolic]
+// CHECK:STDOUT:   %ImplicitAs.type.d88: type = facet_type <@ImplicitAs, @ImplicitAs(%U)> [symbolic]
+// CHECK:STDOUT:   %ImplicitAs.assoc_type.5fe: type = assoc_entity_type @ImplicitAs, @ImplicitAs(%U) [symbolic]
+// CHECK:STDOUT:   %assoc0.a0b: %ImplicitAs.assoc_type.5fe = assoc_entity element0, imports.%Core.import_ref.1c7 [symbolic]
+// CHECK:STDOUT:   %require_complete.6c8: <witness> = require_complete_type %ImplicitAs.type.d88 [symbolic]
+// CHECK:STDOUT:   %assoc0.dc0: %ImplicitAs.assoc_type.ca0 = assoc_entity element0, imports.%Core.import_ref.207 [symbolic]
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
 // CHECK:STDOUT: imports {
 // CHECK:STDOUT:   %Core: <namespace> = namespace file.%Core.import, [concrete] {
-// CHECK:STDOUT:     .ImplicitAs = <poisoned>
+// CHECK:STDOUT:     .ImplicitAs = %Core.ImplicitAs
 // CHECK:STDOUT:     import Core//prelude
+// CHECK:STDOUT:     import Core//prelude/...
 // CHECK:STDOUT:   }
+// CHECK:STDOUT:   %Core.ImplicitAs: %ImplicitAs.type.cc7 = import_ref Core//prelude/parts/as, ImplicitAs, loaded [concrete = constants.%ImplicitAs.generic]
+// CHECK:STDOUT:   %Core.import_ref.492: @ImplicitAs.%ImplicitAs.assoc_type (%ImplicitAs.assoc_type.ca0) = import_ref Core//prelude/parts/as, loc10_35, loaded [symbolic = @ImplicitAs.%assoc0 (constants.%assoc0.dc0)]
+// CHECK:STDOUT:   %Core.import_ref.1c7: @ImplicitAs.%Convert.type (%Convert.type.275) = import_ref Core//prelude/parts/as, loc10_35, loaded [symbolic = @ImplicitAs.%Convert (constants.%Convert.42e)]
+// CHECK:STDOUT:   %Core.import_ref.207 = import_ref Core//prelude/parts/as, loc10_35, unloaded
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
 // CHECK:STDOUT: file {
@@ -66,8 +85,12 @@ fn F(T:! type, U:! type) {
 // CHECK:STDOUT:   %ptr.loc16_11.2: type = ptr_type %T.loc15_6.2 [symbolic = %ptr.loc16_11.2 (constants.%ptr)]
 // CHECK:STDOUT:   %require_complete.loc16: <witness> = require_complete_type %ptr.loc16_11.2 [symbolic = %require_complete.loc16 (constants.%require_complete.6e5)]
 // CHECK:STDOUT:   %pattern_type.loc16: type = pattern_type %ptr.loc16_11.2 [symbolic = %pattern_type.loc16 (constants.%pattern_type.afe)]
-// CHECK:STDOUT:   %require_complete.loc21: <witness> = require_complete_type %U.loc15_16.2 [symbolic = %require_complete.loc21 (constants.%require_complete.b54)]
-// CHECK:STDOUT:   %pattern_type.loc21: type = pattern_type %U.loc15_16.2 [symbolic = %pattern_type.loc21 (constants.%pattern_type.a32)]
+// CHECK:STDOUT:   %require_complete.loc24_10: <witness> = require_complete_type %U.loc15_16.2 [symbolic = %require_complete.loc24_10 (constants.%require_complete.b54)]
+// CHECK:STDOUT:   %pattern_type.loc24: type = pattern_type %U.loc15_16.2 [symbolic = %pattern_type.loc24 (constants.%pattern_type.a32)]
+// CHECK:STDOUT:   %ImplicitAs.type.loc24_14.2: type = facet_type <@ImplicitAs, @ImplicitAs(%U.loc15_16.2)> [symbolic = %ImplicitAs.type.loc24_14.2 (constants.%ImplicitAs.type.d88)]
+// CHECK:STDOUT:   %require_complete.loc24_14: <witness> = require_complete_type %ImplicitAs.type.loc24_14.2 [symbolic = %require_complete.loc24_14 (constants.%require_complete.6c8)]
+// CHECK:STDOUT:   %ImplicitAs.assoc_type: type = assoc_entity_type @ImplicitAs, @ImplicitAs(%U.loc15_16.2) [symbolic = %ImplicitAs.assoc_type (constants.%ImplicitAs.assoc_type.5fe)]
+// CHECK:STDOUT:   %assoc0: @F.%ImplicitAs.assoc_type (%ImplicitAs.assoc_type.5fe) = assoc_entity element0, imports.%Core.import_ref.1c7 [symbolic = %assoc0 (constants.%assoc0.a0b)]
 // CHECK:STDOUT:
 // CHECK:STDOUT:   fn() {
 // CHECK:STDOUT:   !entry:
@@ -82,13 +105,16 @@ fn F(T:! type, U:! type) {
 // CHECK:STDOUT:     }
 // CHECK:STDOUT:     %p: ref @F.%ptr.loc16_11.2 (%ptr) = bind_name p, %p.var
 // CHECK:STDOUT:     name_binding_decl {
-// CHECK:STDOUT:       %n.patt: @F.%pattern_type.loc21 (%pattern_type.a32) = binding_pattern n [concrete]
+// CHECK:STDOUT:       %n.patt: @F.%pattern_type.loc24 (%pattern_type.a32) = binding_pattern n [concrete]
 // CHECK:STDOUT:     }
 // CHECK:STDOUT:     %p.ref: ref @F.%ptr.loc16_11.2 (%ptr) = name_ref p, %p
-// CHECK:STDOUT:     %.loc21_15: @F.%ptr.loc16_11.2 (%ptr) = bind_value %p.ref
-// CHECK:STDOUT:     %.loc21_14.1: ref @F.%T.loc15_6.2 (%T) = deref %.loc21_15
+// CHECK:STDOUT:     %.loc24_15: @F.%ptr.loc16_11.2 (%ptr) = bind_value %p.ref
+// CHECK:STDOUT:     %.loc24_14.1: ref @F.%T.loc15_6.2 (%T) = deref %.loc24_15
 // CHECK:STDOUT:     %U.ref: type = name_ref U, %U.loc15_16.1 [symbolic = %U.loc15_16.2 (constants.%U)]
-// CHECK:STDOUT:     %.loc21_14.2: @F.%U.loc15_16.2 (%U) = converted %.loc21_14.1, <error> [concrete = <error>]
+// CHECK:STDOUT:     %ImplicitAs.type.loc24_14.1: type = facet_type <@ImplicitAs, @ImplicitAs(constants.%U)> [symbolic = %ImplicitAs.type.loc24_14.2 (constants.%ImplicitAs.type.d88)]
+// CHECK:STDOUT:     %.loc24_14.2: @F.%ImplicitAs.assoc_type (%ImplicitAs.assoc_type.5fe) = specific_constant imports.%Core.import_ref.492, @ImplicitAs(constants.%U) [symbolic = %assoc0 (constants.%assoc0.a0b)]
+// CHECK:STDOUT:     %Convert.ref: @F.%ImplicitAs.assoc_type (%ImplicitAs.assoc_type.5fe) = name_ref Convert, %.loc24_14.2 [symbolic = %assoc0 (constants.%assoc0.a0b)]
+// CHECK:STDOUT:     %.loc24_14.3: @F.%U.loc15_16.2 (%U) = converted %.loc24_14.1, <error> [concrete = <error>]
 // CHECK:STDOUT:     %n: @F.%U.loc15_16.2 (%U) = bind_name n, <error> [concrete = <error>]
 // CHECK:STDOUT:     return
 // CHECK:STDOUT:   }
diff --git a/toolchain/check/testdata/impl/fail_impl_bad_type.carbon b/toolchain/check/testdata/impl/fail_impl_bad_type.carbon
index b60f85c2cd79..8caee7ff6f45 100644
--- a/toolchain/check/testdata/impl/fail_impl_bad_type.carbon
+++ b/toolchain/check/testdata/impl/fail_impl_bad_type.carbon
@@ -3,7 +3,8 @@
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 // TODO: Add ranges and switch to "--dump-sem-ir-ranges=only".
-// EXTRA-ARGS: --no-prelude-import --dump-sem-ir-ranges=if-present
+// EXTRA-ARGS: --dump-sem-ir-ranges=if-present
+// INCLUDE-FILE: toolchain/testing/testdata/min_prelude/convert.carbon
 //
 // AUTOUPDATE
 // TIP: To test this file alone, run:
@@ -13,7 +14,10 @@
 
 interface I {}
 
-// CHECK:STDERR: fail_impl_bad_type.carbon:[[@LINE+4]]:6: error: `Core.ImplicitAs` implicitly referenced here, but package `Core` not found [CoreNotFound]
+// CHECK:STDERR: fail_impl_bad_type.carbon:[[@LINE+7]]:6: error: cannot implicitly convert non-type value of type `bool` to `type` [ConversionFailureNonTypeToFacet]
+// CHECK:STDERR: impl true as I {}
+// CHECK:STDERR:      ^~~~
+// CHECK:STDERR: fail_impl_bad_type.carbon:[[@LINE+4]]:6: note: type `bool` does not implement interface `Core.ImplicitAs(type)` [MissingImplInMemberAccessNote]
 // CHECK:STDERR: impl true as I {}
 // CHECK:STDERR:      ^~~~
 // CHECK:STDERR:
@@ -23,24 +27,37 @@ impl true as I {}
 // CHECK:STDOUT:
 // CHECK:STDOUT: constants {
 // CHECK:STDOUT:   %I.type: type = facet_type <@I> [concrete]
-// CHECK:STDOUT:   %Self: %I.type = bind_symbolic_name Self, 0 [symbolic]
+// CHECK:STDOUT:   %Self.826: %I.type = bind_symbolic_name Self, 0 [symbolic]
 // CHECK:STDOUT:   %true: bool = bool_literal true [concrete]
+// CHECK:STDOUT:   %ImplicitAs.type.cc7: type = generic_interface_type @ImplicitAs [concrete]
+// CHECK:STDOUT:   %ImplicitAs.generic: %ImplicitAs.type.cc7 = struct_value () [concrete]
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: imports {
+// CHECK:STDOUT:   %Core: <namespace> = namespace file.%Core.import, [concrete] {
+// CHECK:STDOUT:     .ImplicitAs = %Core.ImplicitAs
+// CHECK:STDOUT:     import Core//prelude
+// CHECK:STDOUT:     import Core//prelude/...
+// CHECK:STDOUT:   }
+// CHECK:STDOUT:   %Core.ImplicitAs: %ImplicitAs.type.cc7 = import_ref Core//prelude/parts/as, ImplicitAs, loaded [concrete = constants.%ImplicitAs.generic]
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
 // CHECK:STDOUT: file {
 // CHECK:STDOUT:   package: <namespace> = namespace [concrete] {
+// CHECK:STDOUT:     .Core = imports.%Core
 // CHECK:STDOUT:     .I = %I.decl
 // CHECK:STDOUT:   }
+// CHECK:STDOUT:   %Core.import = import Core
 // CHECK:STDOUT:   %I.decl: type = interface_decl @I [concrete = constants.%I.type] {} {}
 // CHECK:STDOUT:   impl_decl @impl [concrete] {} {
 // CHECK:STDOUT:     %true: bool = bool_literal true [concrete = constants.%true]
-// CHECK:STDOUT:     %.loc20: type = converted %true, <error> [concrete = <error>]
+// CHECK:STDOUT:     %.loc24: type = converted %true, <error> [concrete = <error>]
 // CHECK:STDOUT:     %I.ref: type = name_ref I, file.%I.decl [concrete = constants.%I.type]
 // CHECK:STDOUT:   }
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
 // CHECK:STDOUT: interface @I {
-// CHECK:STDOUT:   %Self: %I.type = bind_symbolic_name Self, 0 [symbolic = constants.%Self]
+// CHECK:STDOUT:   %Self: %I.type = bind_symbolic_name Self, 0 [symbolic = constants.%Self.826]
 // CHECK:STDOUT:
 // CHECK:STDOUT: !members:
 // CHECK:STDOUT:   .Self = %Self
diff --git a/toolchain/check/testdata/impl/impl_thunk.carbon b/toolchain/check/testdata/impl/impl_thunk.carbon
index fe5979ba5d25..3137df5ee3cb 100644
--- a/toolchain/check/testdata/impl/impl_thunk.carbon
+++ b/toolchain/check/testdata/impl/impl_thunk.carbon
@@ -2,7 +2,7 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// EXTRA-ARGS: --no-prelude-import
+// INCLUDE-FILE: toolchain/testing/testdata/min_prelude/convert.carbon
 //
 // AUTOUPDATE
 // TIP: To test this file alone, run:
@@ -119,7 +119,10 @@ impl A as X {
 library "[[@TEST_NAME]]";
 
 interface I {
-  // CHECK:STDERR: fail_param_type_mismatch.carbon:[[@LINE+3]]:8: error: `Core.ImplicitAs` implicitly referenced here, but package `Core` not found [CoreNotFound]
+  // CHECK:STDERR: fail_param_type_mismatch.carbon:[[@LINE+6]]:8: error: cannot implicitly convert expression of type `A` to `B` [ConversionFailure]
+  // CHECK:STDERR:   fn F(a: Self);
+  // CHECK:STDERR:        ^~~~~~~
+  // CHECK:STDERR: fail_param_type_mismatch.carbon:[[@LINE+3]]:8: note: type `A` does not implement interface `Core.ImplicitAs(B)` [MissingImplInMemberAccessNote]
   // CHECK:STDERR:   fn F(a: Self);
   // CHECK:STDERR:        ^~~~~~~
   fn F(a: Self);
@@ -151,10 +154,13 @@ class A {}
 class B {}
 
 impl A as I {
-  // CHECK:STDERR: fail_return_mismatch.carbon:[[@LINE+7]]:3: error: `Core.ImplicitAs` implicitly referenced here, but package `Core` not found [CoreNotFound]
+  // CHECK:STDERR: fail_return_mismatch.carbon:[[@LINE+10]]:3: error: cannot implicitly convert expression of type `B` to `A` [ConversionFailure]
   // CHECK:STDERR:   fn F() -> B;
   // CHECK:STDERR:   ^~~~~~~~~~~~
-  // CHECK:STDERR: fail_return_mismatch.carbon:[[@LINE-10]]:3: note: while building thunk to match the signature of this function [ThunkSignature]
+  // CHECK:STDERR: fail_return_mismatch.carbon:[[@LINE+7]]:3: note: type `B` does not implement interface `Core.ImplicitAs(A)` [MissingImplInMemberAccessNote]
+  // CHECK:STDERR:   fn F() -> B;
+  // CHECK:STDERR:   ^~~~~~~~~~~~
+  // CHECK:STDERR: fail_return_mismatch.carbon:[[@LINE-13]]:3: note: while building thunk to match the signature of this function [ThunkSignature]
   // CHECK:STDERR:   fn F() -> Self;
   // CHECK:STDERR:   ^~~~~~~~~~~~~~~
   // CHECK:STDERR:
@@ -233,17 +239,20 @@ class B;
 
 class C {
   impl as I(B) {
-    // CHECK:STDERR: fail_param_type_incomplete.carbon:[[@LINE+14]]:5: note: while building thunk calling this function [ThunkCallee]
+    // CHECK:STDERR: fail_param_type_incomplete.carbon:[[@LINE+17]]:5: note: while building thunk calling this function [ThunkCallee]
     // CHECK:STDERR:     fn F(c: C);
     // CHECK:STDERR:     ^~~~~~~~~~~
     // CHECK:STDERR:
-    // CHECK:STDERR: fail_param_type_incomplete.carbon:[[@LINE-14]]:8: error: `Core.ImplicitAs` implicitly referenced here, but package `Core` not found [CoreNotFound]
+    // CHECK:STDERR: fail_param_type_incomplete.carbon:[[@LINE-14]]:8: error: cannot implicitly convert expression of type `B` to `C` [ConversionFailure]
+    // CHECK:STDERR:   fn F(a: T);
+    // CHECK:STDERR:        ^~~~
+    // CHECK:STDERR: fail_param_type_incomplete.carbon:[[@LINE-17]]:8: note: type `B` does not implement interface `Core.ImplicitAs(C)` [MissingImplInMemberAccessNote]
     // CHECK:STDERR:   fn F(a: T);
     // CHECK:STDERR:        ^~~~
     // CHECK:STDERR: fail_param_type_incomplete.carbon:[[@LINE+7]]:10: note: initializing function parameter [InCallToFunctionParam]
     // CHECK:STDERR:     fn F(c: C);
     // CHECK:STDERR:          ^~~~
-    // CHECK:STDERR: fail_param_type_incomplete.carbon:[[@LINE-20]]:3: note: while building thunk to match the signature of this function [ThunkSignature]
+    // CHECK:STDERR: fail_param_type_incomplete.carbon:[[@LINE-23]]:3: note: while building thunk to match the signature of this function [ThunkSignature]
     // CHECK:STDERR:   fn F(a: T);
     // CHECK:STDERR:   ^~~~~~~~~~~
     // CHECK:STDERR:
@@ -322,6 +331,9 @@ impl () as I({}) {
 // CHECK:STDOUT:   %struct: %struct_type.c.d.15a = struct_value (%empty_tuple, %empty_struct) [concrete]
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
+// CHECK:STDOUT: imports {
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
 // CHECK:STDOUT: impl @impl: %.loc8_7.2 as %I.ref {
 // CHECK:STDOUT:   %F.decl.loc10_48.1: %F.type.39e918.1 = fn_decl @F.2 [concrete = constants.%F.c04b92.1] {
 // CHECK:STDOUT:     %y.patt: %pattern_type.231 = binding_pattern y [concrete]
@@ -398,6 +410,9 @@ impl () as I({}) {
 // CHECK:STDOUT:   %F.5161e9.2: %F.type.f1b0b1.2 = struct_value () [concrete]
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
+// CHECK:STDOUT: imports {
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
 // CHECK:STDOUT: impl @impl: %B.ref as %X.ref {
 // CHECK:STDOUT:   %F.decl.loc14_39.1: %F.type.f1b0b1.1 = fn_decl @F.2 [concrete = constants.%F.5161e9.1] {
 // CHECK:STDOUT:     %self.patt: %pattern_type.5f8 = binding_pattern self [concrete]
@@ -469,6 +484,9 @@ impl () as I({}) {
 // CHECK:STDOUT:   %F.5161e9.2: %F.type.f1b0b1.2 = struct_value () [concrete]
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
+// CHECK:STDOUT: imports {
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
 // CHECK:STDOUT: impl @impl: %B.ref as %X.ref {
 // CHECK:STDOUT:   %F.decl.loc13_26.1: %F.type.f1b0b1.1 = fn_decl @F.2 [concrete = constants.%F.5161e9.1] {
 // CHECK:STDOUT:     %self.patt: %pattern_type.c10 = binding_pattern self [concrete]
@@ -525,6 +543,9 @@ impl () as I({}) {
 // CHECK:STDOUT:   %F.5161e9.2: %F.type.f1b0b1.2 = struct_value () [concrete]
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
+// CHECK:STDOUT: imports {
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
 // CHECK:STDOUT: impl @impl: %B.ref as %X.ref {
 // CHECK:STDOUT:   %F.decl.loc14_39.1: %F.type.f1b0b1.1 = fn_decl @F.2 [concrete = constants.%F.5161e9.1] {
 // CHECK:STDOUT:     %self.patt: %pattern_type.5f8 = binding_pattern self [concrete]
@@ -595,6 +616,9 @@ impl () as I({}) {
 // CHECK:STDOUT:   %F.77e9d5.2: %F.type.b24d6f.2 = struct_value () [concrete]
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
+// CHECK:STDOUT: imports {
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
 // CHECK:STDOUT: impl @impl: %A.ref as %X.ref {
 // CHECK:STDOUT:   %F.decl.loc20_14.1: %F.type.b24d6f.1 = fn_decl @F.2 [concrete = constants.%F.77e9d5.1] {
 // CHECK:STDOUT:     %return.patt: %pattern_type.049 = return_slot_pattern [concrete]
@@ -649,6 +673,9 @@ impl () as I({}) {
 // CHECK:STDOUT:   %F.specific_fn: <specific function> = specific_function %F.c04b92.1, @F.2(%ptr.79f) [symbolic]
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
+// CHECK:STDOUT: imports {
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
 // CHECK:STDOUT: impl @impl: %.loc8_7.2 as %I.ref {
 // CHECK:STDOUT:   %F.decl.loc10_29.1: %F.type.39e918.1 = fn_decl @F.2 [concrete = constants.%F.c04b92.1] {
 // CHECK:STDOUT:     %U.patt: %pattern_type.98f = symbolic_binding_pattern U, 0 [concrete]
@@ -746,6 +773,9 @@ impl () as I({}) {
 // CHECK:STDOUT:   %require_complete.4ae: <witness> = require_complete_type %U [symbolic]
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
+// CHECK:STDOUT: imports {
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
 // CHECK:STDOUT: impl @impl: %.loc10_7.2 as %I.ref {
 // CHECK:STDOUT:   %F.decl.loc19_39.1: %F.type.39e918.1 = fn_decl @F.2 [concrete = constants.%F.c04b92.1] {
 // CHECK:STDOUT:     %self.patt: %pattern_type.cb1 = binding_pattern self [concrete]
@@ -834,6 +864,9 @@ impl () as I({}) {
 // CHECK:STDOUT:   %struct: %struct_type.a.b.f95 = struct_value (%empty_struct, %empty_struct) [concrete]
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
+// CHECK:STDOUT: imports {
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
 // CHECK:STDOUT: impl @impl: %.loc8_7.2 as %I.type {
 // CHECK:STDOUT:   %F.decl.loc10_29.1: %F.type.29ab63.1 = fn_decl @F.2 [concrete = constants.%F.975709.1] {
 // CHECK:STDOUT:     %return.patt: %pattern_type.914 = return_slot_pattern [concrete]
diff --git a/toolchain/check/testdata/interop/cpp/class.carbon b/toolchain/check/testdata/interop/cpp/class.carbon
index 60fa0a0a34b2..555b7d4dda56 100644
--- a/toolchain/check/testdata/interop/cpp/class.carbon
+++ b/toolchain/check/testdata/interop/cpp/class.carbon
@@ -2,7 +2,7 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// EXTRA-ARGS: --no-prelude-import
+// INCLUDE-FILE: toolchain/testing/testdata/min_prelude/convert.carbon
 //
 // AUTOUPDATE
 // TIP: To test this file alone, run:
@@ -262,10 +262,13 @@ import Cpp library "inheritance_pointers.h";
 //@dump-sem-ir-begin
 fn MyF1(bar : Cpp.Bar1*);
 // TODO: Support C++ inheritance.
-// CHECK:STDERR: fail_todo_import_inheritance_pointers.carbon:[[@LINE+7]]:33: error: `Core.ImplicitAs` implicitly referenced here, but package `Core` not found [CoreNotFound]
+// CHECK:STDERR: fail_todo_import_inheritance_pointers.carbon:[[@LINE+10]]:33: error: cannot implicitly convert expression of type `Cpp.Bar2*` to `Cpp.Bar1*` [ConversionFailure]
 // CHECK:STDERR: fn MyF2(bar : Cpp.Bar2*) { MyF1(bar); }
 // CHECK:STDERR:                                 ^~~
-// CHECK:STDERR: fail_todo_import_inheritance_pointers.carbon:[[@LINE-5]]:9: note: initializing function parameter [InCallToFunctionParam]
+// CHECK:STDERR: fail_todo_import_inheritance_pointers.carbon:[[@LINE+7]]:33: note: type `Cpp.Bar2*` does not implement interface `Core.ImplicitAs(Cpp.Bar1*)` [MissingImplInMemberAccessNote]
+// CHECK:STDERR: fn MyF2(bar : Cpp.Bar2*) { MyF1(bar); }
+// CHECK:STDERR:                                 ^~~
+// CHECK:STDERR: fail_todo_import_inheritance_pointers.carbon:[[@LINE-8]]:9: note: initializing function parameter [InCallToFunctionParam]
 // CHECK:STDERR: fn MyF1(bar : Cpp.Bar1*);
 // CHECK:STDERR:         ^~~~~~~~~~~~~~~
 // CHECK:STDERR:
@@ -731,7 +734,7 @@ fn MyF(bar: Cpp.Bar*);
 // CHECK:STDOUT:     %bar.param_patt: %pattern_type.92a = value_param_pattern %bar.patt, call_param0 [concrete]
 // CHECK:STDOUT:   } {
 // CHECK:STDOUT:     %bar.param: %ptr.eca = value_param call_param0
-// CHECK:STDOUT:     %.loc16_23: type = splice_block %ptr [concrete = constants.%ptr.eca] {
+// CHECK:STDOUT:     %.loc19_23: type = splice_block %ptr [concrete = constants.%ptr.eca] {
 // CHECK:STDOUT:       %Cpp.ref: <namespace> = name_ref Cpp, imports.%Cpp [concrete = imports.%Cpp]
 // CHECK:STDOUT:       <elided>
 // CHECK:STDOUT:       %Bar2.ref: type = name_ref Bar2, imports.%Bar2.decl [concrete = constants.%Bar2]
@@ -747,7 +750,7 @@ fn MyF(bar: Cpp.Bar*);
 // CHECK:STDOUT: !entry:
 // CHECK:STDOUT:   %MyF1.ref: %MyF1.type = name_ref MyF1, file.%MyF1.decl [concrete = constants.%MyF1]
 // CHECK:STDOUT:   %bar.ref: %ptr.eca = name_ref bar, %bar
-// CHECK:STDOUT:   %.loc16_33: %ptr.f68 = converted %bar.ref, <error> [concrete = <error>]
+// CHECK:STDOUT:   %.loc19_33: %ptr.f68 = converted %bar.ref, <error> [concrete = <error>]
 // CHECK:STDOUT:   %MyF1.call: init %empty_tuple.type = call %MyF1.ref(<error>)
 // CHECK:STDOUT:   return
 // CHECK:STDOUT: }
diff --git a/toolchain/check/testdata/interop/cpp/struct.carbon b/toolchain/check/testdata/interop/cpp/struct.carbon
index c34436cf214f..3cfaf7e79360 100644
--- a/toolchain/check/testdata/interop/cpp/struct.carbon
+++ b/toolchain/check/testdata/interop/cpp/struct.carbon
@@ -2,7 +2,7 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// EXTRA-ARGS: --no-prelude-import
+// INCLUDE-FILE: toolchain/testing/testdata/min_prelude/convert.carbon
 //
 // AUTOUPDATE
 // TIP: To test this file alone, run:
@@ -256,10 +256,13 @@ import Cpp library "inheritance_pointers.h";
 //@dump-sem-ir-begin
 fn MyF1(bar : Cpp.Bar1*);
 // TODO: Support C++ inheritance.
-// CHECK:STDERR: fail_todo_import_inheritance_pointers.carbon:[[@LINE+7]]:33: error: `Core.ImplicitAs` implicitly referenced here, but package `Core` not found [CoreNotFound]
+// CHECK:STDERR: fail_todo_import_inheritance_pointers.carbon:[[@LINE+10]]:33: error: cannot implicitly convert expression of type `Cpp.Bar2*` to `Cpp.Bar1*` [ConversionFailure]
 // CHECK:STDERR: fn MyF2(bar : Cpp.Bar2*) { MyF1(bar); }
 // CHECK:STDERR:                                 ^~~
-// CHECK:STDERR: fail_todo_import_inheritance_pointers.carbon:[[@LINE-5]]:9: note: initializing function parameter [InCallToFunctionParam]
+// CHECK:STDERR: fail_todo_import_inheritance_pointers.carbon:[[@LINE+7]]:33: note: type `Cpp.Bar2*` does not implement interface `Core.ImplicitAs(Cpp.Bar1*)` [MissingImplInMemberAccessNote]
+// CHECK:STDERR: fn MyF2(bar : Cpp.Bar2*) { MyF1(bar); }
+// CHECK:STDERR:                                 ^~~
+// CHECK:STDERR: fail_todo_import_inheritance_pointers.carbon:[[@LINE-8]]:9: note: initializing function parameter [InCallToFunctionParam]
 // CHECK:STDERR: fn MyF1(bar : Cpp.Bar1*);
 // CHECK:STDERR:         ^~~~~~~~~~~~~~~
 // CHECK:STDERR:
@@ -721,7 +724,7 @@ fn MyF(bar: Cpp.Bar*);
 // CHECK:STDOUT:     %bar.param_patt: %pattern_type.92a = value_param_pattern %bar.patt, call_param0 [concrete]
 // CHECK:STDOUT:   } {
 // CHECK:STDOUT:     %bar.param: %ptr.eca = value_param call_param0
-// CHECK:STDOUT:     %.loc16_23: type = splice_block %ptr [concrete = constants.%ptr.eca] {
+// CHECK:STDOUT:     %.loc19_23: type = splice_block %ptr [concrete = constants.%ptr.eca] {
 // CHECK:STDOUT:       %Cpp.ref: <namespace> = name_ref Cpp, imports.%Cpp [concrete = imports.%Cpp]
 // CHECK:STDOUT:       <elided>
 // CHECK:STDOUT:       %Bar2.ref: type = name_ref Bar2, imports.%Bar2.decl [concrete = constants.%Bar2]
@@ -737,7 +740,7 @@ fn MyF(bar: Cpp.Bar*);
 // CHECK:STDOUT: !entry:
 // CHECK:STDOUT:   %MyF1.ref: %MyF1.type = name_ref MyF1, file.%MyF1.decl [concrete = constants.%MyF1]
 // CHECK:STDOUT:   %bar.ref: %ptr.eca = name_ref bar, %bar
-// CHECK:STDOUT:   %.loc16_33: %ptr.f68 = converted %bar.ref, <error> [concrete = <error>]
+// CHECK:STDOUT:   %.loc19_33: %ptr.f68 = converted %bar.ref, <error> [concrete = <error>]
 // CHECK:STDOUT:   %MyF1.call: init %empty_tuple.type = call %MyF1.ref(<error>)
 // CHECK:STDOUT:   return
 // CHECK:STDOUT: }
diff --git a/toolchain/check/testdata/var/fail_init_type_mismatch.carbon b/toolchain/check/testdata/var/fail_init_type_mismatch.carbon
index 443295fb6e90..bd64da16a6e4 100644
--- a/toolchain/check/testdata/var/fail_init_type_mismatch.carbon
+++ b/toolchain/check/testdata/var/fail_init_type_mismatch.carbon
@@ -3,7 +3,8 @@
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 // TODO: Add ranges and switch to "--dump-sem-ir-ranges=only".
-// EXTRA-ARGS: --no-prelude-import --dump-sem-ir-ranges=if-present
+// EXTRA-ARGS: --dump-sem-ir-ranges=if-present
+// INCLUDE-FILE: toolchain/testing/testdata/min_prelude/convert.carbon
 //
 // AUTOUPDATE
 // TIP: To test this file alone, run:
@@ -11,7 +12,10 @@
 // TIP: To dump output, run:
 // TIP:   bazel run //toolchain/testing:file_test -- --dump_output --file_tests=toolchain/check/testdata/var/fail_init_type_mismatch.carbon
 
-// CHECK:STDERR: fail_init_type_mismatch.carbon:[[@LINE+4]]:1: error: `Core.ImplicitAs` implicitly referenced here, but package `Core` not found [CoreNotFound]
+// CHECK:STDERR: fail_init_type_mismatch.carbon:[[@LINE+7]]:1: error: cannot implicitly convert expression of type `()` to `{}` [ConversionFailure]
+// CHECK:STDERR: var x: {} = ();
+// CHECK:STDERR: ^~~~~~~~~
+// CHECK:STDERR: fail_init_type_mismatch.carbon:[[@LINE+4]]:1: note: type `()` does not implement interface `Core.ImplicitAs({})` [MissingImplInMemberAccessNote]
 // CHECK:STDERR: var x: {} = ();
 // CHECK:STDERR: ^~~~~~~~~
 // CHECK:STDERR:
@@ -22,32 +26,42 @@ var x: {} = ();
 // CHECK:STDOUT: constants {
 // CHECK:STDOUT:   %empty_struct_type: type = struct_type {} [concrete]
 // CHECK:STDOUT:   %empty_tuple.type: type = tuple_type () [concrete]
-// CHECK:STDOUT:   %pattern_type: type = pattern_type %empty_struct_type [concrete]
-// CHECK:STDOUT:   %empty_tuple: %empty_tuple.type = tuple_value () [concrete]
+// CHECK:STDOUT:   %pattern_type.a96: type = pattern_type %empty_struct_type [concrete]
+// CHECK:STDOUT:   %ImplicitAs.type.cc7: type = generic_interface_type @ImplicitAs [concrete]
+// CHECK:STDOUT:   %ImplicitAs.generic: %ImplicitAs.type.cc7 = struct_value () [concrete]
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: imports {
+// CHECK:STDOUT:   %Core: <namespace> = namespace file.%Core.import, [concrete] {
+// CHECK:STDOUT:     .ImplicitAs = %Core.ImplicitAs
+// CHECK:STDOUT:     import Core//prelude
+// CHECK:STDOUT:     import Core//prelude/...
+// CHECK:STDOUT:   }
+// CHECK:STDOUT:   %Core.ImplicitAs: %ImplicitAs.type.cc7 = import_ref Core//prelude/parts/as, ImplicitAs, loaded [concrete = constants.%ImplicitAs.generic]
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
 // CHECK:STDOUT: file {
 // CHECK:STDOUT:   package: <namespace> = namespace [concrete] {
+// CHECK:STDOUT:     .Core = imports.%Core
 // CHECK:STDOUT:     .x = %x
 // CHECK:STDOUT:   }
+// CHECK:STDOUT:   %Core.import = import Core
 // CHECK:STDOUT:   name_binding_decl {
-// CHECK:STDOUT:     %x.patt: %pattern_type = binding_pattern x [concrete]
-// CHECK:STDOUT:     %x.var_patt: %pattern_type = var_pattern %x.patt [concrete]
+// CHECK:STDOUT:     %x.patt: %pattern_type.a96 = binding_pattern x [concrete]
+// CHECK:STDOUT:     %x.var_patt: %pattern_type.a96 = var_pattern %x.patt [concrete]
 // CHECK:STDOUT:   }
 // CHECK:STDOUT:   %x.var: ref %empty_struct_type = var %x.var_patt [concrete]
-// CHECK:STDOUT:   %.loc18_9.1: type = splice_block %.loc18_9.3 [concrete = constants.%empty_struct_type] {
-// CHECK:STDOUT:     %.loc18_9.2: %empty_struct_type = struct_literal ()
-// CHECK:STDOUT:     %.loc18_9.3: type = converted %.loc18_9.2, constants.%empty_struct_type [concrete = constants.%empty_struct_type]
+// CHECK:STDOUT:   %.loc22_9.1: type = splice_block %.loc22_9.3 [concrete = constants.%empty_struct_type] {
+// CHECK:STDOUT:     %.loc22_9.2: %empty_struct_type = struct_literal ()
+// CHECK:STDOUT:     %.loc22_9.3: type = converted %.loc22_9.2, constants.%empty_struct_type [concrete = constants.%empty_struct_type]
 // CHECK:STDOUT:   }
 // CHECK:STDOUT:   %x: ref %empty_struct_type = bind_name x, %x.var [concrete = %x.var]
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
 // CHECK:STDOUT: fn @__global_init() {
 // CHECK:STDOUT: !entry:
-// CHECK:STDOUT:   %.loc18_14.1: %empty_tuple.type = tuple_literal ()
-// CHECK:STDOUT:   %empty_tuple: %empty_tuple.type = tuple_value () [concrete = constants.%empty_tuple]
-// CHECK:STDOUT:   %.loc18_14.2: %empty_tuple.type = converted %.loc18_14.1, %empty_tuple [concrete = constants.%empty_tuple]
-// CHECK:STDOUT:   %.loc18_1: %empty_struct_type = converted %.loc18_14.1, <error> [concrete = <error>]
+// CHECK:STDOUT:   %.loc22_14: %empty_tuple.type = tuple_literal ()
+// CHECK:STDOUT:   %.loc22_1: %empty_struct_type = converted %.loc22_14, <error> [concrete = <error>]
 // CHECK:STDOUT:   assign file.%x.var, <error>
 // CHECK:STDOUT:   return
 // CHECK:STDOUT: }