[cfe] Report an error when implementing a base class through a sealed class.

Bug: https://github.com/dart-lang/sdk/issues/51811
Change-Id: Ifad0519332965d79719173702293b48e94231024
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/291040
Reviewed-by: Johnni Winther <johnniwinther@google.com>
Commit-Queue: Kallen Tu <kallentu@google.com>

pkg/_fe_analyzer_shared/lib/src/messages/codes_generated.dart

@@ -367,6 +367,34 @@ Message _withArgumentsBaseClassImplementedOutsideOfLibrary(String name) {
       arguments: {'name': name});
 }
 
+// DO NOT EDIT. THIS FILE IS GENERATED. SEE TOP OF FILE.
+const Template<Message Function(String name, String name2)>
+    templateBaseClassImplementedOutsideOfLibraryCause =
+    const Template<Message Function(String name, String name2)>(
+        problemMessageTemplate:
+            r"""The type '#name' is a subtype of '#name2', and '#name2' is defined here.""",
+        withArguments: _withArgumentsBaseClassImplementedOutsideOfLibraryCause);
+
+// DO NOT EDIT. THIS FILE IS GENERATED. SEE TOP OF FILE.
+const Code<Message Function(String name, String name2)>
+    codeBaseClassImplementedOutsideOfLibraryCause =
+    const Code<Message Function(String name, String name2)>(
+        "BaseClassImplementedOutsideOfLibraryCause",
+        severity: Severity.context);
+
+// DO NOT EDIT. THIS FILE IS GENERATED. SEE TOP OF FILE.
+Message _withArgumentsBaseClassImplementedOutsideOfLibraryCause(
+    String name, String name2) {
+  if (name.isEmpty) throw 'No name provided';
+  name = demangleMixinApplicationName(name);
+  if (name2.isEmpty) throw 'No name provided';
+  name2 = demangleMixinApplicationName(name2);
+  return new Message(codeBaseClassImplementedOutsideOfLibraryCause,
+      problemMessage:
+          """The type '${name}' is a subtype of '${name2}', and '${name2}' is defined here.""",
+      arguments: {'name': name, 'name2': name2});
+}
+
 // DO NOT EDIT. THIS FILE IS GENERATED. SEE TOP OF FILE.
 const Template<
     Message Function(

pkg/front_end/lib/src/fasta/source/source_loader.dart

@@ -2236,7 +2236,8 @@ severity: $severity
 
     // All subtypes of a base or final class or mixin must also be base,
     // final, or sealed. Report an error otherwise.
-    void checkForBaseFinalRestriction(ClassBuilder superclass) {
+    void checkForBaseFinalRestriction(ClassBuilder superclass,
+        {TypeBuilder? implementsBuilder}) {
       if (classToBaseOrFinalSuperClass.containsKey(cls)) {
         // We've already visited this class. Don't check it again.
         return;
@@ -2265,7 +2266,27 @@ severity: $severity
 
       classToBaseOrFinalSuperClass[cls] = baseOrFinalSuperClass;
 
-      if (!cls.isBase &&
+      if (implementsBuilder != null &&
+          superclass.isSealed &&
+          baseOrFinalSuperClass.libraryBuilder.origin !=
+              cls.libraryBuilder.origin) {
+        // It's an error to implement a class if it has a supertype from a
+        // different library which is marked base.
+        if (baseOrFinalSuperClass.isBase) {
+          cls.addProblem(
+              templateBaseClassImplementedOutsideOfLibrary
+                  .withArguments(baseOrFinalSuperClass.fullNameForErrors),
+              implementsBuilder.charOffset ?? TreeNode.noOffset,
+              noLength,
+              context: [
+                templateBaseClassImplementedOutsideOfLibraryCause
+                    .withArguments(superclass.fullNameForErrors,
+                        baseOrFinalSuperClass.fullNameForErrors)
+                    .withLocation(baseOrFinalSuperClass.fileUri,
+                        baseOrFinalSuperClass.charOffset, noLength)
+              ]);
+        }
+      } else if (!cls.isBase &&
           !cls.isFinal &&
           !cls.isSealed &&
           !cls.cls.isAnonymousMixin &&
@@ -2425,7 +2446,8 @@ severity: $severity
               // supertype is a final type used outside of its library.
               // However, we still check the base and final subtyping
               // restriction if we are evaluating a multiple type 'on' clause.
-              checkForBaseFinalRestriction(interfaceDeclaration);
+              checkForBaseFinalRestriction(interfaceDeclaration,
+                  implementsBuilder: interfaceBuilder);
             }
 
             if (cls.libraryBuilder.origin !=

pkg/front_end/messages.yaml

@@ -6277,6 +6277,10 @@ BaseClassImplementedOutsideOfLibrary:
     lib.dart:
       base class A {}
 
+BaseClassImplementedOutsideOfLibraryCause:
+  problemMessage: "The type '#name' is a subtype of '#name2', and '#name2' is defined here."
+  severity: CONTEXT
+
 BaseMixinImplementedOutsideOfLibrary:
   problemMessage: "The mixin '#name' can't be implemented outside of its library because it's a base mixin."
   analyzerCode: BASE_MIXIN_IMPLEMENTED_OUTSIDE_OF_LIBRARY

pkg/front_end/testcases/class_modifiers/base/outside_library_implements/main.dart

@@ -0,0 +1,22 @@
+// Copyright (c) 2023, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+import 'main_lib.dart';
+
+sealed class SealedClassExtendsBase extends A {} /* Ok */
+
+class ClassImplementsIndirectBase
+    implements SealedClassExtendsBase {} /* Error */
+
+final class FinalClassImplementsIndirectBase
+    implements SealedClassExtendsBase {} /* Error */
+
+interface class InterfaceClassImplementsIndirectBase
+    implements SealedClassExtendsBase {} /* Error */
+
+sealed class SealedClassImplementsIndirectBase
+    implements SealedClassExtendsBase {} /* Error */
+
+base class BaseClassImplementsIndirectBase
+    implements SealedClassExtendsBase {} /* Error */

pkg/front_end/testcases/class_modifiers/base/outside_library_implements/main.dart.strong.expect

@@ -0,0 +1,85 @@
+library /*isNonNullableByDefault*/;
+//
+// Problems in library:
+//
+// pkg/front_end/testcases/class_modifiers/base/outside_library_implements/main.dart:10:16: Error: The class 'A' can't be implemented outside of its library because it's a base class.
+//     implements SealedClassExtendsBase {} /* Error */
+//                ^
+// pkg/front_end/testcases/class_modifiers/base/outside_library_implements/main_lib.dart:5:12: Context: The type 'SealedClassExtendsBase' is a subtype of 'A', and 'A' is defined here.
+// base class A {}
+//            ^
+//
+// pkg/front_end/testcases/class_modifiers/base/outside_library_implements/main.dart:13:16: Error: The class 'A' can't be implemented outside of its library because it's a base class.
+//     implements SealedClassExtendsBase {} /* Error */
+//                ^
+// pkg/front_end/testcases/class_modifiers/base/outside_library_implements/main_lib.dart:5:12: Context: The type 'SealedClassExtendsBase' is a subtype of 'A', and 'A' is defined here.
+// base class A {}
+//            ^
+//
+// pkg/front_end/testcases/class_modifiers/base/outside_library_implements/main.dart:16:16: Error: The class 'A' can't be implemented outside of its library because it's a base class.
+//     implements SealedClassExtendsBase {} /* Error */
+//                ^
+// pkg/front_end/testcases/class_modifiers/base/outside_library_implements/main_lib.dart:5:12: Context: The type 'SealedClassExtendsBase' is a subtype of 'A', and 'A' is defined here.
+// base class A {}
+//            ^
+//
+// pkg/front_end/testcases/class_modifiers/base/outside_library_implements/main.dart:19:16: Error: The class 'A' can't be implemented outside of its library because it's a base class.
+//     implements SealedClassExtendsBase {} /* Error */
+//                ^
+// pkg/front_end/testcases/class_modifiers/base/outside_library_implements/main_lib.dart:5:12: Context: The type 'SealedClassExtendsBase' is a subtype of 'A', and 'A' is defined here.
+// base class A {}
+//            ^
+//
+// pkg/front_end/testcases/class_modifiers/base/outside_library_implements/main.dart:22:16: Error: The class 'A' can't be implemented outside of its library because it's a base class.
+//     implements SealedClassExtendsBase {} /* Error */
+//                ^
+// pkg/front_end/testcases/class_modifiers/base/outside_library_implements/main_lib.dart:5:12: Context: The type 'SealedClassExtendsBase' is a subtype of 'A', and 'A' is defined here.
+// base class A {}
+//            ^
+//
+import self as self;
+import "main_lib.dart" as mai;
+import "dart:core" as core;
+
+import "org-dartlang-testcase:///main_lib.dart";
+
+abstract sealed class SealedClassExtendsBase extends mai::A {
+  synthetic constructor •() → self::SealedClassExtendsBase
+    : super mai::A::•()
+    ;
+}
+class ClassImplementsIndirectBase extends core::Object implements self::SealedClassExtendsBase {
+  synthetic constructor •() → self::ClassImplementsIndirectBase
+    : super core::Object::•()
+    ;
+}
+final class FinalClassImplementsIndirectBase extends core::Object implements self::SealedClassExtendsBase {
+  synthetic constructor •() → self::FinalClassImplementsIndirectBase
+    : super core::Object::•()
+    ;
+}
+interface class InterfaceClassImplementsIndirectBase extends core::Object implements self::SealedClassExtendsBase {
+  synthetic constructor •() → self::InterfaceClassImplementsIndirectBase
+    : super core::Object::•()
+    ;
+}
+abstract sealed class SealedClassImplementsIndirectBase extends core::Object implements self::SealedClassExtendsBase {
+  synthetic constructor •() → self::SealedClassImplementsIndirectBase
+    : super core::Object::•()
+    ;
+}
+base class BaseClassImplementsIndirectBase extends core::Object implements self::SealedClassExtendsBase {
+  synthetic constructor •() → self::BaseClassImplementsIndirectBase
+    : super core::Object::•()
+    ;
+}
+
+library /*isNonNullableByDefault*/;
+import self as mai;
+import "dart:core" as core;
+
+base class A extends core::Object {
+  synthetic constructor •() → mai::A
+    : super core::Object::•()
+    ;
+}

pkg/front_end/testcases/class_modifiers/base/outside_library_implements/main.dart.strong.transformed.expect

@@ -0,0 +1,85 @@
+library /*isNonNullableByDefault*/;
+//
+// Problems in library:
+//
+// pkg/front_end/testcases/class_modifiers/base/outside_library_implements/main.dart:10:16: Error: The class 'A' can't be implemented outside of its library because it's a base class.
+//     implements SealedClassExtendsBase {} /* Error */
+//                ^
+// pkg/front_end/testcases/class_modifiers/base/outside_library_implements/main_lib.dart:5:12: Context: The type 'SealedClassExtendsBase' is a subtype of 'A', and 'A' is defined here.
+// base class A {}
+//            ^
+//
+// pkg/front_end/testcases/class_modifiers/base/outside_library_implements/main.dart:13:16: Error: The class 'A' can't be implemented outside of its library because it's a base class.
+//     implements SealedClassExtendsBase {} /* Error */
+//                ^
+// pkg/front_end/testcases/class_modifiers/base/outside_library_implements/main_lib.dart:5:12: Context: The type 'SealedClassExtendsBase' is a subtype of 'A', and 'A' is defined here.
+// base class A {}
+//            ^
+//
+// pkg/front_end/testcases/class_modifiers/base/outside_library_implements/main.dart:16:16: Error: The class 'A' can't be implemented outside of its library because it's a base class.
+//     implements SealedClassExtendsBase {} /* Error */
+//                ^
+// pkg/front_end/testcases/class_modifiers/base/outside_library_implements/main_lib.dart:5:12: Context: The type 'SealedClassExtendsBase' is a subtype of 'A', and 'A' is defined here.
+// base class A {}
+//            ^
+//
+// pkg/front_end/testcases/class_modifiers/base/outside_library_implements/main.dart:19:16: Error: The class 'A' can't be implemented outside of its library because it's a base class.
+//     implements SealedClassExtendsBase {} /* Error */
+//                ^
+// pkg/front_end/testcases/class_modifiers/base/outside_library_implements/main_lib.dart:5:12: Context: The type 'SealedClassExtendsBase' is a subtype of 'A', and 'A' is defined here.
+// base class A {}
+//            ^
+//
+// pkg/front_end/testcases/class_modifiers/base/outside_library_implements/main.dart:22:16: Error: The class 'A' can't be implemented outside of its library because it's a base class.
+//     implements SealedClassExtendsBase {} /* Error */
+//                ^
+// pkg/front_end/testcases/class_modifiers/base/outside_library_implements/main_lib.dart:5:12: Context: The type 'SealedClassExtendsBase' is a subtype of 'A', and 'A' is defined here.
+// base class A {}
+//            ^
+//
+import self as self;
+import "main_lib.dart" as mai;
+import "dart:core" as core;
+
+import "org-dartlang-testcase:///main_lib.dart";
+
+abstract sealed class SealedClassExtendsBase extends mai::A {
+  synthetic constructor •() → self::SealedClassExtendsBase
+    : super mai::A::•()
+    ;
+}
+class ClassImplementsIndirectBase extends core::Object implements self::SealedClassExtendsBase {
+  synthetic constructor •() → self::ClassImplementsIndirectBase
+    : super core::Object::•()
+    ;
+}
+final class FinalClassImplementsIndirectBase extends core::Object implements self::SealedClassExtendsBase {
+  synthetic constructor •() → self::FinalClassImplementsIndirectBase
+    : super core::Object::•()
+    ;
+}
+interface class InterfaceClassImplementsIndirectBase extends core::Object implements self::SealedClassExtendsBase {
+  synthetic constructor •() → self::InterfaceClassImplementsIndirectBase
+    : super core::Object::•()
+    ;
+}
+abstract sealed class SealedClassImplementsIndirectBase extends core::Object implements self::SealedClassExtendsBase {
+  synthetic constructor •() → self::SealedClassImplementsIndirectBase
+    : super core::Object::•()
+    ;
+}
+base class BaseClassImplementsIndirectBase extends core::Object implements self::SealedClassExtendsBase {
+  synthetic constructor •() → self::BaseClassImplementsIndirectBase
+    : super core::Object::•()
+    ;
+}
+
+library /*isNonNullableByDefault*/;
+import self as mai;
+import "dart:core" as core;
+
+base class A extends core::Object {
+  synthetic constructor •() → mai::A
+    : super core::Object::•()
+    ;
+}

pkg/front_end/testcases/class_modifiers/base/outside_library_implements/main.dart.textual_outline.expect

@@ -0,0 +1,16 @@
+import 'main_lib.dart';
+
+sealed class SealedClassExtendsBase extends A {}
+
+class ClassImplementsIndirectBase implements SealedClassExtendsBase {}
+
+final class FinalClassImplementsIndirectBase
+    implements SealedClassExtendsBase {}
+
+interface class InterfaceClassImplementsIndirectBase
+    implements SealedClassExtendsBase {}
+
+sealed class SealedClassImplementsIndirectBase
+    implements SealedClassExtendsBase {}
+
+base class BaseClassImplementsIndirectBase implements SealedClassExtendsBase {}

pkg/front_end/testcases/class_modifiers/base/outside_library_implements/main.dart.textual_outline_modelled.expect

@@ -0,0 +1,16 @@
+import 'main_lib.dart';
+
+base class BaseClassImplementsIndirectBase implements SealedClassExtendsBase {}
+
+class ClassImplementsIndirectBase implements SealedClassExtendsBase {}
+
+final class FinalClassImplementsIndirectBase
+    implements SealedClassExtendsBase {}
+
+interface class InterfaceClassImplementsIndirectBase
+    implements SealedClassExtendsBase {}
+
+sealed class SealedClassExtendsBase extends A {}
+
+sealed class SealedClassImplementsIndirectBase
+    implements SealedClassExtendsBase {}

pkg/front_end/testcases/class_modifiers/base/outside_library_implements/main.dart.weak.expect

@@ -0,0 +1,85 @@
+library /*isNonNullableByDefault*/;
+//
+// Problems in library:
+//
+// pkg/front_end/testcases/class_modifiers/base/outside_library_implements/main.dart:10:16: Error: The class 'A' can't be implemented outside of its library because it's a base class.
+//     implements SealedClassExtendsBase {} /* Error */
+//                ^
+// pkg/front_end/testcases/class_modifiers/base/outside_library_implements/main_lib.dart:5:12: Context: The type 'SealedClassExtendsBase' is a subtype of 'A', and 'A' is defined here.
+// base class A {}
+//            ^
+//
+// pkg/front_end/testcases/class_modifiers/base/outside_library_implements/main.dart:13:16: Error: The class 'A' can't be implemented outside of its library because it's a base class.
+//     implements SealedClassExtendsBase {} /* Error */
+//                ^
+// pkg/front_end/testcases/class_modifiers/base/outside_library_implements/main_lib.dart:5:12: Context: The type 'SealedClassExtendsBase' is a subtype of 'A', and 'A' is defined here.
+// base class A {}
+//            ^
+//
+// pkg/front_end/testcases/class_modifiers/base/outside_library_implements/main.dart:16:16: Error: The class 'A' can't be implemented outside of its library because it's a base class.
+//     implements SealedClassExtendsBase {} /* Error */
+//                ^
+// pkg/front_end/testcases/class_modifiers/base/outside_library_implements/main_lib.dart:5:12: Context: The type 'SealedClassExtendsBase' is a subtype of 'A', and 'A' is defined here.
+// base class A {}
+//            ^
+//
+// pkg/front_end/testcases/class_modifiers/base/outside_library_implements/main.dart:19:16: Error: The class 'A' can't be implemented outside of its library because it's a base class.
+//     implements SealedClassExtendsBase {} /* Error */
+//                ^
+// pkg/front_end/testcases/class_modifiers/base/outside_library_implements/main_lib.dart:5:12: Context: The type 'SealedClassExtendsBase' is a subtype of 'A', and 'A' is defined here.
+// base class A {}
+//            ^
+//
+// pkg/front_end/testcases/class_modifiers/base/outside_library_implements/main.dart:22:16: Error: The class 'A' can't be implemented outside of its library because it's a base class.
+//     implements SealedClassExtendsBase {} /* Error */
+//                ^
+// pkg/front_end/testcases/class_modifiers/base/outside_library_implements/main_lib.dart:5:12: Context: The type 'SealedClassExtendsBase' is a subtype of 'A', and 'A' is defined here.
+// base class A {}
+//            ^
+//
+import self as self;
+import "main_lib.dart" as mai;
+import "dart:core" as core;
+
+import "org-dartlang-testcase:///main_lib.dart";
+
+abstract sealed class SealedClassExtendsBase extends mai::A {
+  synthetic constructor •() → self::SealedClassExtendsBase
+    : super mai::A::•()
+    ;
+}
+class ClassImplementsIndirectBase extends core::Object implements self::SealedClassExtendsBase {
+  synthetic constructor •() → self::ClassImplementsIndirectBase
+    : super core::Object::•()
+    ;
+}
+final class FinalClassImplementsIndirectBase extends core::Object implements self::SealedClassExtendsBase {
+  synthetic constructor •() → self::FinalClassImplementsIndirectBase
+    : super core::Object::•()
+    ;
+}
+interface class InterfaceClassImplementsIndirectBase extends core::Object implements self::SealedClassExtendsBase {
+  synthetic constructor •() → self::InterfaceClassImplementsIndirectBase
+    : super core::Object::•()
+    ;
+}
+abstract sealed class SealedClassImplementsIndirectBase extends core::Object implements self::SealedClassExtendsBase {
+  synthetic constructor •() → self::SealedClassImplementsIndirectBase
+    : super core::Object::•()
+    ;
+}
+base class BaseClassImplementsIndirectBase extends core::Object implements self::SealedClassExtendsBase {
+  synthetic constructor •() → self::BaseClassImplementsIndirectBase
+    : super core::Object::•()
+    ;
+}
+
+library /*isNonNullableByDefault*/;
+import self as mai;
+import "dart:core" as core;
+
+base class A extends core::Object {
+  synthetic constructor •() → mai::A
+    : super core::Object::•()
+    ;
+}

pkg/front_end/testcases/class_modifiers/base/outside_library_implements/main.dart.weak.modular.expect

@@ -0,0 +1,75 @@
+library /*isNonNullableByDefault*/;
+//
+// Problems in library:
+//
+// pkg/front_end/testcases/class_modifiers/base/outside_library_implements/main.dart:10:16: Error: The class 'A' can't be implemented outside of its library because it's a base class.
+//     implements SealedClassExtendsBase {} /* Error */
+//                ^
+// pkg/front_end/testcases/class_modifiers/base/outside_library_implements/main_lib.dart:5:12: Context: The type 'SealedClassExtendsBase' is a subtype of 'A', and 'A' is defined here.
+// base class A {}
+//            ^
+//
+// pkg/front_end/testcases/class_modifiers/base/outside_library_implements/main.dart:13:16: Error: The class 'A' can't be implemented outside of its library because it's a base class.
+//     implements SealedClassExtendsBase {} /* Error */
+//                ^
+// pkg/front_end/testcases/class_modifiers/base/outside_library_implements/main_lib.dart:5:12: Context: The type 'SealedClassExtendsBase' is a subtype of 'A', and 'A' is defined here.
+// base class A {}
+//            ^
+//
+// pkg/front_end/testcases/class_modifiers/base/outside_library_implements/main.dart:16:16: Error: The class 'A' can't be implemented outside of its library because it's a base class.
+//     implements SealedClassExtendsBase {} /* Error */
+//                ^
+// pkg/front_end/testcases/class_modifiers/base/outside_library_implements/main_lib.dart:5:12: Context: The type 'SealedClassExtendsBase' is a subtype of 'A', and 'A' is defined here.
+// base class A {}
+//            ^
+//
+// pkg/front_end/testcases/class_modifiers/base/outside_library_implements/main.dart:19:16: Error: The class 'A' can't be implemented outside of its library because it's a base class.
+//     implements SealedClassExtendsBase {} /* Error */
+//                ^
+// pkg/front_end/testcases/class_modifiers/base/outside_library_implements/main_lib.dart:5:12: Context: The type 'SealedClassExtendsBase' is a subtype of 'A', and 'A' is defined here.
+// base class A {}
+//            ^
+//
+// pkg/front_end/testcases/class_modifiers/base/outside_library_implements/main.dart:22:16: Error: The class 'A' can't be implemented outside of its library because it's a base class.
+//     implements SealedClassExtendsBase {} /* Error */
+//                ^
+// pkg/front_end/testcases/class_modifiers/base/outside_library_implements/main_lib.dart:5:12: Context: The type 'SealedClassExtendsBase' is a subtype of 'A', and 'A' is defined here.
+// base class A {}
+//            ^
+//
+import self as self;
+import "main_lib.dart" as mai;
+import "dart:core" as core;
+
+import "org-dartlang-testcase:///main_lib.dart";
+
+abstract sealed class SealedClassExtendsBase extends mai::A {
+  synthetic constructor •() → self::SealedClassExtendsBase
+    : super mai::A::•()
+    ;
+}
+class ClassImplementsIndirectBase extends core::Object implements self::SealedClassExtendsBase {
+  synthetic constructor •() → self::ClassImplementsIndirectBase
+    : super core::Object::•()
+    ;
+}
+final class FinalClassImplementsIndirectBase extends core::Object implements self::SealedClassExtendsBase {
+  synthetic constructor •() → self::FinalClassImplementsIndirectBase
+    : super core::Object::•()
+    ;
+}
+interface class InterfaceClassImplementsIndirectBase extends core::Object implements self::SealedClassExtendsBase {
+  synthetic constructor •() → self::InterfaceClassImplementsIndirectBase
+    : super core::Object::•()
+    ;
+}
+abstract sealed class SealedClassImplementsIndirectBase extends core::Object implements self::SealedClassExtendsBase {
+  synthetic constructor •() → self::SealedClassImplementsIndirectBase
+    : super core::Object::•()
+    ;
+}
+base class BaseClassImplementsIndirectBase extends core::Object implements self::SealedClassExtendsBase {
+  synthetic constructor •() → self::BaseClassImplementsIndirectBase
+    : super core::Object::•()
+    ;
+}

pkg/front_end/testcases/class_modifiers/base/outside_library_implements/main.dart.weak.outline.expect

@@ -0,0 +1,78 @@
+library /*isNonNullableByDefault*/;
+//
+// Problems in library:
+//
+// pkg/front_end/testcases/class_modifiers/base/outside_library_implements/main.dart:10:16: Error: The class 'A' can't be implemented outside of its library because it's a base class.
+//     implements SealedClassExtendsBase {} /* Error */
+//                ^
+// pkg/front_end/testcases/class_modifiers/base/outside_library_implements/main_lib.dart:5:12: Context: The type 'SealedClassExtendsBase' is a subtype of 'A', and 'A' is defined here.
+// base class A {}
+//            ^
+//
+// pkg/front_end/testcases/class_modifiers/base/outside_library_implements/main.dart:13:16: Error: The class 'A' can't be implemented outside of its library because it's a base class.
+//     implements SealedClassExtendsBase {} /* Error */
+//                ^
+// pkg/front_end/testcases/class_modifiers/base/outside_library_implements/main_lib.dart:5:12: Context: The type 'SealedClassExtendsBase' is a subtype of 'A', and 'A' is defined here.
+// base class A {}
+//            ^
+//
+// pkg/front_end/testcases/class_modifiers/base/outside_library_implements/main.dart:16:16: Error: The class 'A' can't be implemented outside of its library because it's a base class.
+//     implements SealedClassExtendsBase {} /* Error */
+//                ^
+// pkg/front_end/testcases/class_modifiers/base/outside_library_implements/main_lib.dart:5:12: Context: The type 'SealedClassExtendsBase' is a subtype of 'A', and 'A' is defined here.
+// base class A {}
+//            ^
+//
+// pkg/front_end/testcases/class_modifiers/base/outside_library_implements/main.dart:19:16: Error: The class 'A' can't be implemented outside of its library because it's a base class.
+//     implements SealedClassExtendsBase {} /* Error */
+//                ^
+// pkg/front_end/testcases/class_modifiers/base/outside_library_implements/main_lib.dart:5:12: Context: The type 'SealedClassExtendsBase' is a subtype of 'A', and 'A' is defined here.
+// base class A {}
+//            ^
+//
+// pkg/front_end/testcases/class_modifiers/base/outside_library_implements/main.dart:22:16: Error: The class 'A' can't be implemented outside of its library because it's a base class.
+//     implements SealedClassExtendsBase {} /* Error */
+//                ^
+// pkg/front_end/testcases/class_modifiers/base/outside_library_implements/main_lib.dart:5:12: Context: The type 'SealedClassExtendsBase' is a subtype of 'A', and 'A' is defined here.
+// base class A {}
+//            ^
+//
+import self as self;
+import "main_lib.dart" as mai;
+import "dart:core" as core;
+
+import "org-dartlang-testcase:///main_lib.dart";
+
+abstract sealed class SealedClassExtendsBase extends mai::A {
+  synthetic constructor •() → self::SealedClassExtendsBase
+    ;
+}
+class ClassImplementsIndirectBase extends core::Object implements self::SealedClassExtendsBase {
+  synthetic constructor •() → self::ClassImplementsIndirectBase
+    ;
+}
+final class FinalClassImplementsIndirectBase extends core::Object implements self::SealedClassExtendsBase {
+  synthetic constructor •() → self::FinalClassImplementsIndirectBase
+    ;
+}
+interface class InterfaceClassImplementsIndirectBase extends core::Object implements self::SealedClassExtendsBase {
+  synthetic constructor •() → self::InterfaceClassImplementsIndirectBase
+    ;
+}
+abstract sealed class SealedClassImplementsIndirectBase extends core::Object implements self::SealedClassExtendsBase {
+  synthetic constructor •() → self::SealedClassImplementsIndirectBase
+    ;
+}
+base class BaseClassImplementsIndirectBase extends core::Object implements self::SealedClassExtendsBase {
+  synthetic constructor •() → self::BaseClassImplementsIndirectBase
+    ;
+}
+
+library /*isNonNullableByDefault*/;
+import self as mai;
+import "dart:core" as core;
+
+base class A extends core::Object {
+  synthetic constructor •() → mai::A
+    ;
+}

pkg/front_end/testcases/class_modifiers/base/outside_library_implements/main.dart.weak.transformed.expect

@@ -0,0 +1,85 @@
+library /*isNonNullableByDefault*/;
+//
+// Problems in library:
+//
+// pkg/front_end/testcases/class_modifiers/base/outside_library_implements/main.dart:10:16: Error: The class 'A' can't be implemented outside of its library because it's a base class.
+//     implements SealedClassExtendsBase {} /* Error */
+//                ^
+// pkg/front_end/testcases/class_modifiers/base/outside_library_implements/main_lib.dart:5:12: Context: The type 'SealedClassExtendsBase' is a subtype of 'A', and 'A' is defined here.
+// base class A {}
+//            ^
+//
+// pkg/front_end/testcases/class_modifiers/base/outside_library_implements/main.dart:13:16: Error: The class 'A' can't be implemented outside of its library because it's a base class.
+//     implements SealedClassExtendsBase {} /* Error */
+//                ^
+// pkg/front_end/testcases/class_modifiers/base/outside_library_implements/main_lib.dart:5:12: Context: The type 'SealedClassExtendsBase' is a subtype of 'A', and 'A' is defined here.
+// base class A {}
+//            ^
+//
+// pkg/front_end/testcases/class_modifiers/base/outside_library_implements/main.dart:16:16: Error: The class 'A' can't be implemented outside of its library because it's a base class.
+//     implements SealedClassExtendsBase {} /* Error */
+//                ^
+// pkg/front_end/testcases/class_modifiers/base/outside_library_implements/main_lib.dart:5:12: Context: The type 'SealedClassExtendsBase' is a subtype of 'A', and 'A' is defined here.
+// base class A {}
+//            ^
+//
+// pkg/front_end/testcases/class_modifiers/base/outside_library_implements/main.dart:19:16: Error: The class 'A' can't be implemented outside of its library because it's a base class.
+//     implements SealedClassExtendsBase {} /* Error */
+//                ^
+// pkg/front_end/testcases/class_modifiers/base/outside_library_implements/main_lib.dart:5:12: Context: The type 'SealedClassExtendsBase' is a subtype of 'A', and 'A' is defined here.
+// base class A {}
+//            ^
+//
+// pkg/front_end/testcases/class_modifiers/base/outside_library_implements/main.dart:22:16: Error: The class 'A' can't be implemented outside of its library because it's a base class.
+//     implements SealedClassExtendsBase {} /* Error */
+//                ^
+// pkg/front_end/testcases/class_modifiers/base/outside_library_implements/main_lib.dart:5:12: Context: The type 'SealedClassExtendsBase' is a subtype of 'A', and 'A' is defined here.
+// base class A {}
+//            ^
+//
+import self as self;
+import "main_lib.dart" as mai;
+import "dart:core" as core;
+
+import "org-dartlang-testcase:///main_lib.dart";
+
+abstract sealed class SealedClassExtendsBase extends mai::A {
+  synthetic constructor •() → self::SealedClassExtendsBase
+    : super mai::A::•()
+    ;
+}
+class ClassImplementsIndirectBase extends core::Object implements self::SealedClassExtendsBase {
+  synthetic constructor •() → self::ClassImplementsIndirectBase
+    : super core::Object::•()
+    ;
+}
+final class FinalClassImplementsIndirectBase extends core::Object implements self::SealedClassExtendsBase {
+  synthetic constructor •() → self::FinalClassImplementsIndirectBase
+    : super core::Object::•()
+    ;
+}
+interface class InterfaceClassImplementsIndirectBase extends core::Object implements self::SealedClassExtendsBase {
+  synthetic constructor •() → self::InterfaceClassImplementsIndirectBase
+    : super core::Object::•()
+    ;
+}
+abstract sealed class SealedClassImplementsIndirectBase extends core::Object implements self::SealedClassExtendsBase {
+  synthetic constructor •() → self::SealedClassImplementsIndirectBase
+    : super core::Object::•()
+    ;
+}
+base class BaseClassImplementsIndirectBase extends core::Object implements self::SealedClassExtendsBase {
+  synthetic constructor •() → self::BaseClassImplementsIndirectBase
+    : super core::Object::•()
+    ;
+}
+
+library /*isNonNullableByDefault*/;
+import self as mai;
+import "dart:core" as core;
+
+base class A extends core::Object {
+  synthetic constructor •() → mai::A
+    : super core::Object::•()
+    ;
+}

pkg/front_end/testcases/class_modifiers/base/outside_library_implements/main_lib.dart

@@ -0,0 +1,5 @@
+// Copyright (c) 2023, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+base class A {}

pkg/front_end/testcases/class_modifiers/base/outside_library_implements/test.options

@@ -0,0 +1 @@
+main_lib.dart

tests/language/class_modifiers/base_transitivity/base_class_different_library_error_test.dart

@@ -40,53 +40,49 @@ interface class InterfaceSealedExtendExtend extends SealedExtend {}
 
 // Implementing through a sealed class.
 class SimpleSealedExtendImplement implements SealedExtend {}
-//    ^
-// [cfe] The type 'SimpleSealedExtendImplement' must be 'base', 'final' or 'sealed' because the supertype 'BaseClass' is 'base'.
 //                                           ^^^^^^^^^^^^
 // [analyzer] COMPILE_TIME_ERROR.INVALID_USE_OF_TYPE_OUTSIDE_LIBRARY
+// [cfe] The class 'BaseClass' can't be implemented outside of its library because it's a base class.
 
 base class BaseSealedExtendImplement implements SealedExtend {}
 //                                              ^^^^^^^^^^^^
 // [analyzer] COMPILE_TIME_ERROR.INVALID_USE_OF_TYPE_OUTSIDE_LIBRARY
-// [cfe] unspecified
+// [cfe] The class 'BaseClass' can't be implemented outside of its library because it's a base class.
 
 interface class InterfaceSealedExtendImplement implements SealedExtend {}
-//              ^
-// [cfe] The type 'InterfaceSealedExtendImplement' must be 'base', 'final' or 'sealed' because the supertype 'BaseClass' is 'base'.
 //                                                        ^^^^^^^^^^^^
 // [analyzer] COMPILE_TIME_ERROR.INVALID_USE_OF_TYPE_OUTSIDE_LIBRARY
+// [cfe] The class 'BaseClass' can't be implemented outside of its library because it's a base class.
 
 final class FinalSealedExtendImplement implements SealedExtend {}
 //                                                ^^^^^^^^^^^^
 // [analyzer] COMPILE_TIME_ERROR.INVALID_USE_OF_TYPE_OUTSIDE_LIBRARY
-// [cfe] unspecified
+// [cfe] The class 'BaseClass' can't be implemented outside of its library because it's a base class.
 
 sealed class SealedSealedExtendImplement implements SealedExtend {}
 //                                                  ^^^^^^^^^^^^
 // [analyzer] COMPILE_TIME_ERROR.INVALID_USE_OF_TYPE_OUTSIDE_LIBRARY
-// [cfe] unspecified
+// [cfe] The class 'BaseClass' can't be implemented outside of its library because it's a base class.
 
 mixin class MixinClassSealedExtendImplement implements SealedExtend {}
-//          ^
-// [cfe] The type 'MixinClassSealedExtendImplement' must be 'base', 'final' or 'sealed' because the supertype 'BaseClass' is 'base'.
 //                                                     ^^^^^^^^^^^^
 // [analyzer] COMPILE_TIME_ERROR.INVALID_USE_OF_TYPE_OUTSIDE_LIBRARY
+// [cfe] The class 'BaseClass' can't be implemented outside of its library because it's a base class.
 
 base mixin class BaseMixinClassSealedExtendImplement implements SealedExtend {}
 //                                                              ^^^^^^^^^^^^
 // [analyzer] COMPILE_TIME_ERROR.INVALID_USE_OF_TYPE_OUTSIDE_LIBRARY
-// [cfe] unspecified
+// [cfe] The class 'BaseClass' can't be implemented outside of its library because it's a base class.
 
 mixin MixinSealedExtendImplement implements SealedExtend {}
-//    ^
-// [cfe] The type 'MixinSealedExtendImplement' must be 'base', 'final' or 'sealed' because the supertype 'BaseClass' is 'base'.
 //                                          ^^^^^^^^^^^^
 // [analyzer] COMPILE_TIME_ERROR.INVALID_USE_OF_TYPE_OUTSIDE_LIBRARY
+// [cfe] The class 'BaseClass' can't be implemented outside of its library because it's a base class.
 
 base mixin BaseMixinSealedExtendImplement implements SealedExtend {}
 //                                                   ^^^^^^^^^^^^
 // [analyzer] COMPILE_TIME_ERROR.INVALID_USE_OF_TYPE_OUTSIDE_LIBRARY
-// [cfe] unspecified
+// [cfe] The class 'BaseClass' can't be implemented outside of its library because it's a base class.
 
 // Using a sealed class as an `on` type