diff --git a/pkg/analyzer/lib/src/error/base_or_final_type_verifier.dart b/pkg/analyzer/lib/src/error/base_or_final_type_verifier.dart
index ae00190..ae6cd9e 100644
--- a/pkg/analyzer/lib/src/error/base_or_final_type_verifier.dart
+++ b/pkg/analyzer/lib/src/error/base_or_final_type_verifier.dart
@@ -3,6 +3,7 @@
 // BSD-style license that can be found in the LICENSE file.
 
 import 'package:analyzer/dart/analysis/features.dart';
+import 'package:analyzer/dart/ast/ast.dart';
 import 'package:analyzer/dart/element/element.dart';
 import 'package:analyzer/dart/element/type.dart';
 import 'package:analyzer/diagnostic/diagnostic.dart';
@@ -24,16 +25,21 @@
         _errorReporter = errorReporter;
 
   /// Check to ensure the subelement of a base or final element must be base,
-  /// final, or sealed. Otherwise, an error is reported on that element.
+  /// final, or sealed and that base elements are not implemented outside of its
+  /// library. Otherwise, an error is reported on that element.
   ///
   /// See [CompileTimeErrorCode.SUBTYPE_OF_BASE_IS_NOT_BASE_FINAL_OR_SEALED],
-  /// [CompileTimeErrorCode.SUBTYPE_OF_FINAL_IS_NOT_BASE_FINAL_OR_SEALED]
-  void checkElement(ClassOrMixinElementImpl element) {
+  /// [CompileTimeErrorCode.SUBTYPE_OF_FINAL_IS_NOT_BASE_FINAL_OR_SEALED],
+  /// [CompileTimeErrorCode.BASE_CLASS_IMPLEMENTED_OUTSIDE_OF_LIBRARY].
+  void checkElement(
+      ClassOrMixinElementImpl element, ImplementsClause? implementsClause) {
     final supertype = element.supertype;
     if (supertype != null && _checkSupertypes([supertype], element)) {
       return;
     }
-    if (_checkSupertypes(element.interfaces, element)) {
+    if (implementsClause != null &&
+        _checkInterfaceSupertypes(implementsClause.interfaces, element,
+            areImplementedInterfaces: true)) {
       return;
     }
     if (_checkSupertypes(element.mixins, element)) {
@@ -47,6 +53,28 @@
   }
 
   /// Returns true if a 'base' or 'final' subtype modifier error is reported for
+  /// an interface in [interfaces].
+  bool _checkInterfaceSupertypes(
+      List<NamedType> interfaces, ClassOrMixinElementImpl subElement,
+      {bool areImplementedInterfaces = false}) {
+    for (NamedType interface in interfaces) {
+      final interfaceType = interface.type;
+      if (interfaceType is InterfaceType) {
+        final interfaceElement = interfaceType.element;
+        if (interfaceElement is ClassOrMixinElementImpl) {
+          // Return early if an error has been reported to prevent reporting
+          // multiple errors on one element.
+          if (_reportRestrictionError(subElement, interfaceElement,
+              implementsNamedType: interface)) {
+            return true;
+          }
+        }
+      }
+    }
+    return false;
+  }
+
+  /// Returns true if a 'base' or 'final' subtype modifier error is reported for
   /// a supertype in [supertypes].
   bool _checkSupertypes(
       List<InterfaceType> supertypes, ClassOrMixinElementImpl subElement,
@@ -135,7 +163,7 @@
   /// Reports an error based on the modifier of the [superElement].
   bool _reportRestrictionError(
       ClassOrMixinElementImpl element, ClassOrMixinElementImpl superElement,
-      {bool isSuperclassConstraint = false}) {
+      {bool isSuperclassConstraint = false, NamedType? implementsNamedType}) {
     ClassOrMixinElementImpl? baseOrFinalSuperElement;
     if (superElement.isBase || superElement.isFinal) {
       // The 'base' or 'final' modifier may be an induced modifier. Find the
@@ -151,9 +179,6 @@
       return false;
     }
     if (baseOrFinalSuperElement != null &&
-        !element.isBase &&
-        !element.isFinal &&
-        !element.isSealed &&
         !_mayIgnoreClassModifiers(baseOrFinalSuperElement.library)) {
       // The context message links to the explicitly declared 'base' or 'final'
       // super element and is only added onto the error if 'base' or 'final' is
@@ -170,27 +195,44 @@
         )
       ];
 
-      if (baseOrFinalSuperElement.isFinal) {
-        if (!isSuperclassConstraint &&
-            baseOrFinalSuperElement.library != element.library) {
-          // If you can't extend, implement or mix in a final element outside of
-          // its library anyways, it's not helpful to report a subelement
-          // modifier error.
-          return false;
+      // It's an error to implement a class if it has a supertype from a
+      // different library which is marked base.
+      if (implementsNamedType != null &&
+          superElement.isSealed &&
+          baseOrFinalSuperElement.library != element.library) {
+        if (baseOrFinalSuperElement.isBase) {
+          _errorReporter.reportErrorForNode(
+              CompileTimeErrorCode.BASE_CLASS_IMPLEMENTED_OUTSIDE_OF_LIBRARY,
+              implementsNamedType,
+              [baseOrFinalSuperElement.displayName],
+              contextMessage);
+          return true;
         }
-        _errorReporter.reportErrorForElement(
-            CompileTimeErrorCode.SUBTYPE_OF_FINAL_IS_NOT_BASE_FINAL_OR_SEALED,
-            element,
-            [element.displayName, baseOrFinalSuperElement.displayName],
-            superElement.isSealed ? contextMessage : null);
-        return true;
-      } else if (baseOrFinalSuperElement.isBase) {
-        _errorReporter.reportErrorForElement(
-            CompileTimeErrorCode.SUBTYPE_OF_BASE_IS_NOT_BASE_FINAL_OR_SEALED,
-            element,
-            [element.displayName, baseOrFinalSuperElement.displayName],
-            superElement.isSealed ? contextMessage : null);
-        return true;
+      }
+
+      if (!element.isBase && !element.isFinal && !element.isSealed) {
+        if (baseOrFinalSuperElement.isFinal) {
+          if (!isSuperclassConstraint &&
+              baseOrFinalSuperElement.library != element.library) {
+            // If you can't extend, implement or mix in a final element outside of
+            // its library anyways, it's not helpful to report a subelement
+            // modifier error.
+            return false;
+          }
+          _errorReporter.reportErrorForElement(
+              CompileTimeErrorCode.SUBTYPE_OF_FINAL_IS_NOT_BASE_FINAL_OR_SEALED,
+              element,
+              [element.displayName, baseOrFinalSuperElement.displayName],
+              superElement.isSealed ? contextMessage : null);
+          return true;
+        } else if (baseOrFinalSuperElement.isBase) {
+          _errorReporter.reportErrorForElement(
+              CompileTimeErrorCode.SUBTYPE_OF_BASE_IS_NOT_BASE_FINAL_OR_SEALED,
+              element,
+              [element.displayName, baseOrFinalSuperElement.displayName],
+              superElement.isSealed ? contextMessage : null);
+          return true;
+        }
       }
     }
 
diff --git a/pkg/analyzer/lib/src/generated/resolver.dart b/pkg/analyzer/lib/src/generated/resolver.dart
index 680039c..4f1ffad 100644
--- a/pkg/analyzer/lib/src/generated/resolver.dart
+++ b/pkg/analyzer/lib/src/generated/resolver.dart
@@ -2059,8 +2059,8 @@
       enclosingClass = outerType;
     }
 
-    baseOrFinalTypeVerifier
-        .checkElement(node.declaredElement as ClassOrMixinElementImpl);
+    baseOrFinalTypeVerifier.checkElement(
+        node.declaredElement as ClassOrMixinElementImpl, node.implementsClause);
   }
 
   @override
@@ -2068,8 +2068,8 @@
     checkUnreachableNode(node);
     node.visitChildren(this);
     elementResolver.visitClassTypeAlias(node);
-    baseOrFinalTypeVerifier
-        .checkElement(node.declaredElement as ClassOrMixinElementImpl);
+    baseOrFinalTypeVerifier.checkElement(
+        node.declaredElement as ClassOrMixinElementImpl, node.implementsClause);
   }
 
   @override
@@ -2989,8 +2989,8 @@
       enclosingClass = outerType;
     }
 
-    baseOrFinalTypeVerifier
-        .checkElement(node.declaredElement as ClassOrMixinElementImpl);
+    baseOrFinalTypeVerifier.checkElement(
+        node.declaredElement as ClassOrMixinElementImpl, node.implementsClause);
   }
 
   @override
diff --git a/pkg/analyzer/test/src/diagnostics/base_class_implemented_outside_of_library_test.dart b/pkg/analyzer/test/src/diagnostics/base_class_implemented_outside_of_library_test.dart
index 615b25b..4dbd49d 100644
--- a/pkg/analyzer/test/src/diagnostics/base_class_implemented_outside_of_library_test.dart
+++ b/pkg/analyzer/test/src/diagnostics/base_class_implemented_outside_of_library_test.dart
@@ -5,6 +5,7 @@
 import 'package:analyzer/src/error/codes.dart';
 import 'package:test_reflective_loader/test_reflective_loader.dart';
 
+import '../../generated/test_support.dart';
 import '../dart/resolution/context_collection_resolution.dart';
 
 main() {
@@ -37,6 +38,58 @@
     ]);
   }
 
+  test_class_outside_sealed() async {
+    final a = newFile('$testPackageLibPath/a.dart', r'''
+base class A {}
+''');
+
+    await assertErrorsInCode(r'''
+import 'a.dart';
+sealed class B extends A {}
+base class C implements B {}
+''', [
+      error(
+        CompileTimeErrorCode.BASE_CLASS_IMPLEMENTED_OUTSIDE_OF_LIBRARY,
+        69,
+        1,
+        text:
+            "The class 'A' can't be implemented outside of its library because it's a base class.",
+        contextMessages: [
+          ExpectedContextMessage(a.path, 11, 1,
+              text:
+                  "The type 'B' is a subtype of 'A', and 'A' is defined here.")
+        ],
+      ),
+    ]);
+  }
+
+  test_class_outside_sealed_noBase() async {
+    // Instead of emitting [SUBTYPE_OF_BASE_IS_NOT_BASE_FINAL_OR_SEALED], we
+    // tell the user that they can't implement an indirect base supertype.
+    final a = newFile('$testPackageLibPath/a.dart', r'''
+base class A {}
+''');
+
+    await assertErrorsInCode(r'''
+import 'a.dart';
+sealed class B extends A {}
+class C implements B {}
+''', [
+      error(
+        CompileTimeErrorCode.BASE_CLASS_IMPLEMENTED_OUTSIDE_OF_LIBRARY,
+        64,
+        1,
+        text:
+            "The class 'A' can't be implemented outside of its library because it's a base class.",
+        contextMessages: [
+          ExpectedContextMessage(a.path, 11, 1,
+              text:
+                  "The type 'B' is a subtype of 'A', and 'A' is defined here.")
+        ],
+      ),
+    ]);
+  }
+
   test_class_outside_viaTypedef_inside() async {
     newFile('$testPackageLibPath/foo.dart', r'''
 base class Foo {}
@@ -67,6 +120,41 @@
     ]);
   }
 
+  test_classTypeAlias_inside() async {
+    await assertNoErrorsInCode(r'''
+base class A {}
+sealed class B extends A {}
+mixin M {}
+base class C = Object with M implements B;
+''');
+  }
+
+  test_classTypeAlias_outside() async {
+    final a = newFile('$testPackageLibPath/a.dart', r'''
+base class A {}
+''');
+
+    await assertErrorsInCode(r'''
+import 'a.dart';
+sealed class B extends A {}
+mixin M {}
+base class C = Object with M implements B;
+''', [
+      error(
+        CompileTimeErrorCode.BASE_CLASS_IMPLEMENTED_OUTSIDE_OF_LIBRARY,
+        96,
+        1,
+        text:
+            "The class 'A' can't be implemented outside of its library because it's a base class.",
+        contextMessages: [
+          ExpectedContextMessage(a.path, 11, 1,
+              text:
+                  "The type 'B' is a subtype of 'A', and 'A' is defined here.")
+        ],
+      ),
+    ]);
+  }
+
   test_enum_inside() async {
     await assertNoErrorsInCode(r'''
 base class Foo {}
diff --git a/tests/language/class_modifiers/base_transitivity/base_class_different_library_error_test.dart b/tests/language/class_modifiers/base_transitivity/base_class_different_library_error_test.dart
index 9360566..f3ef842 100644
--- a/tests/language/class_modifiers/base_transitivity/base_class_different_library_error_test.dart
+++ b/tests/language/class_modifiers/base_transitivity/base_class_different_library_error_test.dart
@@ -40,48 +40,52 @@
 
 // Implementing through a sealed class.
 class SimpleSealedExtendImplement implements SealedExtend {}
-//    ^^^^^^^^^^^^^^^^^^^^^^^^^^^
-// [analyzer] COMPILE_TIME_ERROR.SUBTYPE_OF_BASE_OR_FINAL_IS_NOT_BASE_FINAL_OR_SEALED
+//    ^
 // [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
 
 base class BaseSealedExtendImplement implements SealedExtend {}
-//    ^^^^^^^^^^^^^^^^^^^^^^^^^^^
-// [analyzer] unspecified
+//                                              ^^^^^^^^^^^^
+// [analyzer] COMPILE_TIME_ERROR.INVALID_USE_OF_TYPE_OUTSIDE_LIBRARY
 // [cfe] unspecified
 
 interface class InterfaceSealedExtendImplement implements SealedExtend {}
-//              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
-// [analyzer] COMPILE_TIME_ERROR.SUBTYPE_OF_BASE_OR_FINAL_IS_NOT_BASE_FINAL_OR_SEALED
+//              ^
 // [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
 
 final class FinalSealedExtendImplement implements SealedExtend {}
-//    ^^^^^^^^^^^^^^^^^^^^^^^^^^^
-// [analyzer] unspecified
+//                                                ^^^^^^^^^^^^
+// [analyzer] COMPILE_TIME_ERROR.INVALID_USE_OF_TYPE_OUTSIDE_LIBRARY
 // [cfe] unspecified
 
 sealed class SealedSealedExtendImplement implements SealedExtend {}
-//    ^^^^^^^^^^^^^^^^^^^^^^^^^^^
-// [analyzer] unspecified
+//                                                  ^^^^^^^^^^^^
+// [analyzer] COMPILE_TIME_ERROR.INVALID_USE_OF_TYPE_OUTSIDE_LIBRARY
 // [cfe] unspecified
 
 mixin class MixinClassSealedExtendImplement implements SealedExtend {}
-//          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
-// [analyzer] COMPILE_TIME_ERROR.SUBTYPE_OF_BASE_OR_FINAL_IS_NOT_BASE_FINAL_OR_SEALED
+//          ^
 // [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
 
 base mixin class BaseMixinClassSealedExtendImplement implements SealedExtend {}
-//    ^^^^^^^^^^^^^^^^^^^^^^^^^^^
-// [analyzer] unspecified
+//                                                              ^^^^^^^^^^^^
+// [analyzer] COMPILE_TIME_ERROR.INVALID_USE_OF_TYPE_OUTSIDE_LIBRARY
 // [cfe] unspecified
 
 mixin MixinSealedExtendImplement implements SealedExtend {}
-//    ^^^^^^^^^^^^^^^^^^^^^^^^^^
-// [analyzer] COMPILE_TIME_ERROR.SUBTYPE_OF_BASE_OR_FINAL_IS_NOT_BASE_FINAL_OR_SEALED
+//    ^
 // [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
 
 base mixin BaseMixinSealedExtendImplement implements SealedExtend {}
-//    ^^^^^^^^^^^^^^^^^^^^^^^^^^^
-// [analyzer] unspecified
+//                                                   ^^^^^^^^^^^^
+// [analyzer] COMPILE_TIME_ERROR.INVALID_USE_OF_TYPE_OUTSIDE_LIBRARY
 // [cfe] unspecified
 
 // Using a sealed class as an `on` type