[analyzer/cfe] Update abstract sealed error message.

State more explicitly that sealed classes are abstract and lead the
user to a better fix with the error message.

Bug: https://github.com/dart-lang/sdk/issues/52073
Change-Id: Id24c6cb187ee5497ca2819f930c48ff5aa8d07fc
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/296025
Reviewed-by: Brian Wilkerson <brianwilkerson@google.com>
Reviewed-by: Paul Berry <paulberry@google.com>
Commit-Queue: Kallen Tu <kallentu@google.com>

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

@@ -151,8 +151,8 @@ const MessageCode messageAbstractSealedClass = const MessageCode(
     "AbstractSealedClass",
     index: 132,
     problemMessage:
-        r"""A class can't be declared both 'sealed' and 'abstract'.""",
-    correctionMessage: r"""Try removing the 'abstract' or 'sealed' keyword.""");
+        r"""A 'sealed' class can't be marked 'abstract' because it's already implicitly abstract.""",
+    correctionMessage: r"""Try removing the 'abstract' keyword.""");
 
 // DO NOT EDIT. THIS FILE IS GENERATED. SEE TOP OF FILE.
 const Code<Null> codeAbstractStaticField = messageAbstractStaticField;

pkg/analyzer/lib/src/dart/error/syntactic_errors.g.dart

@@ -208,8 +208,9 @@ class ParserErrorCode extends ErrorCode {
 
   static const ParserErrorCode ABSTRACT_SEALED_CLASS = ParserErrorCode(
     'ABSTRACT_SEALED_CLASS',
-    "A class can't be declared both 'sealed' and 'abstract'.",
-    correctionMessage: "Try removing the 'abstract' or 'sealed' keyword.",
+    "A 'sealed' class can't be marked 'abstract' because it's already "
+        "implicitly abstract.",
+    correctionMessage: "Try removing the 'abstract' keyword.",
     hasPublishedDocs: true,
   );
 

pkg/analyzer/tool/diagnostics/diagnostics.md

@@ -566,7 +566,8 @@ abstract class C {
 
 ### abstract_sealed_class
 
-_A class can't be declared both 'sealed' and 'abstract'._
+_A 'sealed' class can't be marked 'abstract' because it's already implicitly
+abstract._
 
 #### Description
 

pkg/front_end/messages.yaml

@@ -962,8 +962,8 @@ AbstractLateField:
     - "abstract class C {abstract late var f;}"
 
 AbstractSealedClass:
-  problemMessage: "A class can't be declared both 'sealed' and 'abstract'."
-  correctionMessage: "Try removing the 'abstract' or 'sealed' keyword."
+  problemMessage: "A 'sealed' class can't be marked 'abstract' because it's already implicitly abstract."
+  correctionMessage: "Try removing the 'abstract' keyword."
   analyzerCode: ParserErrorCode.ABSTRACT_SEALED_CLASS
   index: 132
   hasPublishedDocs: true

pkg/front_end/testcases/sealed_class/sealed_abstract_class_declaration.dart.strong.expect

@@ -2,13 +2,13 @@ library /*isNonNullableByDefault*/;
 //
 // Problems in library:
 //
-// pkg/front_end/testcases/sealed_class/sealed_abstract_class_declaration.dart:5:10: Error: A class can't be declared both 'sealed' and 'abstract'.
-// Try removing the 'abstract' or 'sealed' keyword.
+// pkg/front_end/testcases/sealed_class/sealed_abstract_class_declaration.dart:5:10: Error: A 'sealed' class can't be marked 'abstract' because it's already implicitly abstract.
+// Try removing the 'abstract' keyword.
 // abstract sealed class A {}
 //          ^^^^^^
 //
-// pkg/front_end/testcases/sealed_class/sealed_abstract_class_declaration.dart:6:1: Error: A class can't be declared both 'sealed' and 'abstract'.
-// Try removing the 'abstract' or 'sealed' keyword.
+// pkg/front_end/testcases/sealed_class/sealed_abstract_class_declaration.dart:6:1: Error: A 'sealed' class can't be marked 'abstract' because it's already implicitly abstract.
+// Try removing the 'abstract' keyword.
 // sealed abstract class A {}
 // ^^^^^^
 //

pkg/front_end/testcases/sealed_class/sealed_abstract_class_declaration.dart.strong.transformed.expect

@@ -2,13 +2,13 @@ library /*isNonNullableByDefault*/;
 //
 // Problems in library:
 //
-// pkg/front_end/testcases/sealed_class/sealed_abstract_class_declaration.dart:5:10: Error: A class can't be declared both 'sealed' and 'abstract'.
-// Try removing the 'abstract' or 'sealed' keyword.
+// pkg/front_end/testcases/sealed_class/sealed_abstract_class_declaration.dart:5:10: Error: A 'sealed' class can't be marked 'abstract' because it's already implicitly abstract.
+// Try removing the 'abstract' keyword.
 // abstract sealed class A {}
 //          ^^^^^^
 //
-// pkg/front_end/testcases/sealed_class/sealed_abstract_class_declaration.dart:6:1: Error: A class can't be declared both 'sealed' and 'abstract'.
-// Try removing the 'abstract' or 'sealed' keyword.
+// pkg/front_end/testcases/sealed_class/sealed_abstract_class_declaration.dart:6:1: Error: A 'sealed' class can't be marked 'abstract' because it's already implicitly abstract.
+// Try removing the 'abstract' keyword.
 // sealed abstract class A {}
 // ^^^^^^
 //

pkg/front_end/testcases/sealed_class/sealed_abstract_class_declaration.dart.weak.expect

@@ -2,13 +2,13 @@ library /*isNonNullableByDefault*/;
 //
 // Problems in library:
 //
-// pkg/front_end/testcases/sealed_class/sealed_abstract_class_declaration.dart:5:10: Error: A class can't be declared both 'sealed' and 'abstract'.
-// Try removing the 'abstract' or 'sealed' keyword.
+// pkg/front_end/testcases/sealed_class/sealed_abstract_class_declaration.dart:5:10: Error: A 'sealed' class can't be marked 'abstract' because it's already implicitly abstract.
+// Try removing the 'abstract' keyword.
 // abstract sealed class A {}
 //          ^^^^^^
 //
-// pkg/front_end/testcases/sealed_class/sealed_abstract_class_declaration.dart:6:1: Error: A class can't be declared both 'sealed' and 'abstract'.
-// Try removing the 'abstract' or 'sealed' keyword.
+// pkg/front_end/testcases/sealed_class/sealed_abstract_class_declaration.dart:6:1: Error: A 'sealed' class can't be marked 'abstract' because it's already implicitly abstract.
+// Try removing the 'abstract' keyword.
 // sealed abstract class A {}
 // ^^^^^^
 //

pkg/front_end/testcases/sealed_class/sealed_abstract_class_declaration.dart.weak.modular.expect

@@ -2,13 +2,13 @@ library /*isNonNullableByDefault*/;
 //
 // Problems in library:
 //
-// pkg/front_end/testcases/sealed_class/sealed_abstract_class_declaration.dart:5:10: Error: A class can't be declared both 'sealed' and 'abstract'.
-// Try removing the 'abstract' or 'sealed' keyword.
+// pkg/front_end/testcases/sealed_class/sealed_abstract_class_declaration.dart:5:10: Error: A 'sealed' class can't be marked 'abstract' because it's already implicitly abstract.
+// Try removing the 'abstract' keyword.
 // abstract sealed class A {}
 //          ^^^^^^
 //
-// pkg/front_end/testcases/sealed_class/sealed_abstract_class_declaration.dart:6:1: Error: A class can't be declared both 'sealed' and 'abstract'.
-// Try removing the 'abstract' or 'sealed' keyword.
+// pkg/front_end/testcases/sealed_class/sealed_abstract_class_declaration.dart:6:1: Error: A 'sealed' class can't be marked 'abstract' because it's already implicitly abstract.
+// Try removing the 'abstract' keyword.
 // sealed abstract class A {}
 // ^^^^^^
 //

pkg/front_end/testcases/sealed_class/sealed_abstract_class_declaration.dart.weak.outline.expect

@@ -2,13 +2,13 @@ library /*isNonNullableByDefault*/;
 //
 // Problems in library:
 //
-// pkg/front_end/testcases/sealed_class/sealed_abstract_class_declaration.dart:5:10: Error: A class can't be declared both 'sealed' and 'abstract'.
-// Try removing the 'abstract' or 'sealed' keyword.
+// pkg/front_end/testcases/sealed_class/sealed_abstract_class_declaration.dart:5:10: Error: A 'sealed' class can't be marked 'abstract' because it's already implicitly abstract.
+// Try removing the 'abstract' keyword.
 // abstract sealed class A {}
 //          ^^^^^^
 //
-// pkg/front_end/testcases/sealed_class/sealed_abstract_class_declaration.dart:6:1: Error: A class can't be declared both 'sealed' and 'abstract'.
-// Try removing the 'abstract' or 'sealed' keyword.
+// pkg/front_end/testcases/sealed_class/sealed_abstract_class_declaration.dart:6:1: Error: A 'sealed' class can't be marked 'abstract' because it's already implicitly abstract.
+// Try removing the 'abstract' keyword.
 // sealed abstract class A {}
 // ^^^^^^
 //

pkg/front_end/testcases/sealed_class/sealed_abstract_class_declaration.dart.weak.transformed.expect

@@ -2,13 +2,13 @@ library /*isNonNullableByDefault*/;
 //
 // Problems in library:
 //
-// pkg/front_end/testcases/sealed_class/sealed_abstract_class_declaration.dart:5:10: Error: A class can't be declared both 'sealed' and 'abstract'.
-// Try removing the 'abstract' or 'sealed' keyword.
+// pkg/front_end/testcases/sealed_class/sealed_abstract_class_declaration.dart:5:10: Error: A 'sealed' class can't be marked 'abstract' because it's already implicitly abstract.
+// Try removing the 'abstract' keyword.
 // abstract sealed class A {}
 //          ^^^^^^
 //
-// pkg/front_end/testcases/sealed_class/sealed_abstract_class_declaration.dart:6:1: Error: A class can't be declared both 'sealed' and 'abstract'.
-// Try removing the 'abstract' or 'sealed' keyword.
+// pkg/front_end/testcases/sealed_class/sealed_abstract_class_declaration.dart:6:1: Error: A 'sealed' class can't be marked 'abstract' because it's already implicitly abstract.
+// Try removing the 'abstract' keyword.
 // sealed abstract class A {}
 // ^^^^^^
 //

tests/language/sealed_class/sealed_class_syntax_error_test.dart

@@ -42,13 +42,13 @@ abstract class SealedMembers {
 sealed abstract class SealedAndAbstractClass {}
 // [error column 1, length 6]
 // [analyzer] SYNTACTIC_ERROR.ABSTRACT_SEALED_CLASS
-// [cfe] A class can't be declared both 'sealed' and 'abstract'.
+// [cfe] A 'sealed' class can't be marked 'abstract' because it's already implicitly abstract.
 
 
 abstract sealed class SealedAndAbstractClass2 {}
 //       ^^^^^^
 // [analyzer] SYNTACTIC_ERROR.ABSTRACT_SEALED_CLASS
-// [cfe] A class can't be declared both 'sealed' and 'abstract'.
+// [cfe] A 'sealed' class can't be marked 'abstract' because it's already implicitly abstract.
 
 sealed sealed class SealedDuplicateClass {}
 // [error column 1, length 6]