Reject illegal combination of modifiers for sealed types.

* Fixes #2707

org.eclipse.jdt.core.compiler.batch/src/org/eclipse/jdt/core/compiler/IProblem.java

@@ -831,6 +831,8 @@ public interface IProblem {
 	int ShouldImplementHashcode = TypeRelated + 332;
 	/** @since 3.5 */
 	int AbstractMethodsInConcreteClass = TypeRelated + 333;
+	/** @since 3.40 */
+	int IllegalModifierCombinationForType = TypeRelated + 334;
 
 	/** @deprecated - problem is no longer generated, use {@link #UndefinedType} instead */
 	int SuperclassNotFound =  TypeRelated + 329 + ProblemReasons.NotFound; // TypeRelated + 330

org.eclipse.jdt.core.compiler.batch/src/org/eclipse/jdt/internal/compiler/lookup/ClassScope.java

@@ -612,6 +612,13 @@ private void checkAndSetModifiers() {
 		boolean isSealedSupported = JavaFeature.SEALED_CLASSES.isSupported(options);
 		boolean flagSealedNonModifiers = isSealedSupported &&
 				(modifiers & (ExtraCompilerModifiers.AccSealed | ExtraCompilerModifiers.AccNonSealed)) != 0;
+
+		switch (modifiers & (ExtraCompilerModifiers.AccSealed | ExtraCompilerModifiers.AccNonSealed | ClassFileConstants.AccFinal)) {
+			case ExtraCompilerModifiers.AccSealed, ExtraCompilerModifiers.AccNonSealed, ClassFileConstants.AccFinal, ClassFileConstants.AccDefault : break;
+			default :
+				problemReporter().IllegalModifierCombinationForType(sourceType);
+				break;
+		}
 		if (sourceType.isRecord()) {
 			/* JLS 14 Records Sec 8.10 - A record declaration is implicitly final. */
 			modifiers |= ClassFileConstants.AccFinal;
@@ -1192,7 +1199,7 @@ private boolean connectSuperclass() {
 		if (superclass != null) { // is null if a cycle was detected cycle or a problem
 			if (!superclass.isClass() && (superclass.tagBits & TagBits.HasMissingType) == 0) {
 				problemReporter().superclassMustBeAClass(sourceType, superclassRef, superclass);
-			} else if (superclass.isFinal()) {
+			} else if (superclass.isFinal() && !superclass.isSealed()) { // sealed AND final complained about elsewhere, no value in additional complaint here.
 				if (superclass.isRecord()) {
 					problemReporter().classExtendFinalRecord(sourceType, superclassRef, superclass);
 				} else {

org.eclipse.jdt.core.compiler.batch/src/org/eclipse/jdt/internal/compiler/problem/ProblemReporter.java

@@ -3430,6 +3430,15 @@ public void illegalVisibilityModifierCombinationForField(ReferenceBinding type,
 		fieldDecl.sourceStart,
 		fieldDecl.sourceEnd);
 }
+public void IllegalModifierCombinationForType(SourceTypeBinding type) {
+	String[] arguments = new String[] {new String(type.sourceName())};
+	this.handle(
+		IProblem.IllegalModifierCombinationForType,
+		arguments,
+		arguments,
+		type.sourceStart(),
+		type.sourceEnd());
+}
 public void illegalVisibilityModifierCombinationForMemberType(SourceTypeBinding type) {
 	String[] arguments = new String[] {new String(type.sourceName())};
 	this.handle(

org.eclipse.jdt.core.compiler.batch/src/org/eclipse/jdt/internal/compiler/problem/messages.properties

@@ -330,6 +330,7 @@
 331 = Redundant superinterface {0} for the type {1}, already defined by {2}
 332 = The type {0} should also implement hashCode() since it overrides Object.equals()
 333 = The type {0} must be an abstract class to define abstract methods
+334 = The type {0} may have only one modifier out of sealed, non-sealed, and final
 
 ###[obsolete] 330 = {0} cannot be resolved or is not a valid superclass
 ###[obsolete] 331 = Superclass {0} is not visible

org.eclipse.jdt.core.tests.compiler/src/org/eclipse/jdt/core/tests/compiler/regression/CompilerInvocationTests.java

@@ -1364,6 +1364,7 @@ class ProblemAttributes {
 	    expectedProblemAttributes.put("NamedPatternVariablesDisallowedHere", new ProblemAttributes(CategorizedProblem.CAT_INTERNAL));
 	    expectedProblemAttributes.put("OperandStackExceeds64KLimit", new ProblemAttributes(CategorizedProblem.CAT_INTERNAL));
 	    expectedProblemAttributes.put("OperandStackSizeInappropriate", new ProblemAttributes(CategorizedProblem.CAT_INTERNAL));
+	    expectedProblemAttributes.put("IllegalModifierCombinationForType", new ProblemAttributes(CategorizedProblem.CAT_TYPE));
 
 	    StringBuilder failures = new StringBuilder();
 		StringBuilder correctResult = new StringBuilder(70000);
@@ -2497,6 +2498,7 @@ class ProblemAttributes {
 	    expectedProblemAttributes.put("NamedPatternVariablesDisallowedHere", SKIP);
 	    expectedProblemAttributes.put("OperandStackExceeds64KLimit", SKIP);
 	    expectedProblemAttributes.put("OperandStackSizeInappropriate", SKIP);
+	    expectedProblemAttributes.put("IllegalModifierCombinationForType", SKIP);
 
 
 	    Map constantNamesIndex = new HashMap();

org.eclipse.jdt.core.tests.compiler/src/org/eclipse/jdt/core/tests/compiler/regression/SealedTypesTests.java

@@ -1067,11 +1067,16 @@ public void testBug563806_032() {
 			"1. ERROR in p1\\X.java (at line 2)\n" +
 			"	public sealed non-sealed interface X {\n" +
 			"	                                   ^\n" +
-			"Sealed class or interface lacks the permits clause and no class or interface from the same compilation unit declares X as its direct superclass or superinterface\n" +
+			"The type X may have only one modifier out of sealed, non-sealed, and final\n" +
 			"----------\n" +
 			"2. ERROR in p1\\X.java (at line 2)\n" +
 			"	public sealed non-sealed interface X {\n" +
 			"	                                   ^\n" +
+			"Sealed class or interface lacks the permits clause and no class or interface from the same compilation unit declares X as its direct superclass or superinterface\n" +
+			"----------\n" +
+			"3. ERROR in p1\\X.java (at line 2)\n" +
+			"	public sealed non-sealed interface X {\n" +
+			"	                                   ^\n" +
 			"An interface X is declared both sealed and non-sealed\n" +
 			"----------\n");
 	}
@@ -1192,6 +1197,11 @@ public void testBug563806_038() {
 			"1. ERROR in p1\\X.java (at line 4)\n" +
 			"	non-sealed sealed class Y{}\n" +
 			"	                        ^\n" +
+			"The type Y may have only one modifier out of sealed, non-sealed, and final\n" +
+			"----------\n" +
+			"2. ERROR in p1\\X.java (at line 4)\n" +
+			"	non-sealed sealed class Y{}\n" +
+			"	                        ^\n" +
 			"Illegal modifier for the local class Y; only abstract or final is permitted\n" +
 			"----------\n");
 	}
@@ -6322,4 +6332,74 @@ public static void main(String [] args) {
 				},
 				"42");
 	}
+
+	// https://github.com/eclipse-jdt/eclipse.jdt.core/issues/2707
+	// [Sealed types] ECJ allows a class to be declared as both sealed and non-sealed
+	public void testIssue2707() {
+		runNegativeTest(
+				new String[] {
+						"X.java",
+						"""
+						public sealed class X permits Y {
+						}
+
+						sealed non-sealed class Y extends X permits K {}
+
+						final class K extends Y {}
+						"""
+				},
+				"----------\n" +
+				"1. ERROR in X.java (at line 4)\n" +
+				"	sealed non-sealed class Y extends X permits K {}\n" +
+				"	                        ^\n" +
+				"The type Y may have only one modifier out of sealed, non-sealed, and final\n" +
+				"----------\n");
+	}
+
+	// https://github.com/eclipse-jdt/eclipse.jdt.core/issues/2707
+	// [Sealed types] ECJ allows a class to be declared as both sealed and non-sealed
+	public void testIssue2707_2() {
+		runNegativeTest(
+				new String[] {
+						"X.java",
+						"""
+						public sealed class X permits Y {
+						}
+
+						final non-sealed class Y extends X  {}
+						"""
+				},
+				"----------\n" +
+				"1. ERROR in X.java (at line 4)\n" +
+				"	final non-sealed class Y extends X  {}\n" +
+				"	                       ^\n" +
+				"The type Y may have only one modifier out of sealed, non-sealed, and final\n" +
+				"----------\n");
+	}
+
+	// https://github.com/eclipse-jdt/eclipse.jdt.core/issues/2707
+	// [Sealed types] ECJ allows a class to be declared as both sealed and non-sealed
+	public void testIssue2707_3() {
+		runNegativeTest(
+				new String[] {
+						"X.java",
+						"""
+						public final sealed class X permits Y {
+						}
+
+						final non-sealed class Y extends X  {}
+						"""
+				},
+				"----------\n" +
+			    "1. ERROR in X.java (at line 1)\n" +
+			    "	public final sealed class X permits Y {\n" +
+			    "	                          ^\n" +
+			    "The type X may have only one modifier out of sealed, non-sealed, and final\n" +
+			    "----------\n" +
+			    "2. ERROR in X.java (at line 4)\n" +
+			    "	final non-sealed class Y extends X  {}\n" +
+			    "	                       ^\n" +
+			    "The type Y may have only one modifier out of sealed, non-sealed, and final\n" +
+			    "----------\n");
+	}
 }