[FE] Prohibit protected members in final expected or actual classes

^KT-28850 Fixed

analysis/low-level-api-fir/tests/org/jetbrains/kotlin/analysis/low/level/api/fir/diagnostic/compiler/based/DiagnosticCompilerTestFE10TestdataTestGenerated.java

@@ -21232,6 +21232,12 @@ public class DiagnosticCompilerTestFE10TestdataTestGenerated extends AbstractDia
                 runTest("compiler/testData/diagnostics/tests/modifiers/protected.kt");
             }
 
+            @Test
+            @TestMetadata("protectedInExpectActual.kt")
+            public void testProtectedInExpectActual() throws Exception {
+                runTest("compiler/testData/diagnostics/tests/modifiers/protectedInExpectActual.kt");
+            }
+
             @Test
             @TestMetadata("redundantTargets.kt")
             public void testRedundantTargets() throws Exception {

analysis/low-level-api-fir/tests/org/jetbrains/kotlin/analysis/low/level/api/fir/diagnostic/compiler/based/LLFirPreresolvedReversedDiagnosticCompilerFE10TestDataTestGenerated.java

@@ -21232,6 +21232,12 @@ public class LLFirPreresolvedReversedDiagnosticCompilerFE10TestDataTestGenerated
                 runTest("compiler/testData/diagnostics/tests/modifiers/protected.kt");
             }
 
+            @Test
+            @TestMetadata("protectedInExpectActual.kt")
+            public void testProtectedInExpectActual() throws Exception {
+                runTest("compiler/testData/diagnostics/tests/modifiers/protectedInExpectActual.kt");
+            }
+
             @Test
             @TestMetadata("redundantTargets.kt")
             public void testRedundantTargets() throws Exception {

compiler/fir/analysis-tests/tests-gen/org/jetbrains/kotlin/test/runners/FirLightTreeOldFrontendDiagnosticsTestGenerated.java

@@ -21232,6 +21232,12 @@ public class FirLightTreeOldFrontendDiagnosticsTestGenerated extends AbstractFir
                 runTest("compiler/testData/diagnostics/tests/modifiers/protected.kt");
             }
 
+            @Test
+            @TestMetadata("protectedInExpectActual.kt")
+            public void testProtectedInExpectActual() throws Exception {
+                runTest("compiler/testData/diagnostics/tests/modifiers/protectedInExpectActual.kt");
+            }
+
             @Test
             @TestMetadata("redundantTargets.kt")
             public void testRedundantTargets() throws Exception {

compiler/fir/analysis-tests/tests-gen/org/jetbrains/kotlin/test/runners/FirPsiOldFrontendDiagnosticsTestGenerated.java

@@ -21238,6 +21238,12 @@ public class FirPsiOldFrontendDiagnosticsTestGenerated extends AbstractFirPsiDia
                 runTest("compiler/testData/diagnostics/tests/modifiers/protected.kt");
             }
 
+            @Test
+            @TestMetadata("protectedInExpectActual.kt")
+            public void testProtectedInExpectActual() throws Exception {
+                runTest("compiler/testData/diagnostics/tests/modifiers/protectedInExpectActual.kt");
+            }
+
             @Test
             @TestMetadata("redundantTargets.kt")
             public void testRedundantTargets() throws Exception {

compiler/fir/checkers/src/org/jetbrains/kotlin/fir/analysis/checkers/declaration/FirModifierChecker.kt

@@ -22,9 +22,7 @@ import org.jetbrains.kotlin.diagnostics.KtDiagnosticFactory2
 import org.jetbrains.kotlin.diagnostics.reportOn
 import org.jetbrains.kotlin.fir.declarations.*
 import org.jetbrains.kotlin.fir.declarations.impl.FirPrimaryConstructor
-import org.jetbrains.kotlin.fir.declarations.utils.isCompanion
+import org.jetbrains.kotlin.fir.declarations.utils.*
-import org.jetbrains.kotlin.fir.declarations.utils.isInner
-import org.jetbrains.kotlin.fir.declarations.utils.isLocal
 import org.jetbrains.kotlin.fir.languageVersionSettings
 import org.jetbrains.kotlin.lexer.KtModifierKeywordToken
 import org.jetbrains.kotlin.lexer.KtTokens
@@ -85,7 +83,7 @@ object FirModifierChecker : FirBasicDeclarationChecker() {
                 val modifier = secondModifier.token
                 when {
                     !checkTarget(modifierSource, modifier, actualTargets, parent, context, reporter) -> reportedNodes += secondModifier
-                    !checkParent(modifierSource, modifier, actualParents, context, reporter) -> reportedNodes += secondModifier
+                    !checkParent(modifierSource, modifier, actualParents, parent, context, reporter) -> reportedNodes += secondModifier
                 }
             }
         }
@@ -236,6 +234,7 @@ object FirModifierChecker : FirBasicDeclarationChecker() {
         modifierSource: KtSourceElement,
         modifierToken: KtModifierKeywordToken,
         actualParents: List<KotlinTarget>,
+        parent: FirDeclaration?,
         context: CheckerContext,
         reporter: DiagnosticReporter
     ): Boolean {
@@ -251,6 +250,15 @@ object FirModifierChecker : FirBasicDeclarationChecker() {
             return true
         }
 
+        if (modifierToken == KtTokens.PROTECTED_KEYWORD && isFinalExpectOrActualClass(parent)) {
+            reporter.reportOn(
+                modifierSource,
+                FirErrors.WRONG_MODIFIER_CONTAINING_DECLARATION,
+                modifierToken,
+                "final expect or actual class",
+                context,
+            )
+        }
         val possibleParentPredicate = possibleParentTargetPredicateMap[modifierToken] ?: return true
         if (actualParents.any { possibleParentPredicate.isAllowed(it, context.session.languageVersionSettings) }) return true
 
@@ -268,4 +276,8 @@ object FirModifierChecker : FirBasicDeclarationChecker() {
     private fun List<KotlinTarget>.firstOrThis(): String {
         return firstOrNull()?.description ?: "this"
     }
+
+    private fun isFinalExpectOrActualClass(d: FirDeclaration?): Boolean {
+        return d is FirClass && d.isFinal && (d.isExpect || d.isActual)
+    }
 }

compiler/frontend/src/org/jetbrains/kotlin/resolve/ModifiersChecker.kt

@@ -180,6 +180,15 @@ object ModifierCheckerCore {
             )
             return true
         }
+        if (modifier == PROTECTED_KEYWORD && isFinalExpectOrActualClass(parentDescriptor)) {
+            trace.report(
+                Errors.WRONG_MODIFIER_CONTAINING_DECLARATION.on(
+                    node.psi,
+                    modifier,
+                    "final expect or actual class"
+                )
+            )
+        }
         val possibleParentPredicate = possibleParentTargetPredicateMap[modifier] ?: return true
         if (actualParents.any { possibleParentPredicate.isAllowed(it, languageVersionSettings) }) return true
         trace.report(
@@ -237,4 +246,8 @@ object ModifierCheckerCore {
 
         return true
     }
+
+    private fun isFinalExpectOrActualClass(d: DeclarationDescriptor?): Boolean {
+        return d is ClassDescriptor && d.isFinalOrEnum && (d.isExpect || d.isActual)
+    }
 }

compiler/testData/diagnostics/tests/modifiers/protectedInExpectActual.kt

@@ -0,0 +1,48 @@
+// LANGUAGE: +MultiPlatformProjects
+// !DIAGNOSTICS: -NO_ACTUAL_FOR_EXPECT, -ACTUAL_WITHOUT_EXPECT
+// FIR_IDENTICAL
+class SimpleClass {
+    protected fun foo() = Unit
+}
+
+expect class ExpClass {
+    <!WRONG_MODIFIER_CONTAINING_DECLARATION!>protected<!> fun foo()
+
+    <!WRONG_MODIFIER_CONTAINING_DECLARATION!>protected<!> val bar: Int
+}
+
+actual class ActClass {
+    actual <!WRONG_MODIFIER_CONTAINING_DECLARATION!>protected<!> fun foo() = Unit
+
+    actual <!WRONG_MODIFIER_CONTAINING_DECLARATION!>protected<!> val bar: Int = 42
+}
+
+expect open class ExpOpenClass {
+    protected fun foo()
+}
+
+actual open class ActOpenClass {
+    actual protected fun foo() = Unit
+}
+
+enum class SimpleEnum {
+    ENTRY;
+
+    protected fun foo() = Unit
+}
+
+expect enum class ExpEnumClass {
+    ENTRY;
+
+    <!WRONG_MODIFIER_CONTAINING_DECLARATION!>protected<!> fun foo()
+
+    <!WRONG_MODIFIER_CONTAINING_DECLARATION!>protected<!> val bar: Int
+}
+
+actual enum class ActEnumClass {
+    ENTRY;
+
+    actual <!WRONG_MODIFIER_CONTAINING_DECLARATION!>protected<!> fun foo() = Unit
+
+    <!WRONG_MODIFIER_CONTAINING_DECLARATION!>protected<!> val bar: Int = 42
+}

compiler/tests-common-new/tests-gen/org/jetbrains/kotlin/test/runners/DiagnosticTestGenerated.java

@@ -21238,6 +21238,12 @@ public class DiagnosticTestGenerated extends AbstractDiagnosticTest {
                 runTest("compiler/testData/diagnostics/tests/modifiers/protected.kt");
             }
 
+            @Test
+            @TestMetadata("protectedInExpectActual.kt")
+            public void testProtectedInExpectActual() throws Exception {
+                runTest("compiler/testData/diagnostics/tests/modifiers/protectedInExpectActual.kt");
+            }
+
             @Test
             @TestMetadata("redundantTargets.kt")
             public void testRedundantTargets() throws Exception {