[FIR] Relax MULTIPLE_DEFAULTS_* to account for the K1 behavior

^KT-56442
^KT-36188

analysis/analysis-api-fir/src/org/jetbrains/kotlin/analysis/api/fir/diagnostics/KtFirDataClassConverters.kt

@@ -2602,6 +2602,34 @@ internal val KT_DIAGNOSTIC_CONVERTER = KtDiagnosticConverterBuilder.buildConvert
             token,
         )
     }
+    add(FirErrors.MULTIPLE_DEFAULTS_INHERITED_FROM_SUPERTYPES_DEPRECATION.errorFactory) { firDiagnostic ->
+        MultipleDefaultsInheritedFromSupertypesDeprecationErrorImpl(
+            firSymbolBuilder.buildSymbol(firDiagnostic.a),
+            firDiagnostic as KtPsiDiagnostic,
+            token,
+        )
+    }
+    add(FirErrors.MULTIPLE_DEFAULTS_INHERITED_FROM_SUPERTYPES_DEPRECATION.warningFactory) { firDiagnostic ->
+        MultipleDefaultsInheritedFromSupertypesDeprecationWarningImpl(
+            firSymbolBuilder.buildSymbol(firDiagnostic.a),
+            firDiagnostic as KtPsiDiagnostic,
+            token,
+        )
+    }
+    add(FirErrors.MULTIPLE_DEFAULTS_INHERITED_FROM_SUPERTYPES_WHEN_NO_EXPLICIT_OVERRIDE_DEPRECATION.errorFactory) { firDiagnostic ->
+        MultipleDefaultsInheritedFromSupertypesWhenNoExplicitOverrideDeprecationErrorImpl(
+            firSymbolBuilder.buildSymbol(firDiagnostic.a),
+            firDiagnostic as KtPsiDiagnostic,
+            token,
+        )
+    }
+    add(FirErrors.MULTIPLE_DEFAULTS_INHERITED_FROM_SUPERTYPES_WHEN_NO_EXPLICIT_OVERRIDE_DEPRECATION.warningFactory) { firDiagnostic ->
+        MultipleDefaultsInheritedFromSupertypesWhenNoExplicitOverrideDeprecationWarningImpl(
+            firSymbolBuilder.buildSymbol(firDiagnostic.a),
+            firDiagnostic as KtPsiDiagnostic,
+            token,
+        )
+    }
     add(FirErrors.TYPEALIAS_EXPANDS_TO_ARRAY_OF_NOTHINGS) { firDiagnostic ->
         TypealiasExpandsToArrayOfNothingsImpl(
             firSymbolBuilder.typeBuilder.buildKtType(firDiagnostic.a),

analysis/analysis-api-fir/src/org/jetbrains/kotlin/analysis/api/fir/diagnostics/KtFirDiagnostics.kt

@@ -1849,6 +1849,26 @@ sealed interface KtFirDiagnostic<PSI : PsiElement> : KtDiagnosticWithPsi<PSI> {
         val valueParameter: KtSymbol
     }
 
+    interface MultipleDefaultsInheritedFromSupertypesDeprecationError : KtFirDiagnostic<KtElement> {
+        override val diagnosticClass get() = MultipleDefaultsInheritedFromSupertypesDeprecationError::class
+        val valueParameter: KtSymbol
+    }
+
+    interface MultipleDefaultsInheritedFromSupertypesDeprecationWarning : KtFirDiagnostic<KtElement> {
+        override val diagnosticClass get() = MultipleDefaultsInheritedFromSupertypesDeprecationWarning::class
+        val valueParameter: KtSymbol
+    }
+
+    interface MultipleDefaultsInheritedFromSupertypesWhenNoExplicitOverrideDeprecationError : KtFirDiagnostic<KtElement> {
+        override val diagnosticClass get() = MultipleDefaultsInheritedFromSupertypesWhenNoExplicitOverrideDeprecationError::class
+        val valueParameter: KtSymbol
+    }
+
+    interface MultipleDefaultsInheritedFromSupertypesWhenNoExplicitOverrideDeprecationWarning : KtFirDiagnostic<KtElement> {
+        override val diagnosticClass get() = MultipleDefaultsInheritedFromSupertypesWhenNoExplicitOverrideDeprecationWarning::class
+        val valueParameter: KtSymbol
+    }
+
     interface TypealiasExpandsToArrayOfNothings : KtFirDiagnostic<KtElement> {
         override val diagnosticClass get() = TypealiasExpandsToArrayOfNothings::class
         val type: KtType

analysis/analysis-api-fir/src/org/jetbrains/kotlin/analysis/api/fir/diagnostics/KtFirDiagnosticsImpl.kt

@@ -2221,6 +2221,30 @@ internal class MultipleDefaultsInheritedFromSupertypesWhenNoExplicitOverrideImpl
     token: KtLifetimeToken,
 ) : KtAbstractFirDiagnostic<KtElement>(firDiagnostic, token), KtFirDiagnostic.MultipleDefaultsInheritedFromSupertypesWhenNoExplicitOverride
 
+internal class MultipleDefaultsInheritedFromSupertypesDeprecationErrorImpl(
+    override val valueParameter: KtSymbol,
+    firDiagnostic: KtPsiDiagnostic,
+    token: KtLifetimeToken,
+) : KtAbstractFirDiagnostic<KtElement>(firDiagnostic, token), KtFirDiagnostic.MultipleDefaultsInheritedFromSupertypesDeprecationError
+
+internal class MultipleDefaultsInheritedFromSupertypesDeprecationWarningImpl(
+    override val valueParameter: KtSymbol,
+    firDiagnostic: KtPsiDiagnostic,
+    token: KtLifetimeToken,
+) : KtAbstractFirDiagnostic<KtElement>(firDiagnostic, token), KtFirDiagnostic.MultipleDefaultsInheritedFromSupertypesDeprecationWarning
+
+internal class MultipleDefaultsInheritedFromSupertypesWhenNoExplicitOverrideDeprecationErrorImpl(
+    override val valueParameter: KtSymbol,
+    firDiagnostic: KtPsiDiagnostic,
+    token: KtLifetimeToken,
+) : KtAbstractFirDiagnostic<KtElement>(firDiagnostic, token), KtFirDiagnostic.MultipleDefaultsInheritedFromSupertypesWhenNoExplicitOverrideDeprecationError
+
+internal class MultipleDefaultsInheritedFromSupertypesWhenNoExplicitOverrideDeprecationWarningImpl(
+    override val valueParameter: KtSymbol,
+    firDiagnostic: KtPsiDiagnostic,
+    token: KtLifetimeToken,
+) : KtAbstractFirDiagnostic<KtElement>(firDiagnostic, token), KtFirDiagnostic.MultipleDefaultsInheritedFromSupertypesWhenNoExplicitOverrideDeprecationWarning
+
 internal class TypealiasExpandsToArrayOfNothingsImpl(
     override val type: KtType,
     firDiagnostic: KtPsiDiagnostic,

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

@@ -357,6 +357,12 @@ public class DiagnosticCompilerTestFE10TestdataTestGenerated extends AbstractDia
             runTest("compiler/testData/diagnostics/tests/falseNegativeDuplicateDefaultValues1.kt");
         }
 
+        @Test
+        @TestMetadata("falseNegativeDuplicateDefaultValues1_error.kt")
+        public void testFalseNegativeDuplicateDefaultValues1_error() throws Exception {
+            runTest("compiler/testData/diagnostics/tests/falseNegativeDuplicateDefaultValues1_error.kt");
+        }
+
         @Test
         @TestMetadata("falseNegativeDuplicateDefaultValues2.kt")
         public void testFalseNegativeDuplicateDefaultValues2() throws Exception {
@@ -753,12 +759,30 @@ public class DiagnosticCompilerTestFE10TestdataTestGenerated extends AbstractDia
             runTest("compiler/testData/diagnostics/tests/missingIteratorMissing.kt");
         }
 
+        @Test
+        @TestMetadata("missingMultipleDefaultsOnTransitiveInheritance1.kt")
+        public void testMissingMultipleDefaultsOnTransitiveInheritance1() throws Exception {
+            runTest("compiler/testData/diagnostics/tests/missingMultipleDefaultsOnTransitiveInheritance1.kt");
+        }
+
+        @Test
+        @TestMetadata("missingMultipleDefaultsOnTransitiveInheritance2.kt")
+        public void testMissingMultipleDefaultsOnTransitiveInheritance2() throws Exception {
+            runTest("compiler/testData/diagnostics/tests/missingMultipleDefaultsOnTransitiveInheritance2.kt");
+        }
+
         @Test
         @TestMetadata("missingWrongAnnotationTarget.kt")
         public void testMissingWrongAnnotationTarget() throws Exception {
             runTest("compiler/testData/diagnostics/tests/missingWrongAnnotationTarget.kt");
         }
 
+        @Test
+        @TestMetadata("moreThanOneOverriddenMissingK2.kt")
+        public void testMoreThanOneOverriddenMissingK2() throws Exception {
+            runTest("compiler/testData/diagnostics/tests/moreThanOneOverriddenMissingK2.kt");
+        }
+
         @Test
         @TestMetadata("MultilineStringTemplates.kt")
         public void testMultilineStringTemplates() throws Exception {
@@ -771,6 +795,18 @@ public class DiagnosticCompilerTestFE10TestdataTestGenerated extends AbstractDia
             runTest("compiler/testData/diagnostics/tests/MultipleBounds.kt");
         }
 
+        @Test
+        @TestMetadata("multipleDefaultsIncorrectlyAllowed1.kt")
+        public void testMultipleDefaultsIncorrectlyAllowed1() throws Exception {
+            runTest("compiler/testData/diagnostics/tests/multipleDefaultsIncorrectlyAllowed1.kt");
+        }
+
+        @Test
+        @TestMetadata("multipleDefaultsIncorrectlyAllowed2.kt")
+        public void testMultipleDefaultsIncorrectlyAllowed2() throws Exception {
+            runTest("compiler/testData/diagnostics/tests/multipleDefaultsIncorrectlyAllowed2.kt");
+        }
+
         @Test
         @TestMetadata("multipleDefaultsNoSource.kt")
         public void testMultipleDefaultsNoSource() throws Exception {

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

@@ -357,6 +357,12 @@ public class LLFirPreresolvedReversedDiagnosticCompilerFE10TestDataTestGenerated
             runTest("compiler/testData/diagnostics/tests/falseNegativeDuplicateDefaultValues1.kt");
         }
 
+        @Test
+        @TestMetadata("falseNegativeDuplicateDefaultValues1_error.kt")
+        public void testFalseNegativeDuplicateDefaultValues1_error() throws Exception {
+            runTest("compiler/testData/diagnostics/tests/falseNegativeDuplicateDefaultValues1_error.kt");
+        }
+
         @Test
         @TestMetadata("falseNegativeDuplicateDefaultValues2.kt")
         public void testFalseNegativeDuplicateDefaultValues2() throws Exception {
@@ -753,12 +759,30 @@ public class LLFirPreresolvedReversedDiagnosticCompilerFE10TestDataTestGenerated
             runTest("compiler/testData/diagnostics/tests/missingIteratorMissing.kt");
         }
 
+        @Test
+        @TestMetadata("missingMultipleDefaultsOnTransitiveInheritance1.kt")
+        public void testMissingMultipleDefaultsOnTransitiveInheritance1() throws Exception {
+            runTest("compiler/testData/diagnostics/tests/missingMultipleDefaultsOnTransitiveInheritance1.kt");
+        }
+
+        @Test
+        @TestMetadata("missingMultipleDefaultsOnTransitiveInheritance2.kt")
+        public void testMissingMultipleDefaultsOnTransitiveInheritance2() throws Exception {
+            runTest("compiler/testData/diagnostics/tests/missingMultipleDefaultsOnTransitiveInheritance2.kt");
+        }
+
         @Test
         @TestMetadata("missingWrongAnnotationTarget.kt")
         public void testMissingWrongAnnotationTarget() throws Exception {
             runTest("compiler/testData/diagnostics/tests/missingWrongAnnotationTarget.kt");
         }
 
+        @Test
+        @TestMetadata("moreThanOneOverriddenMissingK2.kt")
+        public void testMoreThanOneOverriddenMissingK2() throws Exception {
+            runTest("compiler/testData/diagnostics/tests/moreThanOneOverriddenMissingK2.kt");
+        }
+
         @Test
         @TestMetadata("MultilineStringTemplates.kt")
         public void testMultilineStringTemplates() throws Exception {
@@ -771,6 +795,18 @@ public class LLFirPreresolvedReversedDiagnosticCompilerFE10TestDataTestGenerated
             runTest("compiler/testData/diagnostics/tests/MultipleBounds.kt");
         }
 
+        @Test
+        @TestMetadata("multipleDefaultsIncorrectlyAllowed1.kt")
+        public void testMultipleDefaultsIncorrectlyAllowed1() throws Exception {
+            runTest("compiler/testData/diagnostics/tests/multipleDefaultsIncorrectlyAllowed1.kt");
+        }
+
+        @Test
+        @TestMetadata("multipleDefaultsIncorrectlyAllowed2.kt")
+        public void testMultipleDefaultsIncorrectlyAllowed2() throws Exception {
+            runTest("compiler/testData/diagnostics/tests/multipleDefaultsIncorrectlyAllowed2.kt");
+        }
+
         @Test
         @TestMetadata("multipleDefaultsNoSource.kt")
         public void testMultipleDefaultsNoSource() throws Exception {

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

@@ -357,6 +357,12 @@ public class FirLightTreeOldFrontendDiagnosticsTestGenerated extends AbstractFir
             runTest("compiler/testData/diagnostics/tests/falseNegativeDuplicateDefaultValues1.kt");
         }
 
+        @Test
+        @TestMetadata("falseNegativeDuplicateDefaultValues1_error.kt")
+        public void testFalseNegativeDuplicateDefaultValues1_error() throws Exception {
+            runTest("compiler/testData/diagnostics/tests/falseNegativeDuplicateDefaultValues1_error.kt");
+        }
+
         @Test
         @TestMetadata("falseNegativeDuplicateDefaultValues2.kt")
         public void testFalseNegativeDuplicateDefaultValues2() throws Exception {
@@ -753,12 +759,30 @@ public class FirLightTreeOldFrontendDiagnosticsTestGenerated extends AbstractFir
             runTest("compiler/testData/diagnostics/tests/missingIteratorMissing.kt");
         }
 
+        @Test
+        @TestMetadata("missingMultipleDefaultsOnTransitiveInheritance1.kt")
+        public void testMissingMultipleDefaultsOnTransitiveInheritance1() throws Exception {
+            runTest("compiler/testData/diagnostics/tests/missingMultipleDefaultsOnTransitiveInheritance1.kt");
+        }
+
+        @Test
+        @TestMetadata("missingMultipleDefaultsOnTransitiveInheritance2.kt")
+        public void testMissingMultipleDefaultsOnTransitiveInheritance2() throws Exception {
+            runTest("compiler/testData/diagnostics/tests/missingMultipleDefaultsOnTransitiveInheritance2.kt");
+        }
+
         @Test
         @TestMetadata("missingWrongAnnotationTarget.kt")
         public void testMissingWrongAnnotationTarget() throws Exception {
             runTest("compiler/testData/diagnostics/tests/missingWrongAnnotationTarget.kt");
         }
 
+        @Test
+        @TestMetadata("moreThanOneOverriddenMissingK2.kt")
+        public void testMoreThanOneOverriddenMissingK2() throws Exception {
+            runTest("compiler/testData/diagnostics/tests/moreThanOneOverriddenMissingK2.kt");
+        }
+
         @Test
         @TestMetadata("MultilineStringTemplates.kt")
         public void testMultilineStringTemplates() throws Exception {
@@ -771,6 +795,18 @@ public class FirLightTreeOldFrontendDiagnosticsTestGenerated extends AbstractFir
             runTest("compiler/testData/diagnostics/tests/MultipleBounds.kt");
         }
 
+        @Test
+        @TestMetadata("multipleDefaultsIncorrectlyAllowed1.kt")
+        public void testMultipleDefaultsIncorrectlyAllowed1() throws Exception {
+            runTest("compiler/testData/diagnostics/tests/multipleDefaultsIncorrectlyAllowed1.kt");
+        }
+
+        @Test
+        @TestMetadata("multipleDefaultsIncorrectlyAllowed2.kt")
+        public void testMultipleDefaultsIncorrectlyAllowed2() throws Exception {
+            runTest("compiler/testData/diagnostics/tests/multipleDefaultsIncorrectlyAllowed2.kt");
+        }
+
         @Test
         @TestMetadata("multipleDefaultsNoSource.kt")
         public void testMultipleDefaultsNoSource() throws Exception {

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

@@ -357,6 +357,12 @@ public class FirPsiOldFrontendDiagnosticsTestGenerated extends AbstractFirPsiDia
             runTest("compiler/testData/diagnostics/tests/falseNegativeDuplicateDefaultValues1.kt");
         }
 
+        @Test
+        @TestMetadata("falseNegativeDuplicateDefaultValues1_error.kt")
+        public void testFalseNegativeDuplicateDefaultValues1_error() throws Exception {
+            runTest("compiler/testData/diagnostics/tests/falseNegativeDuplicateDefaultValues1_error.kt");
+        }
+
         @Test
         @TestMetadata("falseNegativeDuplicateDefaultValues2.kt")
         public void testFalseNegativeDuplicateDefaultValues2() throws Exception {
@@ -753,12 +759,30 @@ public class FirPsiOldFrontendDiagnosticsTestGenerated extends AbstractFirPsiDia
             runTest("compiler/testData/diagnostics/tests/missingIteratorMissing.kt");
         }
 
+        @Test
+        @TestMetadata("missingMultipleDefaultsOnTransitiveInheritance1.kt")
+        public void testMissingMultipleDefaultsOnTransitiveInheritance1() throws Exception {
+            runTest("compiler/testData/diagnostics/tests/missingMultipleDefaultsOnTransitiveInheritance1.kt");
+        }
+
+        @Test
+        @TestMetadata("missingMultipleDefaultsOnTransitiveInheritance2.kt")
+        public void testMissingMultipleDefaultsOnTransitiveInheritance2() throws Exception {
+            runTest("compiler/testData/diagnostics/tests/missingMultipleDefaultsOnTransitiveInheritance2.kt");
+        }
+
         @Test
         @TestMetadata("missingWrongAnnotationTarget.kt")
         public void testMissingWrongAnnotationTarget() throws Exception {
             runTest("compiler/testData/diagnostics/tests/missingWrongAnnotationTarget.kt");
         }
 
+        @Test
+        @TestMetadata("moreThanOneOverriddenMissingK2.kt")
+        public void testMoreThanOneOverriddenMissingK2() throws Exception {
+            runTest("compiler/testData/diagnostics/tests/moreThanOneOverriddenMissingK2.kt");
+        }
+
         @Test
         @TestMetadata("MultilineStringTemplates.kt")
         public void testMultilineStringTemplates() throws Exception {
@@ -771,6 +795,18 @@ public class FirPsiOldFrontendDiagnosticsTestGenerated extends AbstractFirPsiDia
             runTest("compiler/testData/diagnostics/tests/MultipleBounds.kt");
         }
 
+        @Test
+        @TestMetadata("multipleDefaultsIncorrectlyAllowed1.kt")
+        public void testMultipleDefaultsIncorrectlyAllowed1() throws Exception {
+            runTest("compiler/testData/diagnostics/tests/multipleDefaultsIncorrectlyAllowed1.kt");
+        }
+
+        @Test
+        @TestMetadata("multipleDefaultsIncorrectlyAllowed2.kt")
+        public void testMultipleDefaultsIncorrectlyAllowed2() throws Exception {
+            runTest("compiler/testData/diagnostics/tests/multipleDefaultsIncorrectlyAllowed2.kt");
+        }
+
         @Test
         @TestMetadata("multipleDefaultsNoSource.kt")
         public void testMultipleDefaultsNoSource() throws Exception {

compiler/fir/checkers/checkers-component-generator/src/org/jetbrains/kotlin/fir/checkers/generator/diagnostics/FirDiagnosticsList.kt

@@ -888,6 +888,18 @@ object DIAGNOSTICS_LIST : DiagnosticList("FirErrors") {
         val MULTIPLE_DEFAULTS_INHERITED_FROM_SUPERTYPES_WHEN_NO_EXPLICIT_OVERRIDE by error<KtElement>(PositioningStrategy.DECLARATION_NAME) {
             parameter<FirValueParameterSymbol>("valueParameter")
         }
+        val MULTIPLE_DEFAULTS_INHERITED_FROM_SUPERTYPES_DEPRECATION by deprecationError<KtElement>(
+            LanguageFeature.ProhibitAllMultipleDefaultsInheritedFromSupertypes,
+            PositioningStrategy.DECLARATION_SIGNATURE_OR_DEFAULT
+        ) {
+            parameter<FirValueParameterSymbol>("valueParameter")
+        }
+        val MULTIPLE_DEFAULTS_INHERITED_FROM_SUPERTYPES_WHEN_NO_EXPLICIT_OVERRIDE_DEPRECATION by deprecationError<KtElement>(
+            LanguageFeature.ProhibitAllMultipleDefaultsInheritedFromSupertypes,
+            PositioningStrategy.DECLARATION_NAME
+        ) {
+            parameter<FirValueParameterSymbol>("valueParameter")
+        }
 
         val TYPEALIAS_EXPANDS_TO_ARRAY_OF_NOTHINGS by error<KtElement> {
             parameter<ConeKotlinType>("type")

compiler/fir/checkers/gen/org/jetbrains/kotlin/fir/analysis/diagnostics/FirErrors.kt

@@ -15,6 +15,7 @@ import org.jetbrains.kotlin.config.LanguageFeature.ForbidExposingTypesInPrimaryC
 import org.jetbrains.kotlin.config.LanguageFeature.ForbidInferringTypeVariablesIntoEmptyIntersection
 import org.jetbrains.kotlin.config.LanguageFeature.ForbidUsingExtensionPropertyTypeParameterInDelegate
 import org.jetbrains.kotlin.config.LanguageFeature.ModifierNonBuiltinSuspendFunError
+import org.jetbrains.kotlin.config.LanguageFeature.ProhibitAllMultipleDefaultsInheritedFromSupertypes
 import org.jetbrains.kotlin.config.LanguageFeature.ProhibitAssigningSingleElementsToVarargsInNamedForm
 import org.jetbrains.kotlin.config.LanguageFeature.ProhibitConfusingSyntaxInWhenBranches
 import org.jetbrains.kotlin.config.LanguageFeature.ProhibitCyclesInAnnotations
@@ -491,6 +492,8 @@ object FirErrors {
     val CANNOT_CHANGE_ACCESS_PRIVILEGE by error3<KtModifierListOwner, Visibility, FirCallableSymbol<*>, Name>(SourceElementPositioningStrategies.VISIBILITY_MODIFIER)
     val MULTIPLE_DEFAULTS_INHERITED_FROM_SUPERTYPES by error1<KtElement, FirValueParameterSymbol>(SourceElementPositioningStrategies.DECLARATION_SIGNATURE_OR_DEFAULT)
     val MULTIPLE_DEFAULTS_INHERITED_FROM_SUPERTYPES_WHEN_NO_EXPLICIT_OVERRIDE by error1<KtElement, FirValueParameterSymbol>(SourceElementPositioningStrategies.DECLARATION_NAME)
+    val MULTIPLE_DEFAULTS_INHERITED_FROM_SUPERTYPES_DEPRECATION by deprecationError1<KtElement, FirValueParameterSymbol>(ProhibitAllMultipleDefaultsInheritedFromSupertypes, SourceElementPositioningStrategies.DECLARATION_SIGNATURE_OR_DEFAULT)
+    val MULTIPLE_DEFAULTS_INHERITED_FROM_SUPERTYPES_WHEN_NO_EXPLICIT_OVERRIDE_DEPRECATION by deprecationError1<KtElement, FirValueParameterSymbol>(ProhibitAllMultipleDefaultsInheritedFromSupertypes, SourceElementPositioningStrategies.DECLARATION_NAME)
     val TYPEALIAS_EXPANDS_TO_ARRAY_OF_NOTHINGS by error1<KtElement, ConeKotlinType>()
     val OVERRIDING_FINAL_MEMBER by error2<KtNamedDeclaration, FirCallableSymbol<*>, Name>(SourceElementPositioningStrategies.OVERRIDE_MODIFIER)
     val RETURN_TYPE_MISMATCH_ON_INHERITANCE by error2<KtClassOrObject, FirCallableSymbol<*>, FirCallableSymbol<*>>(SourceElementPositioningStrategies.DECLARATION_NAME)

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

@@ -5,6 +5,7 @@
 
 package org.jetbrains.kotlin.fir.analysis.checkers.declaration
 
+import org.jetbrains.kotlin.KtSourceElement
 import org.jetbrains.kotlin.diagnostics.DiagnosticReporter
 import org.jetbrains.kotlin.diagnostics.reportOn
 import org.jetbrains.kotlin.fir.analysis.checkers.collectOverriddenFunctionsWhere
@@ -18,6 +19,7 @@ import org.jetbrains.kotlin.fir.declarations.utils.superConeTypes
 import org.jetbrains.kotlin.fir.isSubstitutionOverride
 import org.jetbrains.kotlin.fir.scopes.processAllFunctions
 import org.jetbrains.kotlin.fir.symbols.impl.FirNamedFunctionSymbol
+import org.jetbrains.kotlin.fir.symbols.impl.FirValueParameterSymbol
 
 object FirMultipleDefaultsInheritedFromSupertypesChecker : FirRegularClassChecker() {
     override fun check(declaration: FirRegularClass, context: CheckerContext, reporter: DiagnosticReporter) {
@@ -42,26 +44,59 @@ object FirMultipleDefaultsInheritedFromSupertypesChecker : FirRegularClassChecke
         }
         val isExplicitOverride = function.origin == FirDeclarationOrigin.Source
 
+        val immediateSupertypes = declaration.superConeTypes.mapTo(mutableSetOf()) { it.lookupTag }
+        val overriddenFunctionsK1WouldConsider = overriddenFunctions.filter { it.containingClassLookupTag() in immediateSupertypes }
+
         for ((index, parameter) in function.valueParameterSymbols.withIndex()) {
             val basesWithDefaultValues = overriddenFunctions.count { it.valueParameterSymbols[index].hasDefaultValue }
 
+            if (basesWithDefaultValues <= 1) {
+                continue
+            }
+
+            val k1WouldMiss = overriddenFunctionsK1WouldConsider.count { it.valueParameterSymbols[index].hasDefaultValue } <= 1
+
             when {
-                basesWithDefaultValues <= 1 -> {
-                    continue
-                }
                 !isExplicitOverride -> {
-                    reporter.reportOn(
-                        declaration.source, FirErrors.MULTIPLE_DEFAULTS_INHERITED_FROM_SUPERTYPES_WHEN_NO_EXPLICIT_OVERRIDE,
-                        parameter, context,
-                    )
+                    reportDiagnosticForImplicitOverride(k1WouldMiss, declaration.source, parameter, context, reporter)
                     // Avoid duplicates
                     break
                 }
-                else -> reporter.reportOn(
-                    parameter.source, FirErrors.MULTIPLE_DEFAULTS_INHERITED_FROM_SUPERTYPES,
-                    parameter, context,
-                )
+                else -> reportDiagnosticForExplicitOverride(k1WouldMiss, parameter, context, reporter)
             }
         }
     }
+
+    private fun reportDiagnosticForImplicitOverride(
+        k1WouldMiss: Boolean,
+        source: KtSourceElement?,
+        parameter: FirValueParameterSymbol,
+        context: CheckerContext,
+        reporter: DiagnosticReporter,
+    ): Unit = when {
+        k1WouldMiss -> reporter.reportOn(
+            source, FirErrors.MULTIPLE_DEFAULTS_INHERITED_FROM_SUPERTYPES_WHEN_NO_EXPLICIT_OVERRIDE_DEPRECATION,
+            parameter, context,
+        )
+        else -> reporter.reportOn(
+            source, FirErrors.MULTIPLE_DEFAULTS_INHERITED_FROM_SUPERTYPES_WHEN_NO_EXPLICIT_OVERRIDE,
+            parameter, context,
+        )
+    }
+
+    private fun reportDiagnosticForExplicitOverride(
+        k1WouldMiss: Boolean,
+        parameter: FirValueParameterSymbol,
+        context: CheckerContext,
+        reporter: DiagnosticReporter,
+    ): Unit = when {
+        k1WouldMiss -> reporter.reportOn(
+            parameter.source, FirErrors.MULTIPLE_DEFAULTS_INHERITED_FROM_SUPERTYPES_DEPRECATION,
+            parameter, context,
+        )
+        else -> reporter.reportOn(
+            parameter.source, FirErrors.MULTIPLE_DEFAULTS_INHERITED_FROM_SUPERTYPES,
+            parameter, context,
+        )
+    }
 }

compiler/fir/checkers/src/org/jetbrains/kotlin/fir/analysis/diagnostics/FirErrorsDefaultMessages.kt

@@ -369,6 +369,8 @@ import org.jetbrains.kotlin.fir.analysis.diagnostics.FirErrors.MODIFIER_FORM_FOR
 import org.jetbrains.kotlin.fir.analysis.diagnostics.FirErrors.MULTIPLE_ARGUMENTS_APPLICABLE_FOR_CONTEXT_RECEIVER
 import org.jetbrains.kotlin.fir.analysis.diagnostics.FirErrors.MULTIPLE_DEFAULTS_INHERITED_FROM_SUPERTYPES
 import org.jetbrains.kotlin.fir.analysis.diagnostics.FirErrors.MULTIPLE_DEFAULTS_INHERITED_FROM_SUPERTYPES_WHEN_NO_EXPLICIT_OVERRIDE
+import org.jetbrains.kotlin.fir.analysis.diagnostics.FirErrors.MULTIPLE_DEFAULTS_INHERITED_FROM_SUPERTYPES_DEPRECATION
+import org.jetbrains.kotlin.fir.analysis.diagnostics.FirErrors.MULTIPLE_DEFAULTS_INHERITED_FROM_SUPERTYPES_WHEN_NO_EXPLICIT_OVERRIDE_DEPRECATION
 import org.jetbrains.kotlin.fir.analysis.diagnostics.FirErrors.MULTIPLE_VARARG_PARAMETERS
 import org.jetbrains.kotlin.fir.analysis.diagnostics.FirErrors.MULTI_FIELD_VALUE_CLASS_PRIMARY_CONSTRUCTOR_DEFAULT_PARAMETER
 import org.jetbrains.kotlin.fir.analysis.diagnostics.FirErrors.MUST_BE_INITIALIZED
@@ -1494,9 +1496,29 @@ object FirErrorsDefaultMessages : BaseDiagnosticRendererFactory() {
             FQ_NAMES_IN_TYPES
         )
 
-        val multipleDefaultsMessage = "More than one overridden descriptor declares a default value for ''{0}''. As the compiler can not make sure these values agree, this is not allowed."
-        map.put(MULTIPLE_DEFAULTS_INHERITED_FROM_SUPERTYPES, multipleDefaultsMessage, SYMBOL)
-        map.put(MULTIPLE_DEFAULTS_INHERITED_FROM_SUPERTYPES_WHEN_NO_EXPLICIT_OVERRIDE, multipleDefaultsMessage, SYMBOL)
+        val multipleDefaultsMessage = "More than one overridden function declares a default value for ''{0}''."
+        val multipleDefaultsNotAllowed = " As the compiler can not make sure these values agree, this is not allowed."
+        map.put(
+            MULTIPLE_DEFAULTS_INHERITED_FROM_SUPERTYPES,
+            multipleDefaultsMessage + multipleDefaultsNotAllowed,
+            SYMBOL
+        )
+        map.put(
+            MULTIPLE_DEFAULTS_INHERITED_FROM_SUPERTYPES_WHEN_NO_EXPLICIT_OVERRIDE,
+            multipleDefaultsMessage + multipleDefaultsNotAllowed,
+            SYMBOL
+        )
+        val multipleDefaultsDiscouraged = " As the compiler can not make sure these values agree, this will be prohibited in the Future."
+        map.put(
+            MULTIPLE_DEFAULTS_INHERITED_FROM_SUPERTYPES_DEPRECATION,
+            multipleDefaultsMessage + multipleDefaultsDiscouraged,
+            SYMBOL
+        )
+        map.put(
+            MULTIPLE_DEFAULTS_INHERITED_FROM_SUPERTYPES_WHEN_NO_EXPLICIT_OVERRIDE_DEPRECATION,
+            multipleDefaultsMessage + multipleDefaultsNotAllowed,
+            SYMBOL
+        )
 
         map.put(TYPEALIAS_EXPANDS_TO_ARRAY_OF_NOTHINGS, "Type alias expanded to malformed type ''{0}''", RENDER_TYPE)
 

compiler/testData/codegen/box/bridges/kt46389.kt

@@ -1,6 +1,4 @@
-// IGNORE_BACKEND_K1: JS, JS_IR, JS_IR_ES6
-// IGNORE_BACKEND_K2: ANY
-// STATUS: This must be red code anyway, KT-36188
+// IGNORE_BACKEND: JS, JS_IR, JS_IR_ES6
 
 // The code in this test should be prohibited in the frontend, see KT-36188.
 

compiler/testData/codegen/box/bridges/kt46389_jvmDefault.kt

@@ -1,6 +1,4 @@
-// IGNORE_BACKEND_K1: JS, JS_IR, JS_IR_ES6
-// IGNORE_BACKEND_K2: ANY
-// STATUS: This must be red code anyway, KT-36188
+// IGNORE_BACKEND: JS, JS_IR, JS_IR_ES6
 // !JVM_DEFAULT_MODE: all
 // JVM_TARGET: 1.8
 

compiler/testData/codegen/box/defaultArguments/function/kt36188.kt

@@ -1,6 +1,4 @@
 // IGNORE_BACKEND: JS
-// IGNORE_BACKEND_K2: ANY
-// STATUS: This must be red code anyway, KT-36188
 
 // Test for KT-36188 bug compatibility between non-IR and IR backends
 

compiler/testData/codegen/box/defaultArguments/function/kt36188_2.kt

@@ -1,6 +1,4 @@
 // IGNORE_BACKEND: JS
-// IGNORE_BACKEND_K2: ANY
-// STATUS: This must be red code anyway, KT-36188
 
 // Test for KT-36188 bug compatibility between non-IR and IR backends
 

compiler/testData/diagnostics/tests/falseNegativeDuplicateDefaultValues1.fir.kt

@@ -11,5 +11,5 @@ open class Keker<P> {
 }
 
 class Implementation<P>() : Keker<P>(), SomeRandomOverride<P> {
-    override fun child(<!MULTIPLE_DEFAULTS_INHERITED_FROM_SUPERTYPES!>props: Int<!>) {}
+    override fun child(<!MULTIPLE_DEFAULTS_INHERITED_FROM_SUPERTYPES_DEPRECATION_WARNING!>props: Int<!>) {}
 }

compiler/testData/diagnostics/tests/falseNegativeDuplicateDefaultValues1_error.fir.kt

@@ -0,0 +1,16 @@
+// LANGUAGE: +ProhibitAllMultipleDefaultsInheritedFromSupertypes
+// ISSUE: KT-36188
+
+interface SomeRandomBase<K> {
+    fun child(props: Int = 20)
+}
+
+interface SomeRandomOverride<J> : SomeRandomBase<J>
+
+open class Keker<P> {
+    open fun child(props: Int = 10) {}
+}
+
+class Implementation<P>() : Keker<P>(), SomeRandomOverride<P> {
+    override fun child(<!MULTIPLE_DEFAULTS_INHERITED_FROM_SUPERTYPES_DEPRECATION_ERROR!>props: Int<!>) {}
+}

compiler/testData/diagnostics/tests/falseNegativeDuplicateDefaultValues1_error.kt

@@ -0,0 +1,16 @@
+// LANGUAGE: +ProhibitAllMultipleDefaultsInheritedFromSupertypes
+// ISSUE: KT-36188
+
+interface SomeRandomBase<K> {
+    fun child(props: Int = 20)
+}
+
+interface SomeRandomOverride<J> : SomeRandomBase<J>
+
+open class Keker<P> {
+    open fun child(props: Int = 10) {}
+}
+
+class Implementation<P>() : Keker<P>(), SomeRandomOverride<P> {
+    override fun child(props: Int) {}
+}

compiler/testData/diagnostics/tests/falseNegativeDuplicateDefaultValues2.fir.kt

@@ -13,5 +13,5 @@ open class Keker<P> {
 }
 
 class Implementation<P>() : Keker<P>(), SomeRandomOverride<P> {
-    override fun child(<!MULTIPLE_DEFAULTS_INHERITED_FROM_SUPERTYPES!>props: Int<!>) {}
+    override fun child(<!MULTIPLE_DEFAULTS_INHERITED_FROM_SUPERTYPES_DEPRECATION_WARNING!>props: Int<!>) {}
 }

compiler/testData/diagnostics/tests/falseNegativeDuplicateDefaultValues3.fir.kt

@@ -13,5 +13,5 @@ open class Keker<P> {
 }
 
 class Implementation<P>() : Keker<P>(), SomeRandomOverride<P> {
-    override fun child(<!MULTIPLE_DEFAULTS_INHERITED_FROM_SUPERTYPES!>props: Int<!>) {}
+    override fun child(<!MULTIPLE_DEFAULTS_INHERITED_FROM_SUPERTYPES_DEPRECATION_WARNING!>props: Int<!>) {}
 }

compiler/testData/diagnostics/tests/missingMultipleDefaultsOnTransitiveInheritance1.fir.kt

@@ -0,0 +1,23 @@
+// ISSUE: KT-60269
+// WITH_STDLIB
+
+interface Foo {
+    fun foo(param: Int = 1)
+}
+
+interface Bar {
+    fun foo(param: Int = 2)
+}
+
+interface Baz1 : Bar
+
+class Baz : Baz1, Foo {
+    override fun foo(<!MULTIPLE_DEFAULTS_INHERITED_FROM_SUPERTYPES_DEPRECATION_WARNING!>param: Int<!>) {
+        println(param)
+    }
+}
+
+fun main() {
+    Baz().foo()
+    (Baz() as Foo).foo()
+}

compiler/testData/diagnostics/tests/missingMultipleDefaultsOnTransitiveInheritance1.kt

@@ -0,0 +1,23 @@
+// ISSUE: KT-60269
+// WITH_STDLIB
+
+interface Foo {
+    fun foo(param: Int = 1)
+}
+
+interface Bar {
+    fun foo(param: Int = 2)
+}
+
+interface Baz1 : Bar
+
+class Baz : Baz1, Foo {
+    override fun foo(param: Int) {
+        println(param)
+    }
+}
+
+fun main() {
+    Baz().foo()
+    (Baz() as Foo).foo()
+}

compiler/testData/diagnostics/tests/missingMultipleDefaultsOnTransitiveInheritance2.kt

@@ -0,0 +1,22 @@
+// FIR_IDENTICAL
+// ISSUE: KT-60269
+// WITH_STDLIB
+
+interface Foo {
+    fun foo(param: Int = 1)
+}
+
+interface Bar {
+    fun foo(param: Int = 2)
+}
+
+class Baz : Bar, Foo {
+    override fun foo(<!MULTIPLE_DEFAULTS_INHERITED_FROM_SUPERTYPES!>param: Int<!>) {
+        println(param)
+    }
+}
+
+fun main() {
+    Baz().foo()
+    (Baz() as Foo).foo()
+}

compiler/testData/diagnostics/tests/moreThanOneOverriddenMissingK2.kt

@@ -0,0 +1,19 @@
+// FIR_IDENTICAL
+// ISSUE: KT-57076
+// WITH_STDLIB
+
+interface I01 {
+    fun some(x: Int = 1, y: Int)
+}
+
+open class C01 {
+    open fun some(x: Int = -1, y: Int = 2) {
+        println("x = $x y = $y")
+    }
+}
+
+<!MULTIPLE_DEFAULTS_INHERITED_FROM_SUPERTYPES_WHEN_NO_EXPLICIT_OVERRIDE!>class C02<!>: C01(), I01
+
+fun main(){
+    C02().some() // K2: x = -1 y = 2
+}

compiler/testData/diagnostics/tests/multipleDefaultsIncorrectlyAllowed1.kt

@@ -0,0 +1,14 @@
+// FIR_IDENTICAL
+// ISSUE: KT-36188
+
+interface A {
+    fun foo(a: String = "Fail"): String
+}
+interface B {
+    fun foo(a: String = "OK"): String
+}
+class Impl : A, B {
+    override fun foo(<!MULTIPLE_DEFAULTS_INHERITED_FROM_SUPERTYPES!>a: String<!>) = a
+}
+
+fun box(): String = Impl().foo()

compiler/testData/diagnostics/tests/multipleDefaultsIncorrectlyAllowed2.fir.kt

@@ -0,0 +1,14 @@
+// ISSUE: KT-36188
+
+interface A {
+    fun foo(a: String = "Fail"): String
+}
+interface A2 : A
+interface B {
+    fun foo(a: String = "OK"): String
+}
+class Impl : A2, B {
+    override fun foo(<!MULTIPLE_DEFAULTS_INHERITED_FROM_SUPERTYPES_DEPRECATION_WARNING!>a: String<!>) = a
+}
+
+fun box(): String = Impl().foo()

compiler/testData/diagnostics/tests/multipleDefaultsIncorrectlyAllowed2.kt

@@ -0,0 +1,14 @@
+// ISSUE: KT-36188
+
+interface A {
+    fun foo(a: String = "Fail"): String
+}
+interface A2 : A
+interface B {
+    fun foo(a: String = "OK"): String
+}
+class Impl : A2, B {
+    override fun foo(a: String) = a
+}
+
+fun box(): String = Impl().foo()

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

@@ -357,6 +357,12 @@ public class DiagnosticTestGenerated extends AbstractDiagnosticTest {
             runTest("compiler/testData/diagnostics/tests/falseNegativeDuplicateDefaultValues1.kt");
         }
 
+        @Test
+        @TestMetadata("falseNegativeDuplicateDefaultValues1_error.kt")
+        public void testFalseNegativeDuplicateDefaultValues1_error() throws Exception {
+            runTest("compiler/testData/diagnostics/tests/falseNegativeDuplicateDefaultValues1_error.kt");
+        }
+
         @Test
         @TestMetadata("falseNegativeDuplicateDefaultValues2.kt")
         public void testFalseNegativeDuplicateDefaultValues2() throws Exception {
@@ -753,12 +759,30 @@ public class DiagnosticTestGenerated extends AbstractDiagnosticTest {
             runTest("compiler/testData/diagnostics/tests/missingIteratorMissing.kt");
         }
 
+        @Test
+        @TestMetadata("missingMultipleDefaultsOnTransitiveInheritance1.kt")
+        public void testMissingMultipleDefaultsOnTransitiveInheritance1() throws Exception {
+            runTest("compiler/testData/diagnostics/tests/missingMultipleDefaultsOnTransitiveInheritance1.kt");
+        }
+
+        @Test
+        @TestMetadata("missingMultipleDefaultsOnTransitiveInheritance2.kt")
+        public void testMissingMultipleDefaultsOnTransitiveInheritance2() throws Exception {
+            runTest("compiler/testData/diagnostics/tests/missingMultipleDefaultsOnTransitiveInheritance2.kt");
+        }
+
         @Test
         @TestMetadata("missingWrongAnnotationTarget.kt")
         public void testMissingWrongAnnotationTarget() throws Exception {
             runTest("compiler/testData/diagnostics/tests/missingWrongAnnotationTarget.kt");
         }
 
+        @Test
+        @TestMetadata("moreThanOneOverriddenMissingK2.kt")
+        public void testMoreThanOneOverriddenMissingK2() throws Exception {
+            runTest("compiler/testData/diagnostics/tests/moreThanOneOverriddenMissingK2.kt");
+        }
+
         @Test
         @TestMetadata("MultilineStringTemplates.kt")
         public void testMultilineStringTemplates() throws Exception {
@@ -771,6 +795,18 @@ public class DiagnosticTestGenerated extends AbstractDiagnosticTest {
             runTest("compiler/testData/diagnostics/tests/MultipleBounds.kt");
         }
 
+        @Test
+        @TestMetadata("multipleDefaultsIncorrectlyAllowed1.kt")
+        public void testMultipleDefaultsIncorrectlyAllowed1() throws Exception {
+            runTest("compiler/testData/diagnostics/tests/multipleDefaultsIncorrectlyAllowed1.kt");
+        }
+
+        @Test
+        @TestMetadata("multipleDefaultsIncorrectlyAllowed2.kt")
+        public void testMultipleDefaultsIncorrectlyAllowed2() throws Exception {
+            runTest("compiler/testData/diagnostics/tests/multipleDefaultsIncorrectlyAllowed2.kt");
+        }
+
         @Test
         @TestMetadata("multipleDefaultsNoSource.kt")
         public void testMultipleDefaultsNoSource() throws Exception {

compiler/util/src/org/jetbrains/kotlin/config/LanguageVersionSettings.kt

@@ -349,6 +349,7 @@ enum class LanguageFeature(
     AllowResultInReturnType(null),
     MultiPlatformProjects(sinceVersion = null),
     ProhibitComparisonOfIncompatibleClasses(sinceVersion = null, kind = BUG_FIX),
+    ProhibitAllMultipleDefaultsInheritedFromSupertypes(sinceVersion = null, kind = BUG_FIX),
     ExplicitBackingFields(sinceVersion = null, kind = UNSTABLE_FEATURE),
     FunctionalTypeWithExtensionAsSupertype(sinceVersion = null),
     JsAllowInvalidCharsIdentifiersEscaping(sinceVersion = null, kind = UNSTABLE_FEATURE),