[FIR] Bring conflicting overloads errors closer to K1 behavior

#KT-55860 Fixed

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

@@ -23387,6 +23387,12 @@ public class DiagnosticCompilerTestFE10TestdataTestGenerated extends AbstractDia
                     KtTestUtil.assertAllTestsPresentByMetadataWithExcluded(this.getClass(), new File("compiler/testData/diagnostics/tests/override/clashesOnInheritance"), Pattern.compile("^(.+)\\.kt$"), Pattern.compile("^(.+)\\.(reversed|fir|ll)\\.kts?$"), true, "multiplatform");
                 }
 
+                @Test
+                @TestMetadata("conflictingInherited.kt")
+                public void testConflictingInherited() throws Exception {
+                    runTest("compiler/testData/diagnostics/tests/override/clashesOnInheritance/conflictingInherited.kt");
+                }
+
                 @Test
                 @TestMetadata("covariantOverrides.kt")
                 public void testCovariantOverrides() throws Exception {

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

@@ -23387,6 +23387,12 @@ public class LLFirPreresolvedReversedDiagnosticCompilerFE10TestDataTestGenerated
                     KtTestUtil.assertAllTestsPresentByMetadataWithExcluded(this.getClass(), new File("compiler/testData/diagnostics/tests/override/clashesOnInheritance"), Pattern.compile("^(.+)\\.kt$"), Pattern.compile("^(.+)\\.(reversed|fir|ll)\\.kts?$"), true, "multiplatform");
                 }
 
+                @Test
+                @TestMetadata("conflictingInherited.kt")
+                public void testConflictingInherited() throws Exception {
+                    runTest("compiler/testData/diagnostics/tests/override/clashesOnInheritance/conflictingInherited.kt");
+                }
+
                 @Test
                 @TestMetadata("covariantOverrides.kt")
                 public void testCovariantOverrides() throws Exception {

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

@@ -23387,6 +23387,12 @@ public class FirLightTreeOldFrontendDiagnosticsTestGenerated extends AbstractFir
                     KtTestUtil.assertAllTestsPresentByMetadataWithExcluded(this.getClass(), new File("compiler/testData/diagnostics/tests/override/clashesOnInheritance"), Pattern.compile("^(.+)\\.kt$"), Pattern.compile("^(.+)\\.(reversed|fir|ll)\\.kts?$"), true, "multiplatform");
                 }
 
+                @Test
+                @TestMetadata("conflictingInherited.kt")
+                public void testConflictingInherited() throws Exception {
+                    runTest("compiler/testData/diagnostics/tests/override/clashesOnInheritance/conflictingInherited.kt");
+                }
+
                 @Test
                 @TestMetadata("covariantOverrides.kt")
                 public void testCovariantOverrides() throws Exception {

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

@@ -23393,6 +23393,12 @@ public class FirPsiOldFrontendDiagnosticsTestGenerated extends AbstractFirPsiDia
                     KtTestUtil.assertAllTestsPresentByMetadataWithExcluded(this.getClass(), new File("compiler/testData/diagnostics/tests/override/clashesOnInheritance"), Pattern.compile("^(.*)\\.kts?$"), Pattern.compile("^(.+)\\.(reversed|fir|ll)\\.kts?$"), true, "multiplatform");
                 }
 
+                @Test
+                @TestMetadata("conflictingInherited.kt")
+                public void testConflictingInherited() throws Exception {
+                    runTest("compiler/testData/diagnostics/tests/override/clashesOnInheritance/conflictingInherited.kt");
+                }
+
                 @Test
                 @TestMetadata("covariantOverrides.kt")
                 public void testCovariantOverrides() throws Exception {

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

@@ -6,6 +6,7 @@
 package org.jetbrains.kotlin.fir.analysis.checkers.declaration
 
 import org.jetbrains.kotlin.KtFakeSourceElementKind
+import org.jetbrains.kotlin.KtRealSourceElementKind
 import org.jetbrains.kotlin.descriptors.ClassKind
 import org.jetbrains.kotlin.fir.analysis.checkers.context.CheckerContext
 import org.jetbrains.kotlin.fir.analysis.checkers.unsubstitutedScope
@@ -13,6 +14,7 @@ import org.jetbrains.kotlin.diagnostics.DiagnosticReporter
 import org.jetbrains.kotlin.diagnostics.impl.deduplicating
 import org.jetbrains.kotlin.fir.analysis.diagnostics.FirErrors
 import org.jetbrains.kotlin.diagnostics.reportOn
+import org.jetbrains.kotlin.fir.containingClassLookupTag
 import org.jetbrains.kotlin.fir.declarations.FirClass
 import org.jetbrains.kotlin.fir.declarations.FirRegularClass
 import org.jetbrains.kotlin.fir.declarations.utils.classId
@@ -195,16 +197,14 @@ object FirImplementationMismatchChecker : FirClassChecker() {
     ) {
         val allFunctions = mutableListOf<FirNamedFunctionSymbol>()
         scope.processFunctionsByName(name) { sym ->
-            val declaredInThisClass = sym.callableId.classId == containingClass.classId
+            when (sym) {
-            when {
+                is FirIntersectionOverrideFunctionSymbol -> sym.intersections.mapNotNullTo(allFunctions) { it as? FirNamedFunctionSymbol }
-                sym is FirIntersectionOverrideFunctionSymbol && declaredInThisClass ->
+                else -> allFunctions.add(sym)
-                    sym.intersections.mapNotNullTo(allFunctions) { it as? FirNamedFunctionSymbol }
-                !declaredInThisClass -> allFunctions.add(sym)
             }
         }
 
         val sameArgumentGroups = allFunctions.groupBy { function ->
-            buildList<ConeKotlinType> {
+            buildList {
                 addIfNotNull(function.resolvedReceiverTypeRef?.type)
                 function.valueParameterSymbols.mapTo(this) { it.resolvedReturnTypeRef.coneType }
             }
@@ -216,8 +216,30 @@ object FirImplementationMismatchChecker : FirClassChecker() {
             }
         }
 
-        clashes.forEach {
+        for (clash in clashes) {
-            reporter.reportOn(containingClass.source, FirErrors.CONFLICTING_INHERITED_MEMBERS, containingClass.symbol, it.toList(), context)
+            val (first, second) = clash
+
+            val firstClassLookupTag = first.containingClassLookupTag()
+            val secondClassLookupTag = second.containingClassLookupTag()
+
+            if (firstClassLookupTag == secondClassLookupTag) {
+                // Don't report if both declarations came from the same class because either CONFLICTING_OVERLOADS was reported in the
+                // original class or it's ok to keep compatibility with K1. See KT-55860.
+                continue
+            }
+
+            val thisClassLookupTag = containingClass.symbol.toLookupTag()
+
+            // If one of the declarations is from this class, report CONFLICTING_OVERLOADS, otherwise CONFLICTING_INHERITED_MEMBERS
+            if (firstClassLookupTag == thisClassLookupTag && first.source?.kind is KtRealSourceElementKind) {
+                reporter.reportOn(first.source, FirErrors.CONFLICTING_OVERLOADS, clash.toList(), context)
+            } else if (secondClassLookupTag == thisClassLookupTag && second.source?.kind is KtRealSourceElementKind) {
+                reporter.reportOn(second.source, FirErrors.CONFLICTING_OVERLOADS, clash.toList(), context)
+            } else {
+                reporter.reportOn(
+                    containingClass.source, FirErrors.CONFLICTING_INHERITED_MEMBERS, containingClass.symbol, clash.toList(), context,
+                )
+            }
         }
     }
 

compiler/testData/diagnostics/tests/override/ConflictingFunctionSignatureFromSuperclass.fir.kt

@@ -1,7 +0,0 @@
-open class Aaa() {
-    fun foo() = 1
-}
-
-open class Bbb() : Aaa() {
-    fun <T> foo() = 2
-}

compiler/testData/diagnostics/tests/override/ConflictingFunctionSignatureFromSuperclass.kt

@@ -1,3 +1,4 @@
+// FIR_IDENTICAL
 open class Aaa() {
     fun foo() = 1
 }

compiler/testData/diagnostics/tests/override/clashesOnInheritance/conflictingInherited.fir.kt

@@ -0,0 +1,31 @@
+// SKIP_TXT
+
+open class A {
+    fun <T> some(s: String): T = null!!
+}
+
+interface I {
+    fun some(text: String) = ""
+}
+
+open <!CONFLICTING_INHERITED_MEMBERS!>class X1<!> : A(), I
+open <!CONFLICTING_INHERITED_MEMBERS!>class X2<!> : X1()
+
+// for some reason no error in K1
+open class B {
+    fun <T> some(s: String): T = null!!
+
+    fun some(text: String) = ""
+}
+
+open class X3 : B()
+open class X4 : X3()
+
+open class C {
+    fun <T> some(s: String): T = null!!
+}
+
+open class X5 : C() {
+    <!CONFLICTING_OVERLOADS!>fun some(text: String)<!> = ""
+}
+open <!CONFLICTING_INHERITED_MEMBERS!>class X6<!> : X5()

compiler/testData/diagnostics/tests/override/clashesOnInheritance/conflictingInherited.kt

@@ -0,0 +1,31 @@
+// SKIP_TXT
+
+open class A {
+    fun <T> some(s: String): T = null!!
+}
+
+interface I {
+    fun some(text: String) = ""
+}
+
+open <!CONFLICTING_INHERITED_MEMBERS!>class X1<!> : A(), I
+open class X2 : X1()
+
+// for some reason no error in K1
+open class B {
+    fun <T> some(s: String): T = null!!
+
+    fun some(text: String) = ""
+}
+
+open class X3 : B()
+open class X4 : X3()
+
+open class C {
+    fun <T> some(s: String): T = null!!
+}
+
+open class X5 : C() {
+    <!CONFLICTING_OVERLOADS!>fun some(text: String)<!> = ""
+}
+open class X6 : X5()

compiler/testData/diagnostics/tests/override/clashesOnInheritance/kt9550.fir.kt

@@ -1,13 +0,0 @@
-interface A {
-    fun <T> foo()
-    fun <T> bar()
-}
-
-interface B {
-    fun foo()
-    fun bar()
-}
-
-<!CONFLICTING_INHERITED_MEMBERS!>interface C1<!> : A, B {
-    override fun bar()
-}

compiler/testData/diagnostics/tests/override/clashesOnInheritance/kt9550.kt

@@ -1,3 +1,4 @@
+// FIR_IDENTICAL
 interface A {
     fun <T> foo()
     fun <T> bar()

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

@@ -24135,6 +24135,12 @@ public class DiagnosticTestGenerated extends AbstractDiagnosticTest {
                     KtTestUtil.assertAllTestsPresentByMetadataWithExcluded(this.getClass(), new File("compiler/testData/diagnostics/tests/override/clashesOnInheritance"), Pattern.compile("^(.*)\\.kts?$"), Pattern.compile("^(.+)\\.(reversed|fir|ll)\\.kts?$"), true);
                 }
 
+                @Test
+                @TestMetadata("conflictingInherited.kt")
+                public void testConflictingInherited() throws Exception {
+                    runTest("compiler/testData/diagnostics/tests/override/clashesOnInheritance/conflictingInherited.kt");
+                }
+
                 @Test
                 @TestMetadata("covariantOverrides.kt")
                 public void testCovariantOverrides() throws Exception {