IR: handle conflicts in resolveFakeOverride()

compiler/ir/ir.tree/src/org/jetbrains/kotlin/ir/util/IrUtils.kt

@@ -296,7 +296,12 @@ fun IrSimpleFunction.collectRealOverrides(toSkip: (IrSimpleFunction) -> Boolean
 // This implementation is from kotlin-native
 // TODO: use this implementation instead of any other
 fun IrSimpleFunction.resolveFakeOverride(toSkip: (IrSimpleFunction) -> Boolean = { false }): IrSimpleFunction? {
-    return collectRealOverrides(toSkip).singleOrNull { it.modality != Modality.ABSTRACT }
+    return collectRealOverrides(toSkip)
+        .filter { it.modality != Modality.ABSTRACT }
+        .let { realOverrides ->
+            // Kotlin forbids conflicts between overrides, but they may trickle down from Java.
+            realOverrides.singleOrNull { it.parent.safeAs<IrClass>()?.isInterface != true } ?: realOverrides.singleOrNull()
+        }
 }
 
 fun IrSimpleFunction.isOrOverridesSynthesized(): Boolean {

compiler/testData/codegen/box/traits/multipleImplFromJava.kt

@@ -0,0 +1,20 @@
+// TARGET_BACKEND: JVM
+// SKIP_JDK6
+// FILE: I.java
+interface I {
+    default String ifun() { return "fail"; }
+}
+
+// FILE: Z.java
+public class Z {
+    public String ifun() { return "OK"; }
+}
+
+// FILE: Zz.java
+public class Zz extends Z implements I {}
+
+// FILE: multipleImplFromJava.kt
+
+class Cc : Zz()
+
+fun box(): String = Zz().ifun()

compiler/tests/org/jetbrains/kotlin/codegen/BlackBoxCodegenTestGenerated.java

@@ -28157,6 +28157,11 @@ public class BlackBoxCodegenTestGenerated extends AbstractBlackBoxCodegenTest {
             runTest("compiler/testData/codegen/box/traits/multiple.kt");
         }
 
+        @TestMetadata("multipleImplFromJava.kt")
+        public void testMultipleImplFromJava() throws Exception {
+            runTest("compiler/testData/codegen/box/traits/multipleImplFromJava.kt");
+        }
+
         @TestMetadata("noPrivateDelegation.kt")
         public void testNoPrivateDelegation() throws Exception {
             runTest("compiler/testData/codegen/box/traits/noPrivateDelegation.kt");

compiler/tests/org/jetbrains/kotlin/codegen/LightAnalysisModeTestGenerated.java

@@ -26974,6 +26974,11 @@ public class LightAnalysisModeTestGenerated extends AbstractLightAnalysisModeTes
             runTest("compiler/testData/codegen/box/traits/multiple.kt");
         }
 
+        @TestMetadata("multipleImplFromJava.kt")
+        public void testMultipleImplFromJava() throws Exception {
+            runTest("compiler/testData/codegen/box/traits/multipleImplFromJava.kt");
+        }
+
         @TestMetadata("noPrivateDelegation.kt")
         public void testNoPrivateDelegation() throws Exception {
             runTest("compiler/testData/codegen/box/traits/noPrivateDelegation.kt");

compiler/tests/org/jetbrains/kotlin/codegen/ir/FirBlackBoxCodegenTestGenerated.java

@@ -26661,6 +26661,11 @@ public class FirBlackBoxCodegenTestGenerated extends AbstractFirBlackBoxCodegenT
             runTest("compiler/testData/codegen/box/traits/multiple.kt");
         }
 
+        @TestMetadata("multipleImplFromJava.kt")
+        public void testMultipleImplFromJava() throws Exception {
+            runTest("compiler/testData/codegen/box/traits/multipleImplFromJava.kt");
+        }
+
         @TestMetadata("noPrivateDelegation.kt")
         public void testNoPrivateDelegation() throws Exception {
             runTest("compiler/testData/codegen/box/traits/noPrivateDelegation.kt");

compiler/tests/org/jetbrains/kotlin/codegen/ir/IrBlackBoxCodegenTestGenerated.java

@@ -26661,6 +26661,11 @@ public class IrBlackBoxCodegenTestGenerated extends AbstractIrBlackBoxCodegenTes
             runTest("compiler/testData/codegen/box/traits/multiple.kt");
         }
 
+        @TestMetadata("multipleImplFromJava.kt")
+        public void testMultipleImplFromJava() throws Exception {
+            runTest("compiler/testData/codegen/box/traits/multipleImplFromJava.kt");
+        }
+
         @TestMetadata("noPrivateDelegation.kt")
         public void testNoPrivateDelegation() throws Exception {
             runTest("compiler/testData/codegen/box/traits/noPrivateDelegation.kt");