[JVM IR] Maintain KT-36188 bug compatibility between non-IR and IR

backends.

compiler/fir/fir2ir/tests/org/jetbrains/kotlin/codegen/ir/FirBlackBoxCodegenTestGenerated.java

@@ -8535,6 +8535,16 @@ public class FirBlackBoxCodegenTestGenerated extends AbstractFirBlackBoxCodegenT
                 runTest("compiler/testData/codegen/box/defaultArguments/function/innerExtentionFunctionManyArgs.kt");
             }
 
+            @TestMetadata("kt36188.kt")
+            public void testKt36188() throws Exception {
+                runTest("compiler/testData/codegen/box/defaultArguments/function/kt36188.kt");
+            }
+
+            @TestMetadata("kt36188_2.kt")
+            public void testKt36188_2() throws Exception {
+                runTest("compiler/testData/codegen/box/defaultArguments/function/kt36188_2.kt");
+            }
+
             @TestMetadata("kt5232.kt")
             public void testKt5232() throws Exception {
                 runTest("compiler/testData/codegen/box/defaultArguments/function/kt5232.kt");

compiler/ir/backend.jvm/src/org/jetbrains/kotlin/backend/jvm/lower/InterfaceLowering.kt

@@ -96,12 +96,16 @@ internal class InterfaceLowering(val context: JvmBackendContext) : IrElementTran
                  *    ```
                  */
                 function.origin == IrDeclarationOrigin.FAKE_OVERRIDE -> {
-                    val implementation = function.resolveFakeOverride()!!
+                    // We check to see if this is a default stub function BEFORE finding the implementation because of a front-end bug
+                    // (KT-36188) where there could be multiple implementations. (resolveFakeOverride() only returns the implementation if
+                    // there's only one.)
+                    if (function.name.asString().endsWith("\$default")) {
+                        continue@loop
+                    }
+                    val implementation = function.resolveFakeOverride() ?: error("No single implementation found for: ${function.render()}")
 
                     when {
-                        Visibilities.isPrivate(implementation.visibility) ||
+                        Visibilities.isPrivate(implementation.visibility) || implementation.isMethodOfAny() ->
-                                implementation.isMethodOfAny() ||
-                                implementation.origin == IrDeclarationOrigin.FUNCTION_FOR_DEFAULT_PARAMETER ->
                             continue@loop
                         !implementation.hasJvmDefault() -> {
                             val defaultImpl = createDefaultImpl(function)

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

@@ -0,0 +1,20 @@
+// IGNORE_BACKEND_FIR: JVM_IR
+// IGNORE_BACKEND: JS
+
+// Test for KT-36188 bug compatibility between non-IR and IR backends
+
+interface A {
+    fun foo(a: String = "OK"): String
+}
+
+interface A2 : A
+
+interface B {
+    fun foo(a: String = "Fail"): String
+}
+
+class Impl : A2, B {
+    override fun foo(a: String) = a
+}
+
+fun box(): String = Impl().foo()

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

@@ -0,0 +1,22 @@
+// IGNORE_BACKEND_FIR: JVM_IR
+// IGNORE_BACKEND: JS
+
+// Test for KT-36188 bug compatibility between non-IR and IR backends
+
+interface A {
+    fun foo(a: String = "OK"): String
+}
+
+interface A2 : A
+
+interface B {
+    fun foo(a: String = "Fail"): String
+}
+
+interface C : A2, B
+
+class Impl : C {
+    override fun foo(a: String) = a
+}
+
+fun box(): String = Impl().foo()

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

@@ -9680,6 +9680,16 @@ public class BlackBoxCodegenTestGenerated extends AbstractBlackBoxCodegenTest {
                 runTest("compiler/testData/codegen/box/defaultArguments/function/innerExtentionFunctionManyArgs.kt");
             }
 
+            @TestMetadata("kt36188.kt")
+            public void testKt36188() throws Exception {
+                runTest("compiler/testData/codegen/box/defaultArguments/function/kt36188.kt");
+            }
+
+            @TestMetadata("kt36188_2.kt")
+            public void testKt36188_2() throws Exception {
+                runTest("compiler/testData/codegen/box/defaultArguments/function/kt36188_2.kt");
+            }
+
             @TestMetadata("kt5232.kt")
             public void testKt5232() throws Exception {
                 runTest("compiler/testData/codegen/box/defaultArguments/function/kt5232.kt");

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

@@ -9680,6 +9680,16 @@ public class LightAnalysisModeTestGenerated extends AbstractLightAnalysisModeTes
                 runTest("compiler/testData/codegen/box/defaultArguments/function/innerExtentionFunctionManyArgs.kt");
             }
 
+            @TestMetadata("kt36188.kt")
+            public void testKt36188() throws Exception {
+                runTest("compiler/testData/codegen/box/defaultArguments/function/kt36188.kt");
+            }
+
+            @TestMetadata("kt36188_2.kt")
+            public void testKt36188_2() throws Exception {
+                runTest("compiler/testData/codegen/box/defaultArguments/function/kt36188_2.kt");
+            }
+
             @TestMetadata("kt5232.kt")
             public void testKt5232() throws Exception {
                 runTest("compiler/testData/codegen/box/defaultArguments/function/kt5232.kt");

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

@@ -8535,6 +8535,16 @@ public class IrBlackBoxCodegenTestGenerated extends AbstractIrBlackBoxCodegenTes
                 runTest("compiler/testData/codegen/box/defaultArguments/function/innerExtentionFunctionManyArgs.kt");
             }
 
+            @TestMetadata("kt36188.kt")
+            public void testKt36188() throws Exception {
+                runTest("compiler/testData/codegen/box/defaultArguments/function/kt36188.kt");
+            }
+
+            @TestMetadata("kt36188_2.kt")
+            public void testKt36188_2() throws Exception {
+                runTest("compiler/testData/codegen/box/defaultArguments/function/kt36188_2.kt");
+            }
+
             @TestMetadata("kt5232.kt")
             public void testKt5232() throws Exception {
                 runTest("compiler/testData/codegen/box/defaultArguments/function/kt5232.kt");

js/js.tests/test/org/jetbrains/kotlin/js/test/ir/semantics/IrJsCodegenBoxTestGenerated.java

@@ -7245,6 +7245,16 @@ public class IrJsCodegenBoxTestGenerated extends AbstractIrJsCodegenBoxTest {
                 runTest("compiler/testData/codegen/box/defaultArguments/function/innerExtentionFunctionManyArgs.kt");
             }
 
+            @TestMetadata("kt36188.kt")
+            public void testKt36188() throws Exception {
+                runTest("compiler/testData/codegen/box/defaultArguments/function/kt36188.kt");
+            }
+
+            @TestMetadata("kt36188_2.kt")
+            public void testKt36188_2() throws Exception {
+                runTest("compiler/testData/codegen/box/defaultArguments/function/kt36188_2.kt");
+            }
+
             @TestMetadata("kt5232.kt")
             public void testKt5232() throws Exception {
                 runTest("compiler/testData/codegen/box/defaultArguments/function/kt5232.kt");

js/js.tests/test/org/jetbrains/kotlin/js/test/semantics/JsCodegenBoxTestGenerated.java

@@ -7245,6 +7245,16 @@ public class JsCodegenBoxTestGenerated extends AbstractJsCodegenBoxTest {
                 runTest("compiler/testData/codegen/box/defaultArguments/function/innerExtentionFunctionManyArgs.kt");
             }
 
+            @TestMetadata("kt36188.kt")
+            public void testKt36188() throws Exception {
+                runTest("compiler/testData/codegen/box/defaultArguments/function/kt36188.kt");
+            }
+
+            @TestMetadata("kt36188_2.kt")
+            public void testKt36188_2() throws Exception {
+                runTest("compiler/testData/codegen/box/defaultArguments/function/kt36188_2.kt");
+            }
+
             @TestMetadata("kt5232.kt")
             public void testKt5232() throws Exception {
                 runTest("compiler/testData/codegen/box/defaultArguments/function/kt5232.kt");