K2: Do not use KFunctionN as representation type for adapted references

Beside some corner cases, it's already prohibited in K1 because
adaptation have a bit strange nature
(they don't represent any existing real function exactly)

^KT-55137 Fixed

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

@@ -3117,6 +3117,12 @@ public class FirOldFrontendDiagnosticsTestGenerated extends AbstractFirDiagnosti
                     KtTestUtil.assertAllTestsPresentByMetadataWithExcluded(this.getClass(), new File("compiler/testData/diagnostics/tests/callableReference/adapted"), Pattern.compile("^(.*)\\.kts?$"), Pattern.compile("^(.+)\\.fir\\.kts?$"), true);
                 }
 
+                @Test
+                @TestMetadata("noKFunctionForAdaptation.kt")
+                public void testNoKFunctionForAdaptation() throws Exception {
+                    runTest("compiler/testData/diagnostics/tests/callableReference/adapted/noKFunctionForAdaptation.kt");
+                }
+
                 @Test
                 @TestMetadata("simpleAdaptationOutsideOfCall.kt")
                 public void testSimpleAdaptationOutsideOfCall() throws Exception {

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

@@ -3111,6 +3111,12 @@ public class FirOldFrontendDiagnosticsWithLightTreeTestGenerated extends Abstrac
                     KtTestUtil.assertAllTestsPresentByMetadataWithExcluded(this.getClass(), new File("compiler/testData/diagnostics/tests/callableReference/adapted"), Pattern.compile("^(.+)\\.kt$"), Pattern.compile("^(.+)\\.fir\\.kts?$"), true);
                 }
 
+                @Test
+                @TestMetadata("noKFunctionForAdaptation.kt")
+                public void testNoKFunctionForAdaptation() throws Exception {
+                    runTest("compiler/testData/diagnostics/tests/callableReference/adapted/noKFunctionForAdaptation.kt");
+                }
+
                 @Test
                 @TestMetadata("simpleAdaptationOutsideOfCall.kt")
                 public void testSimpleAdaptationOutsideOfCall() throws Exception {

compiler/fir/fir2ir/src/org/jetbrains/kotlin/fir/backend/generators/AdapterGenerator.kt

@@ -98,8 +98,7 @@ internal class AdapterGenerator(
      * At the use site, instead of referenced, we can put the suspend lambda as an adapter.
      */
     private fun needSuspendConversion(type: IrSimpleType, function: IrFunction): Boolean =
-        // TODO: should refer to LanguageVersionSettings.SuspendConversion
-        type.isKSuspendFunction() && !function.isSuspend
+        type.isSuspendFunction() && !function.isSuspend
 
     /**
      * For example,
@@ -133,9 +132,6 @@ internal class AdapterGenerator(
         }
     }
 
-    internal fun ConeKotlinType.kFunctionTypeToFunctionType(): IrSimpleType =
-        reflectFunctionTypeToNonReflectFunctionType(session).toIrType() as IrSimpleType
-
     internal fun generateAdaptedCallableReference(
         callableReferenceAccess: FirCallableReferenceAccess,
         explicitReceiverExpression: IrExpression?,

compiler/fir/fir2ir/src/org/jetbrains/kotlin/fir/backend/generators/CallAndReferenceGenerator.kt

@@ -156,7 +156,8 @@ class CallAndReferenceGenerator(
                     if (adapterGenerator.needToGenerateAdaptedCallableReference(callableReferenceAccess, type, function)) {
                         // Receivers are being applied inside
                         with(adapterGenerator) {
-                            val adaptedType = callableReferenceAccess.typeRef.coneType.kFunctionTypeToFunctionType()
+                            // TODO: Figure out why `adaptedType` is different from the `type`?
+                            val adaptedType = callableReferenceAccess.typeRef.coneType.toIrType() as IrSimpleType
                             generateAdaptedCallableReference(callableReferenceAccess, explicitReceiverExpression, symbol, adaptedType)
                         }
                     } else {

compiler/fir/fir2ir/tests-gen/org/jetbrains/kotlin/test/runners/codegen/FirBlackBoxCodegenTestGenerated.java

@@ -18319,6 +18319,12 @@ public class FirBlackBoxCodegenTestGenerated extends AbstractFirBlackBoxCodegenT
             runTest("compiler/testData/codegen/box/fir/assertEqualsFakeOverride.kt");
         }
 
+        @Test
+        @TestMetadata("callableReferenceConversionTopLevel.kt")
+        public void testCallableReferenceConversionTopLevel() throws Exception {
+            runTest("compiler/testData/codegen/box/fir/callableReferenceConversionTopLevel.kt");
+        }
+
         @Test
         @TestMetadata("callableReferenceToJavaField.kt")
         public void testCallableReferenceToJavaField() throws Exception {

compiler/fir/resolve/src/org/jetbrains/kotlin/fir/resolve/calls/CallableReferenceResolution.kt

@@ -136,7 +136,7 @@ private fun buildReflectionType(
                 ?: FunctionTypeKind.Function
 
             return createFunctionType(
-                baseFunctionTypeKind.reflectKind(),
+                if (callableReferenceAdaptation == null) baseFunctionTypeKind.reflectKind() else baseFunctionTypeKind.nonReflectKind(),
                 parameters,
                 receiverType = receiverType.takeIf { fir.receiverParameter != null },
                 rawReturnType = returnType,
@@ -239,9 +239,11 @@ private fun BodyResolveComponents.getCallableReferenceAdaptation(
         mappedArguments[valueParameter] = ResolvedCallArgument.VarargArgument(varargElements)
     }
 
+    var isThereVararg = mappedVarargElements.isNotEmpty()
     for (valueParameter in function.valueParameters) {
         if (valueParameter.isVararg && valueParameter !in mappedArguments) {
             mappedArguments[valueParameter] = ResolvedCallArgument.VarargArgument(emptyList())
+            isThereVararg = true
         }
     }
 
@@ -264,6 +266,14 @@ private fun BodyResolveComponents.getCallableReferenceAdaptation(
         CallableReferenceConversionStrategy.NoConversion
     }
 
+    if (defaults == 0 && !isThereVararg &&
+        coercionStrategy == CoercionStrategy.NO_COERCION && conversionStrategy == CallableReferenceConversionStrategy.NoConversion
+    ) {
+        // Do not create adaptation for trivial (id) conversion as it makes resulting type FunctionN instead of KFunctionN
+        // It happens because adapted references do not support reflection (see KT-40406)
+        return null
+    }
+
     @Suppress("UNCHECKED_CAST")
     return CallableReferenceAdaptation(
         mappedArgumentTypes as Array<ConeKotlinType>,