Do not force coercion to Unit for nullable lambda return type

#KT-41005 Fixed

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

@@ -12469,6 +12469,11 @@ public class FirBlackBoxCodegenTestGenerated extends AbstractFirBlackBoxCodegenT
             runTest("compiler/testData/codegen/box/inference/capturedStarProjection.kt");
         }
 
+        @TestMetadata("coercionToUnitForLambdaReturnTypeWithFlexibleConstraint.kt")
+        public void testCoercionToUnitForLambdaReturnTypeWithFlexibleConstraint() throws Exception {
+            runTest("compiler/testData/codegen/box/inference/coercionToUnitForLambdaReturnTypeWithFlexibleConstraint.kt");
+        }
+
         @TestMetadata("coercionToUnitWithLastLambdaExpression.kt")
         public void testCoercionToUnitWithLastLambdaExpression() throws Exception {
             runTest("compiler/testData/codegen/box/inference/coercionToUnitWithLastLambdaExpression.kt");
@@ -12529,6 +12534,11 @@ public class FirBlackBoxCodegenTestGenerated extends AbstractFirBlackBoxCodegenT
             runTest("compiler/testData/codegen/box/inference/lastExpressionOfLambdaWithNothingConstraint.kt");
         }
 
+        @TestMetadata("noCoercionToUniForNullableLambdaReturnType.kt")
+        public void testNoCoercionToUniForNullableLambdaReturnType() throws Exception {
+            runTest("compiler/testData/codegen/box/inference/noCoercionToUniForNullableLambdaReturnType.kt");
+        }
+
         @TestMetadata("noNothingValueInsideSpecialCall.kt")
         public void testNoNothingValueInsideSpecialCall() throws Exception {
             runTest("compiler/testData/codegen/box/inference/noNothingValueInsideSpecialCall.kt");

compiler/resolution/src/org/jetbrains/kotlin/resolve/calls/components/PostponedArgumentsAnalyzer.kt

@@ -13,6 +13,7 @@ import org.jetbrains.kotlin.descriptors.annotations.Annotations
 import org.jetbrains.kotlin.descriptors.annotations.FilteredAnnotations
 import org.jetbrains.kotlin.resolve.calls.inference.ConstraintSystemBuilder
 import org.jetbrains.kotlin.resolve.calls.inference.addSubsystemFromArgument
+import org.jetbrains.kotlin.resolve.calls.inference.components.NewTypeSubstitutorByConstructorMap
 import org.jetbrains.kotlin.resolve.calls.inference.model.ConstraintStorage
 import org.jetbrains.kotlin.resolve.calls.inference.model.CoroutinePosition
 import org.jetbrains.kotlin.resolve.calls.inference.model.LambdaArgumentConstraintPosition
@@ -122,7 +123,7 @@ class PostponedArgumentsAnalyzer(
             c.canBeProper(rawReturnType) -> substitute(rawReturnType)
 
             // For Unit-coercion
-            c.hasUpperOrEqualUnitConstraint(rawReturnType) -> builtIns.unitType
+            !rawReturnType.isMarkedNullable && c.hasUpperOrEqualUnitConstraint(rawReturnType) -> builtIns.unitType
 
             else -> null
         }

compiler/testData/codegen/box/inference/coercionToUnitForLambdaReturnTypeWithFlexibleConstraint.kt

@@ -0,0 +1,22 @@
+// IGNORE_BACKEND_FIR: JVM_IR
+// TARGET_BACKEND: JVM
+
+// FILE: TestJ.java
+
+public class TestJ {
+    public static <T> In<T> materialize() {
+        return null;
+    }
+}
+
+// FILE: test.kt
+
+class In<in T>
+
+fun <T> inferred(e: In<T>?, l: () -> T): T = l()
+
+fun box(): String {
+    // coercion to Unit for T!
+    val inferred = (inferred(TestJ.materialize<Unit>(), { null })).toString()
+    return if (inferred == "kotlin.Unit") "OK" else "fail : $inferred"
+}

compiler/testData/codegen/box/inference/noCoercionToUniForNullableLambdaReturnType.kt

@@ -0,0 +1,7 @@
+fun <T> nullableK(m: () -> T?) = m()
+
+fun box(): String {
+    // no coercion to Unit for T?
+    val nullableK = (nullableK<Unit> { null }).toString()
+    return if (nullableK == "null") "OK" else  "fail: $nullableK"
+}

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

@@ -13694,6 +13694,11 @@ public class BlackBoxCodegenTestGenerated extends AbstractBlackBoxCodegenTest {
             runTest("compiler/testData/codegen/box/inference/capturedStarProjection.kt");
         }
 
+        @TestMetadata("coercionToUnitForLambdaReturnTypeWithFlexibleConstraint.kt")
+        public void testCoercionToUnitForLambdaReturnTypeWithFlexibleConstraint() throws Exception {
+            runTest("compiler/testData/codegen/box/inference/coercionToUnitForLambdaReturnTypeWithFlexibleConstraint.kt");
+        }
+
         @TestMetadata("coercionToUnitWithLastLambdaExpression.kt")
         public void testCoercionToUnitWithLastLambdaExpression() throws Exception {
             runTest("compiler/testData/codegen/box/inference/coercionToUnitWithLastLambdaExpression.kt");
@@ -13754,6 +13759,11 @@ public class BlackBoxCodegenTestGenerated extends AbstractBlackBoxCodegenTest {
             runTest("compiler/testData/codegen/box/inference/lastExpressionOfLambdaWithNothingConstraint.kt");
         }
 
+        @TestMetadata("noCoercionToUniForNullableLambdaReturnType.kt")
+        public void testNoCoercionToUniForNullableLambdaReturnType() throws Exception {
+            runTest("compiler/testData/codegen/box/inference/noCoercionToUniForNullableLambdaReturnType.kt");
+        }
+
         @TestMetadata("noNothingValueInsideSpecialCall.kt")
         public void testNoNothingValueInsideSpecialCall() throws Exception {
             runTest("compiler/testData/codegen/box/inference/noNothingValueInsideSpecialCall.kt");

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

@@ -13694,6 +13694,11 @@ public class LightAnalysisModeTestGenerated extends AbstractLightAnalysisModeTes
             runTest("compiler/testData/codegen/box/inference/capturedStarProjection.kt");
         }
 
+        @TestMetadata("coercionToUnitForLambdaReturnTypeWithFlexibleConstraint.kt")
+        public void testCoercionToUnitForLambdaReturnTypeWithFlexibleConstraint() throws Exception {
+            runTest("compiler/testData/codegen/box/inference/coercionToUnitForLambdaReturnTypeWithFlexibleConstraint.kt");
+        }
+
         @TestMetadata("coercionToUnitWithLastLambdaExpression.kt")
         public void testCoercionToUnitWithLastLambdaExpression() throws Exception {
             runTest("compiler/testData/codegen/box/inference/coercionToUnitWithLastLambdaExpression.kt");
@@ -13754,6 +13759,11 @@ public class LightAnalysisModeTestGenerated extends AbstractLightAnalysisModeTes
             runTest("compiler/testData/codegen/box/inference/lastExpressionOfLambdaWithNothingConstraint.kt");
         }
 
+        @TestMetadata("noCoercionToUniForNullableLambdaReturnType.kt")
+        public void testNoCoercionToUniForNullableLambdaReturnType() throws Exception {
+            runTest("compiler/testData/codegen/box/inference/noCoercionToUniForNullableLambdaReturnType.kt");
+        }
+
         @TestMetadata("noNothingValueInsideSpecialCall.kt")
         public void testNoNothingValueInsideSpecialCall() throws Exception {
             runTest("compiler/testData/codegen/box/inference/noNothingValueInsideSpecialCall.kt");

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

@@ -12469,6 +12469,11 @@ public class IrBlackBoxCodegenTestGenerated extends AbstractIrBlackBoxCodegenTes
             runTest("compiler/testData/codegen/box/inference/capturedStarProjection.kt");
         }
 
+        @TestMetadata("coercionToUnitForLambdaReturnTypeWithFlexibleConstraint.kt")
+        public void testCoercionToUnitForLambdaReturnTypeWithFlexibleConstraint() throws Exception {
+            runTest("compiler/testData/codegen/box/inference/coercionToUnitForLambdaReturnTypeWithFlexibleConstraint.kt");
+        }
+
         @TestMetadata("coercionToUnitWithLastLambdaExpression.kt")
         public void testCoercionToUnitWithLastLambdaExpression() throws Exception {
             runTest("compiler/testData/codegen/box/inference/coercionToUnitWithLastLambdaExpression.kt");
@@ -12529,6 +12534,11 @@ public class IrBlackBoxCodegenTestGenerated extends AbstractIrBlackBoxCodegenTes
             runTest("compiler/testData/codegen/box/inference/lastExpressionOfLambdaWithNothingConstraint.kt");
         }
 
+        @TestMetadata("noCoercionToUniForNullableLambdaReturnType.kt")
+        public void testNoCoercionToUniForNullableLambdaReturnType() throws Exception {
+            runTest("compiler/testData/codegen/box/inference/noCoercionToUniForNullableLambdaReturnType.kt");
+        }
+
         @TestMetadata("noNothingValueInsideSpecialCall.kt")
         public void testNoNothingValueInsideSpecialCall() throws Exception {
             runTest("compiler/testData/codegen/box/inference/noNothingValueInsideSpecialCall.kt");

js/js.tests/test/org/jetbrains/kotlin/js/test/es6/semantics/IrJsCodegenBoxES6TestGenerated.java

@@ -10784,6 +10784,11 @@ public class IrJsCodegenBoxES6TestGenerated extends AbstractIrJsCodegenBoxES6Tes
             runTest("compiler/testData/codegen/box/inference/lastExpressionOfLambdaWithNothingConstraint.kt");
         }
 
+        @TestMetadata("noCoercionToUniForNullableLambdaReturnType.kt")
+        public void testNoCoercionToUniForNullableLambdaReturnType() throws Exception {
+            runTest("compiler/testData/codegen/box/inference/noCoercionToUniForNullableLambdaReturnType.kt");
+        }
+
         @TestMetadata("plusAssignInsideLambda.kt")
         public void testPlusAssignInsideLambda() throws Exception {
             runTest("compiler/testData/codegen/box/inference/plusAssignInsideLambda.kt");

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

@@ -10784,6 +10784,11 @@ public class IrJsCodegenBoxTestGenerated extends AbstractIrJsCodegenBoxTest {
             runTest("compiler/testData/codegen/box/inference/lastExpressionOfLambdaWithNothingConstraint.kt");
         }
 
+        @TestMetadata("noCoercionToUniForNullableLambdaReturnType.kt")
+        public void testNoCoercionToUniForNullableLambdaReturnType() throws Exception {
+            runTest("compiler/testData/codegen/box/inference/noCoercionToUniForNullableLambdaReturnType.kt");
+        }
+
         @TestMetadata("plusAssignInsideLambda.kt")
         public void testPlusAssignInsideLambda() throws Exception {
             runTest("compiler/testData/codegen/box/inference/plusAssignInsideLambda.kt");

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

@@ -10849,6 +10849,11 @@ public class JsCodegenBoxTestGenerated extends AbstractJsCodegenBoxTest {
             runTest("compiler/testData/codegen/box/inference/lastExpressionOfLambdaWithNothingConstraint.kt");
         }
 
+        @TestMetadata("noCoercionToUniForNullableLambdaReturnType.kt")
+        public void testNoCoercionToUniForNullableLambdaReturnType() throws Exception {
+            runTest("compiler/testData/codegen/box/inference/noCoercionToUniForNullableLambdaReturnType.kt");
+        }
+
         @TestMetadata("plusAssignInsideLambda.kt")
         public void testPlusAssignInsideLambda() throws Exception {
             runTest("compiler/testData/codegen/box/inference/plusAssignInsideLambda.kt");