lambda: do not mark cast redundant when functional interface won't be found otherwise

java/java-psi-api/src/com/intellij/psi/LambdaUtil.java

@@ -250,7 +250,7 @@ public class LambdaUtil {
     return null;
   }
 
-  public static int getLambdaIdx(PsiExpressionList expressionList, final PsiLambdaExpression element) {
+  public static int getLambdaIdx(PsiExpressionList expressionList, final PsiElement element) {
     PsiExpression[] expressions = expressionList.getExpressions();
     for (int i = 0; i < expressions.length; i++) {
       PsiExpression expression = expressions[i];
@@ -329,7 +329,7 @@ public class LambdaUtil {
   }
 
   @Nullable
-  public static PsiType getFunctionalInterfaceType(PsiLambdaExpression expression, final boolean tryToSubstitute) {
+  public static PsiType getFunctionalInterfaceType(PsiElement expression, final boolean tryToSubstitute) {
     PsiElement parent = expression.getParent();
     while (parent instanceof PsiParenthesizedExpression) {
       parent = parent.getParent();

java/java-tests/testData/inspection/redundantCast/lambda/ExpectedSupertype/expected.xml

@@ -0,0 +1,3 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<problems>
+</problems>

java/java-tests/testData/inspection/redundantCast/lambda/ExpectedSupertype/src/Test.java

@@ -0,0 +1,31 @@
+/*
+ * Copyright 2000-2012 JetBrains s.r.o.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+public class Test {
+  interface I {
+  }
+
+  interface Bar extends I {
+    int compare(String o1, String o2);
+  }
+
+  {
+    I bar2 = (Bar) (o1, o2) -> 0;
+    g((Bar) (o1, o2) -> 0);
+    ((Bar) (o1, o2) -> 0).getClass();
+  }
+
+  void g(I i){}
+}

java/java-tests/testSrc/com/intellij/codeInspection/RedundantCast18Test.java

@@ -33,6 +33,7 @@ public class RedundantCast18Test extends InspectionTestCase {
   }
 
   public void testLambdaContext() throws Exception { doTest(); }
+  public void testExpectedSupertype() throws Exception { doTest(); }
 
   protected Sdk getTestProjectSdk() {
     Sdk sdk = JavaSdkImpl.getMockJdk17();

java/openapi/src/com/intellij/psi/util/RedundantCastUtil.java

@@ -362,13 +362,13 @@ public class RedundantCastUtil {
 
       PsiElement expr = deparenthesizeExpression(operand);
 
+      final PsiType topCastType = typeCast.getType();
       if (expr instanceof PsiTypeCastExpression) {
         PsiTypeElement typeElement = ((PsiTypeCastExpression)expr).getCastType();
         if (typeElement == null) return;
         PsiType castType = typeElement.getType();
         final PsiExpression innerOperand = ((PsiTypeCastExpression)expr).getOperand();
         final PsiType operandType = innerOperand != null ? innerOperand.getType() : null;
-        final PsiType topCastType = typeCast.getType();
         if (!(castType instanceof PsiPrimitiveType)) {
           if (operandType != null && topCastType != null && TypeConversionUtil.areTypesConvertible(operandType, topCastType)) {
             addToResults((PsiTypeCastExpression)expr);
@@ -389,8 +389,13 @@ public class RedundantCastUtil {
           }
         } else if (parent instanceof PsiSynchronizedStatement && (expr instanceof PsiExpression && ((PsiExpression)expr).getType() instanceof PsiPrimitiveType)) {
           return;
-        } else if (expr instanceof PsiLambdaExpression && parent instanceof PsiParenthesizedExpression && parent.getParent() instanceof PsiReferenceExpression) {
-          return;
+        } else if (expr instanceof PsiLambdaExpression) {
+          if (parent instanceof PsiParenthesizedExpression && parent.getParent() instanceof PsiReferenceExpression) {
+            return;
+          }
+
+          final PsiType functionalInterfaceType = LambdaUtil.getFunctionalInterfaceType(typeCast, true);
+          if (topCastType != null && functionalInterfaceType != null && !TypeConversionUtil.isAssignable(topCastType, functionalInterfaceType, false)) return;
         }
         processAlreadyHasTypeCast(typeCast);
       }