redundant cast: process instanceof (IDEA-128493)

java/java-psi-api/src/com/intellij/psi/util/RedundantCastUtil.java

@@ -541,8 +541,17 @@ public class RedundantCastUtil {
             }
           }
         }
-        if (parent instanceof PsiInstanceOfExpression || (TypeConversionUtil.isAssignable(castTo, opType, false) && 
-                                                          (expectedTypeByParent == null || TypeConversionUtil.isAssignable(expectedTypeByParent, opType, false)))) {
+        if (parent instanceof PsiInstanceOfExpression) {
+          //15.20.2. Type Comparison Operator instanceof:
+          //If a cast (§15.16) of the RelationalExpression to the ReferenceType would be rejected as a compile-time error,
+          //then the instanceof relational expression likewise produces a compile-time error.
+          final PsiTypeElement checkTypeElement = ((PsiInstanceOfExpression)parent).getCheckType();
+          if (checkTypeElement != null && TypeConversionUtil.areTypesConvertible(opType, checkTypeElement.getType())) {
+            addToResults(typeCast);
+          }
+        }
+        else if (TypeConversionUtil.isAssignable(castTo, opType, false) &&
+                 (expectedTypeByParent == null || TypeConversionUtil.isAssignable(expectedTypeByParent, opType, false))) {
           addToResults(typeCast);
         }
       }

java/java-tests/testData/codeInsight/daemonCodeAnalyzer/advHighlighting7/TypeCastInInstanceof.java

@@ -0,0 +1,13 @@
+@SuppressWarnings({"UnusedDeclaration"})
+class C {
+  boolean foo(final ConfigurableField<String> nameField) {
+    return (Formatter<?>)nameField.getFormatter() instanceof DefaultFormatter;
+  }
+}
+
+@SuppressWarnings({"UnusedDeclaration"})
+interface Formatter<V>{}
+class DefaultFormatter implements Formatter<Object>{}
+interface ConfigurableField<V> {
+  Formatter<V> getFormatter();
+}

java/java-tests/testSrc/com/intellij/codeInsight/daemon/LightAdvHighlightingJdk7Test.java

@@ -145,6 +145,7 @@ public class LightAdvHighlightingJdk7Test extends LightDaemonAnalyzerTestCase {
   public void testPreciseRethrow() { doTest(false, false); }
   public void testImprovedCatchAnalysis() { doTest(true, false); }
   public void testPolymorphicTypeCast() { doTest(true, false); }
+  public void testTypeCastInInstanceof() { doTest(true, false); }
   public void testErasureClashConfusion() { doTest(true, false, UnusedDeclarationInspection.class); }
   public void testUnused() { doTest(true, false, UnusedDeclarationInspection.class); }
   public void testSuperBound() { doTest(false, false); }