don't cast when instaceofing List<String> to ArrayList

java/java-impl/src/com/intellij/codeInsight/completion/JavaCompletionUtil.java

@@ -52,6 +52,7 @@ import com.intellij.psi.tree.IElementType;
 import com.intellij.psi.util.PsiFormatUtil;
 import com.intellij.psi.util.PsiTreeUtil;
 import com.intellij.psi.util.PsiUtil;
+import com.intellij.psi.util.TypeConversionUtil;
 import com.intellij.psi.xml.XmlToken;
 import com.intellij.psi.xml.XmlTokenType;
 import com.intellij.util.IncorrectOperationException;
@@ -510,8 +511,16 @@ public class JavaCompletionUtil {
         PsiType castType = castTypeItem.getPsiType();
         if (plainQualifier instanceof PsiClassType && castType instanceof PsiClassType) {
           PsiMethod method = (PsiMethod)o;
-          PsiSubstitutor plainSub = ((PsiClassType)plainQualifier).resolveGenerics().getSubstitutor();
-          PsiSubstitutor castSub = ((PsiClassType)castType).resolveGenerics().getSubstitutor();
+          PsiClassType.ClassResolveResult plainResult = ((PsiClassType)plainQualifier).resolveGenerics();
+          PsiClass plainClass = plainResult.getElement();
+          PsiClass castClass = ((PsiClassType)castType).resolveGenerics().getElement();
+
+          if (castClass == null || plainClass == null || !castClass.isInheritor(plainClass, true)) {
+            return item;
+          }
+
+          PsiSubstitutor plainSub = plainResult.getSubstitutor();
+          PsiSubstitutor castSub = TypeConversionUtil.getSuperClassSubstitutor(plainClass, (PsiClassType)castType);
           if (method.getSignature(plainSub).equals(method.getSignature(castSub)) &&
               plainSub.substitute(method.getReturnType()).equals(castSub.substitute(method.getReturnType()))) {
             return item;

java/java-tests/testData/codeInsight/completion/normal/ListArrayListCast.java

@@ -0,0 +1,10 @@
+import java.util.List;
+import java.util.ArrayList;
+
+public class A {
+  void foo(List<String> l) {
+    if (l instanceof ArrayList) {
+      l.ge<caret>
+    }
+  }
+}

java/java-tests/testData/codeInsight/completion/normal/ListArrayListCast_after.java

@@ -0,0 +1,10 @@
+import java.util.List;
+import java.util.ArrayList;
+
+public class A {
+  void foo(List<String> l) {
+    if (l instanceof ArrayList) {
+      l.get(<caret>)
+    }
+  }
+}

java/java-tests/testSrc/com/intellij/codeInsight/completion/NormalCompletionTest.groovy

@@ -1324,4 +1324,6 @@ public class ListUtils {
 
   }
 
+  public void testListArrayListCast() { doTest('\n') }
+
 }