method ref: introduce method ref variable; accept method ref type separately

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

@@ -28,4 +28,7 @@ public interface PsiMethodReferenceExpression extends PsiReferenceExpression {
    */
   @Nullable
   PsiTypeElement getQualifierType();
+  
+  @Nullable
+  PsiType getFunctionalInterfaceType();
 }

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

@@ -57,7 +57,7 @@ public class PsiMethodReferenceType extends PsiType {
 
   @Override
   public <A> A accept(@NotNull final PsiTypeVisitor<A> visitor) {
-    return visitor.visitType(this);
+    return visitor.visitMethodReferenceType(this);
   }
 
   @Override

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

@@ -75,4 +75,11 @@ public class PsiTypeVisitor<A> {
     if (interfaceType != null) return interfaceType.accept(this);
     return visitType(interfaceType);
   }
+  
+  public A visitMethodReferenceType(PsiMethodReferenceType methodReferenceType) {
+    final PsiMethodReferenceExpression expression = methodReferenceType.getExpression();
+    final PsiType interfaceType = expression.getFunctionalInterfaceType();
+    if (interfaceType != null) return interfaceType.accept(this);
+    return visitType(methodReferenceType);
+  }
 }

java/java-tests/testData/refactoring/introduceVariable/MethodRef.after.java

@@ -0,0 +1,15 @@
+class Test {
+  {
+      Bar c = Test::length;
+      bar(c);
+  }
+
+  public static Integer length(String s) {
+    return s.length();
+  }
+
+    static void bar(Bar bar) {}
+  interface Bar {
+    Integer _(String s);
+  }
+}

java/java-tests/testData/refactoring/introduceVariable/MethodRef.java

@@ -0,0 +1,14 @@
+class Test {
+  {
+      bar(<selection>Test::length</selection>);
+  }
+
+  public static Integer length(String s) {
+    return s.length();
+  }
+
+    static void bar(Bar bar) {}
+  interface Bar {
+    Integer _(String s);
+  }
+}

java/java-tests/testSrc/com/intellij/refactoring/IntroduceVariableTest.java

@@ -373,6 +373,10 @@ public class IntroduceVariableTest extends LightCodeInsightTestCase {
     doTest(new MockIntroduceVariableHandler("c", false, false, false, "SAM<java.lang.Integer>"));
   }
 
+  public void testMethodRef() throws Exception {
+    doTest(new MockIntroduceVariableHandler("c", false, false, false, "Test.Bar"));
+  }
+
   public void testLambdaExprNotAccepted() throws Exception {
     doTest(new MockIntroduceVariableHandler("c", false, false, false, "SAM<java.lang.String>"));
   }