functional interface: introduce acceptability for both lambda and method references

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

@@ -25,4 +25,9 @@ public interface PsiFunctionalExpression extends PsiExpression, Iconable, Naviga
    */
   @Nullable
   PsiType getFunctionalInterfaceType();
+
+  /**
+   * @return true if assignment SAM s = expr is correctly shaped
+   */
+  boolean isAcceptable(PsiType left);
 }

java/java-psi-impl/src/com/intellij/psi/impl/source/tree/java/PsiLambdaExpressionImpl.java

@@ -167,6 +167,11 @@ public class PsiLambdaExpressionImpl extends ExpressionPsiElement implements Psi
     return true;
   }
 
+  @Override
+  public boolean isAcceptable(PsiType left) {
+    return isAcceptable(left, false);
+  }
+
   @Override
   public boolean isAcceptable(PsiType leftType, boolean checkReturnType) {
     if (leftType instanceof PsiIntersectionType) {