"Potential compatibility" is specified as follows for a lambda expression:

A lambda expression (15.27) is potentially compatible with a functional interface type (9.8) if all of the following are true:
- The arity of the targeted type's function type is the same as the arity of the lambda expression.
- If the targeted type's function type has a void return, then the lambda body is either a statement expression (14.8) or a void-compatible block (15.27.2).
- If the targeted type's function type has a (non-void) return type, then the lambda body is either an expression or a value-compatible block (15.27.2).

The current javac implementation seems to only take the lambda expression's arity into account, completely ignoring the lambda body.

Example:

 interface I {
  String foo(String [] x, String y);
 }

 interface J {
  void foo(int x, int y);
 }

 public class X {
     static void goo(J j) {
         System.out.println("goo(J)");
     }
     static void goo(I i) {
         System.out.println("goo(I)");
     }
     public static void main(String[] args) throws InterruptedException {
        goo((x, y) -> { return x[0] += 1; }); // expected: print goo(I); actual: ambiguity error
     }
 }

Originally reported on lambda-dev@openjdk.java.net:
http://mail.openjdk.java.net/pipermail/lambda-dev/2013-November/011394.html