"more specific method" w.r.t function return type

[Zhong Yu]

The following code does not compile in jdk8b120

public class Tmp
{
    <T> void foo(Function<String,T> function){}

    void foo(Consumer<String> consumer){}

    void test()
    {
        foo( s->System::gc ); // javac: foo is ambiguous
    }
}

however I think that foo#1 should be more specific than foo#2, because
Consumer returns void.

Is this a javac bug or my understanding is incorrect?

(If foo#1 is indeed the most specific method, the code still won't
compile because gc() returns void)

Zhong Yu