Method overload with function parameter returning Ambiguous method call error

Question

I have two overload method and a test method

<T, R> R ifNotEmpty(T o, Function<T, R> function)
<T> void ifNotEmpty(T o, Consumer<T> consumer)
void test(String) {
//do nothing
}

when I call

ifNotEmpty("aaa", x -> test(x));

I get an error

Ambiguous method call. Both
ifNotEmpty(String,Function<String, R>) and
ifNotEmpty(String,Consumer<String>) match

Why?

And when I write like this,it's correct

ifNotEmpty("aaa", this::test);

like this also correct

ifNotEmpty("aaa", (String x) -> test(x));

Why?

I used JDK-1.8 and IDEA-2017.3.5-community edition