The following code is fine in Java 7, but poses a problem in Java 8.

static class A {
  B c(D d) { return null; }
  static B c(A a, D d) { return null; }
}

The method reference A::c of the instance method c has an implicit first parameter for the this pointer. So both methods that A::c could resolve to are compatible with BiFunction<A, D, B>, and the method reference is ambiguous.

void f(BiFunction<A, D, B> f) { ... }

error: incompatible types: invalid method reference
    f(A::c);
      ^
    reference to c is ambiguous
      both method c(A,D) in A and method c(D) in A match

Consider renaming one of the methods to avoid the ambiguity.