C++17’s {} impeding SC for new method overloads

Are you the C++ experienced reader to solve the following challenge?

Given a class C (edit: covered by binary compatibility needs) with the overloaded methods foo() and foo(A a):

    class C
    {
    public:
        void foo();
        void foo(A a);
    };

Now you want to add an overload, to serve further use-cases as requested by API consumers, and going for another overload matches your API guidelines :

        void foo(B b);

But there is existing consumer code, making use of C++17, which calls

    C c;
    c.foo({});

So the new overload will not be source-compatible and break existing code, due to the ambiguity whether {} should be turned into an instance of A or B.

What could be done about this?

The goals here would be:

1. to enable API consumers to write code which works as if there are the two old and the one new overloads declared
2. any calls using the {} expression as argument are resolved to a method which emits a compiler warning to avoid that ambiguous argument and which on any next compatibility breaking occasion can be simply removed

While asking around, some approaches have been proposed, but so far none could satisfy the second goal, catching existing ambiguous argument expressions to hint API consumers to adapt them.

Would you have an idea?

Edit: Note that we cannot change A or B (might be types/classes from elsewhere), and only can add new API to C, due to binary compatibility needs.

Edit: On a second thought, similar problems also exist before C++17 already when an argument has a type which can be implicitly converted both to A and B, by implicit constructors or type operator methods.