Why does this template argument deduction/overload resolution fail?

Question

This code snippet fails to compile in MSVC, Clang and Gcc, (they give different error message):

int foo(int a, int b) {
    return a + b;
}

template <class Ret, class A, class B>
void foo(Ret (*)(A, B)) {

}

int main() {
    foo(foo);
    return 0;
}

Shouldn't this compile? I can't see why it fails in resolving the overloaded function or in deducing the template arguments. Any help is welcome, thanks.

PS: It compiles if the template is replaced with void foo(int (*)(int, int)), or if we rename one of foo to avoid overloading.

Answer 1

For simplicity, let's call the first overload foo1 and call the second overload foo2.

With templates, the problem is that you cannot deduce the template arguments for the outer foo, according to [temp.deduct.call]/6:

If the argument is an overload set containing one or more function templates, the parameter is treated as a non-deduced context.

Without templates, the program considers all possibilities foo1(foo1), foo1(foo2), foo2(foo1), foo2(foo2) and chooses the only viable one foo2(foo1), according to [over.over]/6:

[ Note: If f() and g() are both overloaded functions, the cross product of possibilities must be considered to resolve f(&g), or the equivalent expression f(g). — end note ]