Unexpected overload resolution with default function template parameter

I experience overload behavior that seems very unexpected. The following code is rejected with an ambiguity error for both gcc and clang:

template <typename T>
struct A
{
    typedef T key_type;
};

template <typename T>
void foo(A<T> rng, T val);

template <typename T, typename U = T>
void foo(T, typename U::key_type);

int main()
{
    A<int> i;
    foo(i, 0);
}

Error:

test.cpp:16:5: error: call to 'foo' is ambiguous
    foo(i, 0);
    ^~~
test.cpp:8:6: note: candidate function [with T = int]
void foo(A<T> rng, T val);
     ^
test.cpp:11:6: note: candidate function [with T = A<int>, U = A<int>]
void foo(T, typename U::key_type);
     ^

I would expect both to be exact matches, but the first overload is to win in partial order, since the first parameter is A<T>more specialized than T.

Which makes me think that if I change the second signature to:

template <typename T, typename U = T>
void foo(T, typename T::key_type);

and gcc and clang now take the code and select the first overload, as I expected initially.

, : , , , (U) (T).

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