Help needed debugging std::is_convertible problem (PR 65760)

[Jonathan Wakely]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65760

I don't understand why commenting out *any one* of the lines marked
(1), (2), (3), (4) causes this to compile:

#include <functional>

struct C {
    C() = default;

    C(std::function<C(int)>);  // (1)
    C(std::function<C(int, int)>); // (2)
    template <class T> C operator () (T&&); // (3)
    template <class T> C operator () (T&&, T&&); // (4)
};

int main() {
    C c = C();
}

My understanding is that the SFINAE constraint on the std::function
constructor needs to check std::is_convertible<C, C> which requires C
to be complete, but why does it only complain that C is incomplete
when all of (1) - (4) are present?

G++, Clang and EDG all agree on this behaviour, so I don't think it's
a compiler bug. The libc++ std::function works fine in this case, I
don't know what it does differently so that it works.

To fix it I am considering short-circuiting the constraint to not use
is_convertible<C, C>, by doing:

__or_<is_same<From, To>, is_convertible<From, To>>

which doesn't require the types to be complete when they're the same.
Before doing that I'd really like to understand the problem properly.

I've attached a reduced version with no header dependencies, based on
the relevant parts of <type_traits> and <functional> from 4.9.2.
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