[Bug c++/57043] [4.7/4.8/4.9 Regression] converting overloaded complex function pow in C++11 is ambiguous

[jason at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=57043

Jason Merrill <jason at gcc dot gnu.org> changed:

           What    |Removed                     |Added
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                 CC|                            |jason at gcc dot gnu.org

--- Comment #3 from Jason Merrill <jason at gcc dot gnu.org> ---
OK, this is puzzling.

The issue is that GCC can't tell that the first pow is more specialized than
the second; deduction happens fine, but when we substitute in Tp(1) for Tp(2)
and Up we still have complex<typename promote_2<Tp(1), Tp(1)>::type> for the
return type, which looks different from complex<Tp(1)>, so deduction fails and
we decide it isn't more specialized.

I can't figure out what interpretation of the standard clang/EDG are using to
make this work.  Are they actually instantiating promote_2?  That doesn't seem
to be it, as they also accept the following, which seems clearly unordered to
me:

template<typename D> struct complex { };

template <class T> struct promote_1 { typedef T type; };
template<typename T, typename U> struct promote_2 { typedef T type; };

template<typename Tp>
complex<typename promote_1<Tp>::type>
pow(const complex<Tp>& x, const complex<Tp>& y);

template<typename Tp, typename Up>
complex<typename promote_2<Tp, Up>::type>
pow(const complex<Tp>& x, const complex<Up>& y);

complex<double> (*powcc)(const complex<double>&, const complex<double>&) = pow;