Problems with PR 21210

[Zack Weinberg]

> This also depends on somebody not myself doing a not-insubstantial
> amount of core compiler work to make it happen; and somebody else
> jimmying the libstdc++ headers to match, presumably a lesser task.

I gotta wonder how much of the way there it would get us if we could
write

  template <typename T> class complex;
  template <>
  typedef _Complex double complex<double>;

(yes, that's a typedef partially specializing a class template.)

zw