Problems with PR 21210

[Paolo Carlini]

Mark Mitchell wrote:

> 1. Overload resolution
>
>   void f(std::complex<double>);
>   void f(__complex__ double);
>
>   double d;
>   ... f(d) ...
>
> which f should get called?  Under the semantics I used (conversion
> from double to __complex__ double is a standard conversion) we'd pick
> the second "f".  I'd say either we should use the second "f", which
> seems right to me; the other plausible choice would be to consider the
> call ambiguous.

I must be missing something, either technical about the language itself
or about the present discussion: I don't see how would be correct
picking the second 'f'; ambiguity seems also wrong. My reasoning is
that, according to the current C++ standard, we have a constructor from
a pair of doubles, each one *with a default*: the first f, by itself
certainly works. Then, we add a *non-standard* constructor, from
__complex__ double, and... boomer! it gets *preferred*!?! Please explain
in better detail.

Thanks in advance,
Paolo.