Re: Possible 4.4.0 regression (can't match template ctor of template class where...)

[Sebastian Redl <sebastian dot redl at getdesigned dot at>]

Simon Hill wrote:
> g++ doesn't seem able to match a template ctor of a template class
> where the ctor input is an internal class defined inside any template
> class.
>
> I briefly skimmed the titles of the current regression issues and I
> didn't notice any that matched this, but I'm not 100% sure.
>
> I first posted this here thinking it was my code that was somehow wrong.
> http://www.gamedev.net/community/forums/topic.asp?topic_id=526736
>
> demo.cpp
> ========================
> template <typename T>
> class CFoo
>   {
>     public:
>       class CZep {};
>
>       CFoo(int); // line 7.
>       template <typename Z> CFoo(typename CFoo<Z>::CZep); // ## ctor I want ##
>       ~CFoo();
>
>       CZep zep();
>   };
>   
Imagine the compiler has to deal with this specialization of CFoo:

template <>
class CFoo<something> {
  public:
    typedef CFoo<int>::CZep CZep;

    // otherwise identical
};

then the call

CFoo<something>::CZep cz;
CFoo<something> cf(cz);

is ambiguous - both of these are valid:
CFoo<something>::CFoo<something>(CFoo<something>::CZep)
CFoo<something>::CFoo<int>(CFoo<int>::CZep)



The language cuts this short, though. Containing types of types in the
argument do not take part in template argument deduction. Z cannot be
deduced from the arguments; it must be specified explicitly. Since
that's not possible for a constructor (or at least I don't know how),
the constructor is completely unusable.

GCC 4 is right here.

Sebastian