[Bug c++/45873] Parameter packs not expanding consistently in function return types

[redi at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=45873

Jonathan Wakely <redi at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
      Known to fail|                            |4.4.3, 4.5.1, 4.6.0

--- Comment #1 from Jonathan Wakely <redi at gcc dot gnu.org> 2010-10-04 
08:11:07 UTC ---
(You didn't say which version this PR is for, although it doesn't work with any
I tried)

Here's a self-contained testcase, with a fix so the workaround actually works
(it was missing the template argument on baz<pair>)

template<class, class>
struct pair
{
    template<class T1, class U1>
        pair(T1&& t, U1&& u) { }
};

template <template <class ...> class T, class ...Args>
auto foo(Args... args) -> T<Args...>
{ return T<Args...>(args...); }

template <template <class ...> class T, class ...Args>
auto bar(Args... args) -> T<int, double>
{ return T<Args...>(args...); }


// The following work-around seems to work:
template <template <class ...> class T, class ...Args> struct expand
{ typedef T<Args...> type; };

template <template <class ...> class T, class ...Args>
auto baz(Args... args) -> typename expand<T, Args...>::type
{ return T<Args...>(args...); }


int main()
{
   // error: no matching function for call to 'foo(int, double)'
   foo<pair>(1, 2.0);
   bar<pair>(1, 2.0); // OK, returns pair<int, double>
   baz<pair>(1, 2.0); // OK, returns pair<int, double>
}


I think deduction fails for foo<pair>(1, 2.0), though I'm not sure if it
should.

For baz<pair> deduction succeeds, the template parameters appear in a
nested-name-specifier which is a non-deduced context.