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

[sausage at tehsausage dot com]

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

           Summary: Parameter packs not expanding consistently in function
                    return types
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
        AssignedTo: unassig...@gcc.gnu.org
        ReportedBy: saus...@tehsausage.com


Non-variadic templates passed to a function as "template <class ...> class"
have inconsistent behavior when provided with a parameter pack as an argument
and used as a return type than when used in the body of the function in the
same way. I am unsure on the expected behavior or whether passing templates in
such a way is even allowed.


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...); }

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


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()
{
   baz(1, 2.0); // OK, returns pair<int, double>
}