[Bug c++/68309] New: [C++14] Expanding a captured parameter pack with std::forward(args) fails.

[ralph.tandetzky at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=68309

            Bug ID: 68309
           Summary: [C++14] Expanding a captured parameter pack with
                    std::forward<decltype(args)>(args) fails.
           Product: gcc
           Version: 4.9.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: ralph.tandetzky at gmail dot com
  Target Milestone: ---

Here's a minimal not-working example:

    #include <iostream>

    using namespace std;

    template <typename ...Ts>
    void print( Ts &&... args )
    {
        [&]
        {
            const auto _ = { 
                ((cout << forward<decltype(args)>(args) << endl),0)... };
            (void)_;
        }();    
    }

    int main()
    {
        print( 1, "blub", std::to_string(3.1415), 4, 5.2 );
    }

This code fails with GCC 4.9. GCC 5.2 even crashes with a segfault. Clang 3.6
compiles it and gives the expected console output:

    1
    blub
    3.141500
    4
    5.2

The curious thing is: When I remove the wrapping lambda, i.e. the line 

    [&]
    {
    }();

then is compiles fine in all cases. Also when I replace decltype(args) with Ts,
then it compiles fine and produces the correct console output. Go figure!