[Bug c++/64395] New: void_t doesn't work as expected

[benzejaa at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64395

            Bug ID: 64395
           Summary: void_t doesn't work as expected
           Product: gcc
           Version: 4.9.2
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: benzejaa at gmail dot com

Hey all!

I have what I *think* is a bug in gcc-4.9.2.  I haven't tested it against the
development version (mostly due to a lack of free time).

I was messing around with "void_t concepts", and GCC didn't give me the
expected behavior.  I compared to clang, which did give me what I expected, so
my assumption is that of a bug in GCC.

This may be a duplicate under a different name (I find it hard to believe I was
the first person to find the bug), but I couldn't find one.

Here's my test code:

    #include<iostream>
    #include<utility>
    #include<type_traits>

    template< class... > using void_t = void;
    template< class, class = void >
    struct hasSize : std::false_type {};

    template< class T >
    struct hasSize< T, void_t< decltype( std::declval<const T&>().size()) > > :
std::true_type {};

    int main()
    {
       std::cout << hasSize< std::string >::value << std::endl;
       std::cout << hasSize< char >::value << std::endl;
       return 0;
    }

compiled as "g++ -std=c++11 main.cpp", I get the output:

    1
    1

and compiled as "clang++ -std=c++11 main.cpp", I get the output:

    1
    0

As one would expect the second value to be "false", this to me indicates a bug
in g++.

Cheers!