[Bug c++/56065] New: Constant expression incorrectly recognized not as such

[lucdanton at free dot fr]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=56065

             Bug #: 56065
           Summary: Constant expression incorrectly recognized not as such
    Classification: Unclassified
           Product: gcc
           Version: 4.8.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: lucdanton@free.fr


$ gcc-snapshot --version
gcc-snapshot (Debian 20130113-1) 4.8.0 20130113 (experimental) [trunk revision
195136]

$ cat main.cpp 
#include <type_traits>

template<typename T>
constexpr T id(T t) { return t; }

template<int I>
using Alias = std::integral_constant<int, I>;

template<int I>
std::integral_constant<int, id(I)> f()
{ return {}; }

template<int I>
Alias<id(I)> g()
{ return {}; }

int main()
{
    // Fine
    constexpr auto I = id(5);
    (void)I;

    // Also fine
    f<0>();

    // error: integral expression 'id<int>(0)' is not constant
    g<0>();
}