[Bug c++/70543] New: [6 Regression] wrong non-const error for enable_if and constexpr function

[olegendo at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=70543

            Bug ID: 70543
           Summary: [6 Regression] wrong non-const error for enable_if and
                    constexpr function
           Product: gcc
           Version: 6.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: olegendo at gcc dot gnu.org
  Target Milestone: ---

The code below compiles fine on GCC 5 (with -std=c++11 or -std=c++14), while on
GCC 6 (r234732) it says:

error: size of array 'foo' is not an integral constant-expression
   char foo[value];



#include <type_traits>
#include <tuple>

template <typename Tuple> struct X
{
  template <unsigned int I = 0> static constexpr
  typename std::enable_if<I == std::tuple_size<Tuple>::value,
                          unsigned int>::type
  calc (void)
  {
    return 0;
  }

  template <unsigned int I = 0> static constexpr
  typename std::enable_if<I < std::tuple_size<Tuple>::value,
                          unsigned int>::type
  calc (void)
  {
    return sizeof (typename std::tuple_element<I, Tuple>::type)
           + calc<I + 1> ();
  }

  static constexpr unsigned int value = calc ();

  char foo[value];
};