[Bug c++/68585] New: [5/6 Regression] c++14 code accepted by 4.9 not accepted by 5 and 6

[doko at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=68585

            Bug ID: 68585
           Summary: [5/6 Regression] c++14 code accepted by 4.9 not
                    accepted by 5 and 6
           Product: gcc
           Version: 5.2.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: doko at gcc dot gnu.org
  Target Milestone: ---

is this code which shouldn't be accepted by 4.9?

$ cat tst.cc
#include <array>
#include <utility>

struct Pos
{
  unsigned l,c,a;

  //fix g++
// constexpr Pos(unsigned l, unsigned c, unsigned a)
// : l(l), c(c), a(a)
// {}
};

template<class T, T... Ints>
constexpr std::array<Pos, 9*9> make_grid_position(std::integer_sequence<T,
Ints...>)
{
  return std::array<Pos, 9*9>{Pos{Ints / 9, Ints % 9, (Ints / 9) / 3 * 3 +
(Ints % 9) / 3}...};
}

constexpr std::array<Pos, 9*9> make_grid_positions()
{
  return make_grid_position(std::make_index_sequence<9*9>{});
}

template<class T>
void generate_sudoku(T)
{
  constexpr std::array<Pos, 9*9> positions = make_grid_positions(); // fail
}

int main()
{
  constexpr std::array<Pos, 9*9> positions = make_grid_positions(); // ok
  generate_sudoku(1);
}

$ g++ -std=gnu++14 tst.cc 
tst.cc: In instantiation of 'void generate_sudoku(T) [with T = int]':
tst.cc:34:20:   required from here
tst.cc:28:66: error: 'std::array<Pos, 81ul>{std::__array_traits<Pos,
81ul>::_Type{Pos{0u, 0u, 0u}, Pos{0u, 1u, 0u}, Pos{0u, 2u, 0u}, Pos{0u, 3u,
1u}, Pos{0u, 4u, 1u}, Pos{0u, 5u, 1u}, Pos{0u, 6u, 2u}, Pos{0u, 7u, 2u},
Pos{0u, 8u, 2u}, Pos{1u, 0u, 0u}, Pos{1u, 1u, 0u}, Pos{1u, 2u, 0u}, Pos{1u, 3u,
1u}, Pos{1u, 4u, 1u}, Pos{1u, 5u, 1u}, Pos{1u, 6u, 2u}, Pos{1u, 7u, 2u},
Pos{1u, 8u, 2u}, Pos{2u, 0u, 0u}, Pos{2u, 1u, 0u}, Pos{2u, 2u, 0u}, Pos{2u, 3u,
1u}, Pos{2u, 4u, 1u}, Pos{2u, 5u, 1u}, Pos{2u, 6u, 2u}, Pos{2u, 7u, 2u},
Pos{2u, 8u, 2u}, Pos{3u, 0u, 3u}, Pos{3u, 1u, 3u}, Pos{3u, 2u, 3u}, Pos{3u, 3u,
4u}, Pos{3u, 4u, 4u}, Pos{3u, 5u, 4u}, Pos{3u, 6u, 5u}, Pos{3u, 7u, 5u},
Pos{3u, 8u, 5u}, Pos{4u, 0u, 3u}, Pos{4u, 1u, 3u}, Pos{4u, 2u, 3u}, Pos{4u, 3u,
4u}, Pos{4u, 4u, 4u}, Pos{4u, 5u, 4u}, Pos{4u, 6u, 5u}, Pos{4u, 7u, 5u},
Pos{4u, 8u, 5u}, Pos{5u, 0u, 3u}, Pos{5u, 1u, 3u}, Pos{5u, 2u, 3u}, Pos{5u, 3u,
4u}, Pos{5u, 4u, 4u}, Pos{5u, 5u, 4u}, Pos{5u, 6u, 5u}, Pos{5u, 7u, 5u},
Pos{5u, 8u, 5u}, Pos{6u, 0u, 6u}, Pos{6u, 1u, 6u}, Pos{6u, 2u, 6u}, Pos{6u, 3u,
7u}, Pos{6u, 4u, 7u}, Pos{6u, 5u, 7u}, Pos{6u, 6u, 8u}, Pos{6u, 7u, 8u},
Pos{6u, 8u, 8u}, Pos{7u, 0u, 6u}, Pos{7u, 1u, 6u}, Pos{7u, 2u, 6u}, Pos{7u, 3u,
7u}, Pos{7u, 4u, 7u}, Pos{7u, 5u, 7u}, Pos{7u, 6u, 8u}, Pos{7u, 7u, 8u},
Pos{7u, 8u, 8u}, Pos{8u, 0u, 6u}, Pos{8u, 1u, 6u}, Pos{8u, 2u, 6u}, Pos{8u, 3u,
7u}, Pos{8u, 4u, 7u}, Pos{8u, 5u, 7u}, Pos{8u, 6u, 8u}, Pos{8u, 7u, 8u},
Pos{8u, 8u, 8u}}}' is not a constant expression
   constexpr std::array<Pos, 9*9> positions = make_grid_positions(); // fail
                                                                  ^