[Bug c++/70351] New: [C++11] static constexpr member cannot define with a const qualifier

[kariya_mitsuru at hotmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=70351

            Bug ID: 70351
           Summary: [C++11] static constexpr member cannot define with a
                    const qualifier
           Product: gcc
           Version: 6.0
            Status: UNCONFIRMED
          Severity: minor
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: kariya_mitsuru at hotmail dot com
  Target Milestone: ---

Please see the sample code below.

========================== sample code ==========================
struct T {};

struct S {
    static constexpr T m{};
};

const T S::m;

int main(){}
========================== sample code ==========================
========================= error message =========================
prog.cc:7:12: error: 'constexpr' needed for in-class initialization of static
data member 'const T S::m' of non-integral type [-fpermissive]
 const T S::m;
            ^
========================= error message =========================
cf. http://melpon.org/wandbox/permlink/TWKgHmFWnK07lCdm


I think that the code should be compiled successfully.
(The constexpr specifier has no effect on the type of a variable
except that it qualifies the variable as a const.)


Note that it is compiled successfully if T is an integral type.

========================== sample code ==========================
using T = int;

struct S {
    static constexpr T m{};
};

const T S::m;

int main(){}
========================== sample code ==========================

cf. http://melpon.org/wandbox/permlink/59kDO82vFfFEsLJF


Note also that it is compiled successfully if a constexpr specifier
is used in the definition.

========================== sample code ==========================
struct T {};

struct S {
    static constexpr T m{};
};

constexpr T S::m;

int main(){}
========================== sample code ==========================

cf. http://melpon.org/wandbox/permlink/pywyTowtvOVJ6sB5