Mixing constexpr declarations and const definitions

Question

I came across the following situation:

struct Foo
{
    static constexpr char s[] = "Hello world";
};

const char Foo::s[];

This code snippet compiles with Clang 3.7 (with -std=c++11 and -std=c++14 ), but GCC (4.8, 6.0, same language settings) gives the error I would have expected:

GCC 4.8:

in.cpp:6:19: error: redeclaration ‘Foo::s’ differs in ‘constexpr’
 const char Foo::s[];
                   ^
in.cpp:3:27: error: from previous declaration ‘Foo::s’
     static constexpr char s[] = "Hello world";
                           ^
in.cpp:6:19: error: declaration of ‘constexpr const char Foo::s [12]’ outside of class is not definition [-fpermissive]
 const char Foo::s[];

GCC 6.0:

‘constexpr’ needed for in-class initialization of static data member ‘const char Foo::s [12]’ of non-integral type [-fpermissive]

I found this old question that seems to discuss mixing constexpr and const , but it focusses on whether initializers are constant expressions, rather on whether definition and declaration can differ with regard to constness.

Is it allowed to provide the definition for a constexpr T static data member as a const T ?

Answer 1

Your code is well-formed. The constexpr -specifier is not itself part of the type but adds const ([dcl.constexpr]/9), which is present in your second declaration. Although different declarations of one function (or function template) have to agree in constexpr -ness as per [dcl.constexpr]/1, no such rule exists for variable declarations.

See bug #58541 , which basically uses your example.