An incomplete nested static constant. Really C ++ or not?

Question

Clang and GCC disagree on whether the following C++11 code is valid or not:

 struct Thing { int value; static const Thing thing; }; constexpr Thing Thing::thing {3}; 

Clang compiles it, and GNU GCC version 4.7.2 says:
error: redeclaration 'Thing::thing' differs in 'constexpr' constexpr Thing Thing::thing {3};

Which compiler interprets the standard correctly? And is it likely that the C++14 standard will have something new to say about this issue?

Answer 1

Introduction

The snippet is legal C ++ 11, and there are no changes in C ++ 14.

^{Note : gcc 4.9.0 correctly accepts the fragment, i.e. refusing a fragment was a bug in previous versions of the compiler.}

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Explanation

The constexpr specifier certainly affects the object being defined (it will be declared as const), but, in particular, the specifier indicates that the declared object

• is a literal type and
• has an initializer and;
• that the initializer is a constant expression.

 struct Thing { int value; static const Thing thing; // (A) }; constexpr Thing Thing::thing {3}; // (B) 

This means that the definition of (B) refers to an object of type Thing const with the name thing, which matches (A), therefore; this piece is legal.

7.1.5p9 constexpr [dcl.constexpr] ( n3337 )

A constexpr qualifier used in declaring an object declares the object as const. . Such an object must be of literal type and must be initialized.