Can `reinterpret_cast` be used to initialize a `constexpr` static const void pointer?

constexpr and Initialization of a Static const Void Pointer with reinterpret cast: Compiler Discrepancy

The following code snippet illustrates the issue:

struct foo {
  static constexpr const void* ptr = reinterpret_cast<const void*>(0x1);
};

Compiler Discrepancy

While this code compiles successfully in g v4.9, it fails to compile in clang v3.4. Clang generates the error:

error: constexpr variable 'ptr' must be initialized by a constant expression

Standard Compliance

According to the C 11 standard, a constexpr variable must be initialized by a constant expression. Reinterpret_cast is not a constant expression, as stated in section 5.19 paragraph 2 of the draft C 11 standard. Therefore, clang is correct in rejecting the code.

Proper Declaration

The proper way to declare a constexpr variable with a reinterpret cast is to use the __builtin_constant_p macro. This macro allows constant folding of non-constant expressions. The following modified code compiles in both g and clang:

static constexpr const void* ptr = 
  __builtin_constant_p( reinterpret_cast<const void*>(0x1) ) ? 
    reinterpret_cast<const void*>(0x1) : reinterpret_cast<const void*>(0x1)  ;

This code uses the ternary conditional operator to test if the reinterpret cast expression is a constant expression. If true, the cast is performed, otherwise the evaluation fails.

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