Returning a void?

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This looks valid by the draft C++11 standard, if we look at section 5.2.3 Explicit type conversion (functional notation) paragraph 2 says (emphasis mine):

The expression T(), where T is a simple-type-specifier or
typename-specifier for a non-array complete object type or the
(possibly cv-qualified) void type, creates a prvalue of the
specified type, whose value is that produced by value-initializing
(8.5) an object of type T; no initialization is done for the void()
case.[…]

the wording is pretty similar pre C++11 as well.

This okay in a constexpr even though section 7.1.5 paragraph 3 says:

The definition of a constexpr function shall satisfy the following
constraints:

and includes this bullet:

its return type shall be a literal type;

and void is not a literal in C++11 as per section 3.9 paragraph 10, but if we then look at paragraph 6 it gives an exception that fits this case, it says:

If the instantiated template specialization of a constexpr function
template or member function of a class template would fail to satisfy
the requirements for a constexpr function or constexpr constructor,
that specialization is not a constexpr function or constexpr
constructor. [ Note: If the function is a member function it will
still be const as described below. —end note ] If no specialization of
the template would yield a constexpr function or constexpr
constructor, the program is ill-formed; no diagnostic required.

As Casey noted in the C++14 draft standard void is a literal, this is section 3.9 Types paragraph 10 says:

A type is a literal type if it is:

and includes:

— void; or

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