[Bug c++/50785] [C++0x] static constexpr double undefined reference

["redi at gcc dot gnu.org" <gcc-bugzilla at gcc dot gnu dot org>]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=50785

--- Comment #9 from Jonathan Wakely <redi at gcc dot gnu.org> 2011-10-19 13:04:29 UTC ---
(In reply to comment #8)
> I agree that the test case should require the definition of the static member,
> the actual reason being that the constraint in 3.2 p2,
> 
> "[..] unless it is an object that satisfies the requirements for appearing in a
> constant expression [..]"

The rest of the sentence is relevant:
"... and the lvalue-to-rvalue conversion (4.1) is immediately applied."

> is not satisfied, because the operator* of complex is not constexpr.

I disagree.  It is odr-used because the lvalue-to-rvalue conversions is not
immediately applied.

In (1*test::value) the lvalue-to-rvalue conversion is immediately applied.

> Changing
> the reference to std::complex by
> 
> template<class T>
> struct complex {
> private:
>     T data[2];
> public:
>     constexpr complex(T r = 0, T i = 0) : data{r, i} {}
>     constexpr T real() { return data[0]; }
>     constexpr T imag() { return data[1]; }
> };
> 
> template<class T>
> constexpr complex<T> operator*(const complex<T>& lhs, const T& rhs)
> {
>     return complex<T>(lhs.real() * rhs, lhs.imag() * rhs);
> }
> 
> also fixes the initial problem, because now the expression
> 
> complex<double>(0,1)*test::value
> 
> satisfies the criteria to appear in a constant expression.

It is the _object_ (i.e. test:value) that needs to satisfy the criteria for
appearing in a constant expression, not the expression containing it.

test::value satisfies the criteria.