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

["daniel.kruegler at googlemail dot com" <gcc-bugzilla at gcc dot gnu dot org>]

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

--- Comment #8 from Daniel KrÃgler <daniel.kruegler at googlemail dot com> 2011-10-19 12:54:24 UTC ---
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 [..]"

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

But I think that the replacement code

complex<double> x = complex<double>(0,1)*(1*test::value);

should also require the definition of test::value. It could be, that the
compiler now reduces the scope of the analysis to the sub-expression
(1*test::value). If so, this is either a compiler defect or a core language
defect, I'm not sure. It is arguable that the description

"[..] unless it is an object that satisfies the requirements for appearing in a
constant expression [..]"

is somewhat misleading, because it focuses on the object and not on the
expression where it is part of. This could be related to CWG #712 in this case.