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From: "daniel.kruegler at googlemail dot com" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug c++/50785] [C++0x] static constexpr double undefined reference
Date: Wed, 19 Oct 2011 12:55:00 -0000
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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D50785

--- Comment #8 from Daniel Kr=C3=BCgler <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 memb=
er,
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. Changi=
ng
the reference to std::complex by

template<class T>
struct complex {
private:
    T data[2];
public:
    constexpr complex(T r =3D 0, T i =3D 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 =3D 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 c=
ase.