constexpr operator overloading of class templates

Question

I am working with template meta programming techniques, and I'm just playing around with different methods of doing things at the moment. Here is the code:

template<const int A>
struct iwrapper
{
    static const int num = A;
};

template<int A, int B>
constexpr iwrapper<A+B> operator+(iwrapper<A>, iwrapper<B>)
{
    return iwrapper<iwrapper<A>::num + iwrapper<B>::num>();
}

int main()
{
    constexpr iwrapper<2> first;
    constexpr iwrapper<4> second;

    constexpr auto answer = first + second;
}

When I try to run this, it gives me this error message:

error: the value of 'first' is not usable in a constant expression

Can someone help me figure out why? Thanks.

max66 · Accepted Answer · 2018-08-07 12:57:06Z

1

I don't see problems in your code and it compiles without problems with my clang++ 3.8.1.

But I have your same error with my g++ 6.3.0.

Trying with newer versions of g++ (starting from g++ 7.1.0) the error disappear.

So I suppose that the error is a bug in olds versions of g++, corrected from g++ 7.1.0.

answered Aug 7, 2018 at 12:57

max66

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2 Comments

Julius Over a year ago

I am observing the same behavior of g++ 6.3.0. Interestingly, the error message disappears after explicitly defaulting the constructor iwrapper() = default;. Alternatively, passing the arguments to operator+ by value helps, too.

badola Over a year ago

The error also goes away by using constexpr iwrapper<2> first{}; i.e. changing first to first{}. Might be related to this GCC bug

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