Variable template in template class - unexpected e

问题:

Having:

struct Value
{
    template<class T>
    static constexpr T value{0};
};

---

(0) ideone

template<typename TValue>
struct Something
{
    void x()
    {
        static_assert(TValue::template value<int> == 0, "");
    }
};

int main() { Something<Value>{}.x(); return 0; } 

• Does not compile with clang++ 3.6.

  error: cannot refer to variable template 'value' without a template argument list

• Does not compile with g++ 5.2.

  error: ‘template constexpr const T Value::value’ is not a function template

---

(1) ideone

Compiles with both clang++ and g++.

struct Something
{
    void x()
    {
        static_assert(Value::template value<int> == 0, "");
    }
};

int main() { Something{}.x(); return 0; } 

---

Why does (0) fail to compile?

It seems that the issue occurs if the variable template is accessed through a template parameter (in this case, TValue). Defining a type alias for TValue or using the typename keyword does not fix the issue.

What's going on here?

回答1:

This is definitely a gcc and clang bug in their treatment of variable templates as dependent names. I submitted gcc 67248 and clang 24473.

As a workaround for now, both compilers support the old way of doing variable templates, namely if you added:

struct Value
{
    template<class T>
    static constexpr T value = 0;

    template <typename T>
    struct variable_template_ish {
        static constexpr T value = Value::value<T>;
    };
};

then the following compiles:

template<typename TValue>
struct Something
{
    void foo() {
        static_assert(TValue::template variable_template_ish<int>::value == 0, "");
    }
};

int main() { 
    Something<Value>{}.foo();
}

回答2:

I've had some headaches before when creating template class header files in c++.

Make sure you implementation of static constexpr T value{0}; is in the same header file as the declaration.