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A similar question of non-templated class

For a template class,

template <typename T>
struct Test {
    T data;

    static const Test constant;
};

it's fine when defining a static constexpr member variable of specialized type:

template <>
inline constexpr Test<int> Test<int>::constant {42};

https://godbolt.org/z/o4c4YojMf

While results are diverged by compilers when defining a static constexpr member directly from a template class without instantiation:

template <typename T>
inline constexpr Test<T> Test<T>::constant {42};

https://godbolt.org/z/M8jdx3WzM

GCC compiles.
clang ignores the constexpr specifier in definition.
MSVC... /std:c++17 works well, but /std:c++20 rejects due to redefinition.

I have learnt that constexpr can be applied to the definition a variable or variable template

And in this example, which one is right? Why?

zjyhjqs
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  • This is not a member variable template. It is just a member variable. Only `Test` is a template. So naively I don't see why it should be any different from the non-template case. – user17732522 May 27 '22 at 17:52
  • @user17732522 You have to instantiate it for every template type that gets used. Except with `inline constexpr` you should be able to do that in the class so it happens automatically. – Goswin von Brederlow May 27 '22 at 17:56
  • It seems that Clang thinks that it doesn't need to implicitly instantiate the definition to determine whether a initializing declaration is reachable. With an explicit instantiation it doesn't complain anymore: https://godbolt.org/z/r15EEnEnY. MSVC's C++20 behavior looks like a bug to me. – user17732522 May 27 '22 at 18:20
  • @user17732522 MSVC vigorously follows the idea that `const` and `constexpr` aren't same thing. Member variable specialization have to have same storage qualifiers and member functions - same exception qualifiers.It's consistent among other attempts to specialize a template member. – Swift - Friday Pie May 28 '22 at 15:01

1 Answers1

3

I think GCC is correct in accepting the given example. This is because the static data member named constant is an ordinary data member variable(although it is still considered a templated entity).

And since constant is an ordinary data member variable, dcl.constexpr#1.sentence-1 is applicable to it:

The constexpr specifier shall be applied only to the definition of a variable or variable template or the declaration of a function or function template.

(emphasis mine)

Thus, as the construct that you have provided after the class template is nothing but an out-of-class definition for the ordinary static data member constant, the given example is well-formed.

template <typename T>
constexpr Test<T> Test<T>::constant {42};  //this is an out-of-class definition for the ordinary static data member variable `constant`

Note

Note that dcl.constexpr#1.sentence-1 doesn't mention templated entity, but instead only mention "variable" and "variable template" and since constant is a variable(ordinary), the same should be applicable to constant.

Jason
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