clang vs gcc CRTP：constexpr 变量不能有非文字类型

Question

我这里有一个 CRTP 模板 class：

template <typename S>
class Base
{
    public:
    constexpr static S NOT_SET{0};
};

struct Derived : public Base<Derived>
{
};

Clang (5.0.0) 不接受这个：

5 : <source>:5:24: error: constexpr variable cannot have non-literal type 'const Derived'
    constexpr static S NOT_SET{0};
                       ^
8 : <source>:8:25: note: in instantiation of template class 'Base<Derived>' requested here
struct Derived : public Base<Derived>
                        ^
5 : <source>:5:24: note: incomplete type 'const Derived' is not a literal type
    constexpr static S NOT_SET{0};
                       ^
8 : <source>:8:8: note: definition of 'Derived' is not complete until the closing '}'
struct Derived : public Base<Derived>
       ^
1 error generated.
Compiler exited with result code 1

但是 gcc（在 4.9.2 和 6.2 上测试）接受它就好了。

如何在 clang 中执行此操作？

Answer 1

当您尝试在 class 模板 Base 中使用

Derived 时，它不是一个完整的类型，因此您实际上不能那样使用它。那是因为类型必须是完整的才能声明该类型的变量。没有办法解决它。
总而言之，一个类型在结束时完成 }（以及与您的情况无关的其他异常，例如在其成员函数中）。
标准是这样说的(working draft):

A class is considered a completely-defined object type (or complete type) at the closing } of the class-specifier.
Within the class member-specification, the class is regarded as complete within function bodies, default arguments, noexcept-specifiers, and default member initializers (including such things in nested classes).
Otherwise it is regarded as incomplete within its own class member-specification.

因此 clang 是正确的，错误说的差不多。

如评论中所述，存在一种解决方法。只要派生类型是（让我说）constexpr constructible，你就可以在基础 class 中定义一个 constexpr 函数，returns 你是它的 未设置版本（不管它是什么意思）。

clang vs gcc CRTP：constexpr 变量不能有非文字类型

clang vs gcc CRTP: constexpr variable cannot have non-literal type

c++

gcc

clang

constexpr

crtp