如何让MSVC在C++20编译期识别CRTP中的static constexpr成员？

背景 #

尝试在CRTP基类的编译期访问派生类的constexpr静态成员，以下是最小可复现代码：

template <class T>
struct CRTP {
    static constexpr int get_value() { 
        return T::value ; 
    }

    static constexpr int crtp_value = get_value();
};

struct Derived: CRTP<Derived> {
     static constexpr int value = 2;
};

int main() {
    static_assert(Derived::crtp_value == 2, "Should result in 2");
    return 0; 
}

该代码在GCC、Clang中可正常编译，但在MSVC中失败。MSVC的报错信息如下：

example.cpp
<source>(3): error C2039: 'value': is not a member of 'Derived'
<source>(7): note: see declaration of 'Derived'
<source>(3): note: the template instantiation context (the oldest one first) is
<source>(7): note: see reference to class template instantiation 'CRTP<Derived>' being compiled
<source>(3): note: while compiling class template member function 'int CRTP<Derived>::get_value(void)'
<source>(3): error C2065: 'value': undeclared identifier
<source>(4): error C3615: constexpr function 'CRTP<Derived>::get_value' cannot result in a constant expression
<source>(3): note: failure was caused by control reaching the end of a constexpr function
<source>(12): error C2131: expression did not evaluate to a constant
<source>(12): note: failure was caused by a read of an uninitialized symbol
<source>(12): note: see usage of 'crtp_value'
Compiler returned: 2

原因 #

MSVC对类模板实例化时的类型完整性检查更严格：当实例化CRTP<Derived>时，Derived类还处于不完全定义状态（因为它继承自CRTP<Derived>，基类的初始化会优先于派生类成员的解析），此时编译器无法找到Derived::value。而GCC和Clang会延迟对T::value的查找，直到Derived类完全定义后再解析。

解决方案 #

方案1：将静态变量改为constexpr函数 #

把crtp_value从静态变量改为constexpr成员函数，函数调用会触发延迟查找，直到Derived完全定义：

template <class T>
struct CRTP {
    static constexpr int get_value() { 
        return T::value ; 
    }

    static constexpr int crtp_value() { return get_value(); }
};

struct Derived: CRTP<Derived> {
     static constexpr int value = 2;
};

int main() {
    static_assert(Derived::crtp_value() == 2, "Should result in 2");
    return 0; 
}

方案2：使用inline constexpr变量（C++17及以上） #

将crtp_value声明为inline constexpr，C++17标准允许inline静态变量的初始化延迟到类完全实例化后，MSVC会正确处理这种延迟初始化：

template <class T>
struct CRTP {
    static constexpr int get_value() { 
        return T::value ; 
    }

    static inline constexpr int crtp_value = get_value();
};

struct Derived: CRTP<Derived> {
     static constexpr int value = 2;
};

int main() {
    static_assert(Derived::crtp_value == 2, "Should result in 2");
    return 0; 
}

方案3：通过依赖类型的表达式触发延迟查找 #

修改get_value，用依赖于T的表达式让编译器延迟查找value：

#include <utility>

template <class T>
struct CRTP {
    static constexpr int get_value() { 
        return decltype(T::value){}; 
    }

    static constexpr int crtp_value = get_value();
};

struct Derived: CRTP<Derived> {
     static constexpr int value = 2;
};

int main() {
    static_assert(Derived::crtp_value == 2, "Should result in 2");
    return 0; 
}

内容的提问来源于stack exchange，提问作者Krupip