MSVC 无法编译 SFINAE 检查

Question

是否有一些我应该注意的编译器标志？根据 GCC 和 Clang 但不是 MSVC，以下代码工作得很好并且符合预期。是什么让 std::allocator 的分配在这里如此不同？

#include <type_traits>
#include <memory>

template <typename T, typename = std::void_t<>>
constexpr bool declares_allocate = false;

template <typename T>
constexpr bool declares_allocate<T, std::void_t<decltype(&T::allocate)>> = true;

struct normal_struct {

    auto allocate() -> void {}

};

template <typename T>
struct struct_template {

    auto allocate() -> void {}

};

template <typename T>
class class_template_public {

    public:
        auto allocate() -> void {}

};

template <typename T>
class class_template_private {

    private:
        auto allocate() -> void {}

};


auto main() -> int {

    auto allocator = std::allocator<float>();
    auto memory = allocator.allocate(1024);

    static_assert(declares_allocate<normal_struct>);                    // pass
    static_assert(declares_allocate<struct_template<float>>);           // pass
    static_assert(declares_allocate<class_template_public<float>>);     // pass
//  static_assert(declares_allocate<class_template_private<float>>);    // fails
    static_assert(declares_allocate<std::allocator<float>>);            // fails when compiled by MSVC but not Clang or GCC

    allocator.deallocate(memory, 1024);

    return 0;

}

https://godbolt.org/z/GVyNQZ

Answer 1

根据 C++ 标准实现，std::allocator<T>::allocate 可以定义为：

T* allocate(size_t n, const void* hint = 0);

T* allocate(size_t n);
T* allocate(size_t n, const void* hint); // deprecating usage of hint

T* allocate(size_t n);

也就是第二种实现把allocate变成了一个重载的成员函数。而那个目前用于 MSVC's standard library:

_NODISCARD __declspec(allocator) _Ty* allocate(_CRT_GUARDOVERFLOW const size_t _Count) {
    return static_cast<_Ty*>(_Allocate<_New_alignof<_Ty>>(_Get_size_of_n<sizeof(_Ty)>(_Count)));
}

_CXX17_DEPRECATE_OLD_ALLOCATOR_MEMBERS _NODISCARD __declspec(allocator) _Ty* allocate(
    _CRT_GUARDOVERFLOW const size_t _Count, const void*) {
    return allocate(_Count);
}

这样的实现使得 &T::allocate 模棱两可，因此在替换过程中被拒绝。

MSVC 无法编译 SFINAE 检查

MSVC fails to compile SFINAE checks

c++

template-meta-programming

c++17