不满足需要静态模板方法的模板模板概念的约束

Question

我正在尝试使用C++概念实现Functor和其他各种范畴论概念，但遇到编译错误：

http://coliru.stacked-crooked.com/a/e8b6eb387229bddf

这是我的完整代码(我知道要求fmap<int, int>不会验证任何两种类型的fmap，我计划将其更改为fmap<int, std::string>或其他东西以实现稍强的测试-或者相反，可能更改Functor概念，以便除了F之外，还需要两种类型T和U并验证fmap<T, U>的存在， 但这一切都是在我想出如何解决我得到的错误之后(：

#include <functional>
#include <iostream>
#include <vector>
// empty Functor_Impl struct - specialize for each functor
template<template<class> class F> struct Functor_Impl {};
// std::vector Functor implementation
template<>
struct Functor_Impl<std::vector> {
template<class T, class U>
static std::vector<U> fmap(std::vector<T> x, std::function<U(T)> f) {
std::vector<U> out;
out.reserve(x.size());
for (int i = 0; i < x.size(); i++) {
out.push_back(f(x[i]));
}
return out;
}
};
// Functor concept requires Functor_Impl<F> to have fmap
template<template<class> class F>
concept bool Functor = requires(F<int> x) {
{Functor_Impl<F>::template fmap<int, int>(x)} -> F<int>;
};
// Test function using constraint.
template<template<class> class F, class T>
requires Functor<F>
F<T> mult_by_2(F<T> a) {
return Functor_Impl<F>::template fmap<T, T>(a, [](T x) {
return x * 2;
});
}
int main() {
std::vector<int> x = {1, 2, 3};
std::vector<int> x2 = mult_by_2(x);
for (int i = 0; i < x2.size(); i++) {
std::cout << x2[i] << std::endl;
}
}

和编译错误：

lol@foldingmachinebox:~/p/website-editor$ g++ foo.cpp -std=c++17 -fconcepts -o foo
foo.cpp: In function ‘int main()’:
foo.cpp:39:38: error: cannot call function ‘F<T> mult_by_2(F<T>) [with F = std::vector; T = int]’
std::vector<int> x2 = mult_by_2(x);
^
foo.cpp:31:6: note:   constraints not satisfied
F<T> mult_by_2(F<T> a) {
^~~~~~~~~
foo.cpp:24:14: note: within ‘template<template<class> class F> concept const bool Functor<F> [with F = std::vector]’
concept bool Functor = requires(F<int> x) {
^~~~~~~
foo.cpp:24:14: note:     with ‘std::vector<int> x’
foo.cpp:24:14: note: the required expression ‘Functor_Impl<F>::fmap<int, int>(x)’ would be ill-formed

我猜我对这个概念本身的语法是错误的 - 它将变量视为函数，反之亦然，因为我不太熟悉concept语法，此外，cppreference.com上的一些示例代码无法在 GCC 的实现下编译(例如concept EqualityComparable不编译，则必须更改为concept bool EqualityComparable(。

如果我从mult_by_2函数声明中删除requires Functor<F>，则代码将编译并运行。

Answer 1

问题正是错误消息所说的：Functor_Impl<F>::template fmap<int, int>(x)不是有效的表达式。Functor_Impl<std::vector>::fmap有两个参数，而不是一个。