概念要求和非直接上下文

Question

我正在学习 C++ 概念，并试图理解为什么 the following does not compile（这只是一个证明这一点的琐碎且毫无意义的示例；已使用 GCC-11.1 进行测试）：

#include <iostream>

struct non_negatable {
};

template <class T>
auto negate(T a) {
    return -a;
}

template <class T>
concept is_negatable = requires(T t) {
    //just `-a;` would do the job of course
    negate(t);
};

template <class T>
auto do_negate(T) {
    std::cout << "here we are\n";
}

template <is_negatable T>
auto do_negate(T t) {
    return negate(t);
}

int main() {
    non_negatable x;
    do_negate(x);
}

上述概念试图调用函数模板，其实现将无法编译。这个事实会导致“硬错误”，而不是使概念失败。

我猜它是这样工作的，因为概念要求可能仅因表达式的“直接上下文”格式不正确（就像我们可以在 SFINAE 中观察到的那样）而失败。

我的理解对吗？标准在哪里描述了这一点？有没有一种方法可以使基于功能模板实现格式不正确的概念“失败”？

Answer 1

首先，正如您所提到的，negate<non_negatable> 不是替换失败，因为错误不在 C++20 标准的 immediate context of the function, as such it's ill-formed. From 13.10.3.1/8 中：

If a substitution results in an invalid type or expression, type deduction fails.

An invalid type or expression is one that would be ill-formed, with a diagnostic required, if written using the substituted arguments.

[Note 4: If no diagnostic is required, the program is still ill-formed. Access checking is done as part of the substitution process. — end note]

Only invalid types and expressions in the immediate context of the function type, its template parameter types, and its explicit-specifier can result in a deduction failure.

[Note 5: The substitution into types and expressions can result in effects such as the instantiation of class template specializations and/or function template specializations, the generation of implicitly-defined functions, etc. Such effects are not in the “immediate context” and can result in the program being ill-formed. — end note]

其次，7.5.7.1/6 说：

[...]

[Note 1: If a requires-expression contains invalid types or expressions in its requirements, and it does not appear within the declaration of a templated entity, then the program is ill-formed. — end note]

[...]

所以，你的理解对我来说似乎是正确的。但是，您在上一个问题中提出的问题归结为是否有一种方法可以使模板函数主体中的无效表达式导致替换失败而不是 ill-formed 程序。我觉得不可能。

概念要求和非直接上下文

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