Talk:cpp/concepts/constructible from

I wonder if the C++ concepts are meant to be friendship-boundary resistant. According to the C++20 friend statement description on cppreference.com, you cannot befriend a concept. But then there are concepts like this one.

According to this reference, the std::constructible_from concept depends on the struct/class std::is_constructible_v but this struct is not friendship-boundary resistant. For example, you could have class A and B which are friends of each-other and B has its constructors private. If A where to query B if it can construct it using some parameters, std::is_constructible_v MUST return false because the struct or class has no access to B's constructors. But I wonder, since you cannot befriend concepts, this access denial should not happen by using std::constructible_from? The access check should be done against the location of where the concept is being used and not the point in the source where the concept was defined. 2001:16B8:8A:3A00:E405:76DE:DAFB:F609 17:29, 3 October 2021 (PDT)