Concept subsumption working for functions, but not for structs-CodePudding

Apologies for potentially wrong title, that is my best guess what is happening.

I was learning some basic concepts and tried this:

#include <concepts>
#include <iostream>
#include <memory>

template<typename T>
concept eight = sizeof(T) == 8;

template<typename T>
concept basic = std::is_trivial_v<T>;

template<typename T>
requires eight<T>
constexpr void ff(){
    std::cout << "eight" << std::endl;
}

template<typename T>
requires eight<T> && basic<T>
constexpr void ff(){
    std::cout << "eight and basic" << std::endl;
}

template<typename T>
requires eight<T>
struct ffs{
};

template<typename T>
requires eight<T> && basic<T>
struct ffs{
};

What is insane to me is that I get error for struct when same stuff works for functions.

:29:10: error: requires clause differs in template redeclaration requires eight && basic ^ :24:10: note: previous template declaration is here requires eight

It could be that I am just UB NDRing in both cases but compiler does not mind in first case(not that when I remove structs from code my code seems to run as expected, including distinguishing properly what to invoke based on concepts), but that seems unlikely.

P.S. if somebody wonders why I just don't use requires instead of trivial concepts here is the answer.

CodePudding user response：

Overloading template classes wasn't allowed before concepts, and it still not allowed even with concepts. Use partial specialization:

template <typename T>
requires eight<T>
struct ffs {};

template <typename T>
requires basic<T>
struct ffs<T> {};

Or with the terse syntax:

template <eight T>
struct ffs {};

template <basic T>
struct ffs<T> {};

Note that in both cases, the constraints on the primary template automatically apply to all specializations.