concepts diagn

main.cpp

template<typename T> using id=T;

template<typename T>
concept bool A = requires(T a) { a==a; };

template<typename T, typename U>
concept bool B =
  A<T> && // (1) basic usage
  A<T> && // (2) duplicate concept check
  A<U> && // (3) different concept check that happens to work on the same type here
  A<id<T>>; // (4) same as above

template<typename T> requires B<T, T> void f(T){}

struct S{} s; // operator== not defined
int main(){ f(s); }

/*
main.cpp: In function ‘int main()’:
main.cpp:16:16: error: cannot call function ‘void f(T) [with T = S]’
 int main(){ f(s); }
                ^
main.cpp:13:44: note:   constraints not satisfied
 template<typename T> requires B<T, T> void f(T){}
                                            ^
main.cpp:7:14: note: within ‘template<class T, class U> concept const bool B<T, U> [with T = S; U = S]’
 concept bool B =
              ^
main.cpp:4:14: note: within ‘template<class T> concept const bool A<T> [with T = S]’
 concept bool A = requires(T a) { a==a; };
              ^
main.cpp:4:14: note:     with ‘S a’
main.cpp:4:14: note: the required expression ‘(a == a)’ would be ill-formed
main.cpp:4:14: note: within ‘template<class T> concept const bool A<T> [with T = S]’
main.cpp:4:14: note:     with ‘S a’
main.cpp:4:14: note: the required expression ‘(a == a)’ would be ill-formed
main.cpp:4:14: note: within ‘template<class T> concept const bool A<T> [with T = S]’
main.cpp:4:14: note:     with ‘S a’
main.cpp:4:14: note: the required expression ‘(a == a)’ would be ill-formed
main.cpp:4:14: note: within ‘template<class T> concept const bool A<T> [with T = S]’
main.cpp:4:14: note:     with ‘S a’
main.cpp:4:14: note: the required expression ‘(a == a)’ would be ill-formed
*/