https://gcc.gnu.org/bugzilla/show_bug.cgi?id=67595
--- Comment #3 from Markus Trippelsdorf <trippels at gcc dot gnu.org> ---
Or even more:
template <class X> concept bool allocatable = requires{{new X}->X * };
template <class X> concept bool semiregular = allocatable<X>;
template <class X> concept bool readable = requires{requires semiregular<X>};
template <class> int weak_input_iterator = requires{{0}->readable};
template <class X> bool input_iterator{weak_input_iterator<X>};
template <class X> bool forward_iterator{input_iterator<X>};
template <class X> bool bidirectional_iterator{forward_iterator<X>};
template <class X>
concept bool random_access_iterator{bidirectional_iterator<X>};
void fn1(random_access_iterator);
int main() { fn1(0); }
