Created attachment 36077 [details]
Preprocessed test case

r226205 fails to compile this correct program:

#include <stl2/iterator.hpp>

template <stl2::WeakOutputIterator<int> I>
  requires !stl2::WeakInputIterator<I>()
constexpr bool dispatch() { return false; }

template <stl2::WeakInputIterator I>
constexpr bool dispatch() { return true; }

template <stl2::WeakOutputIterator<int> I>
constexpr bool is_weak_out() { return true; }

template <stl2::WeakInputIterator I>
constexpr bool is_weak_in() { return true; }

int main() {
  static_assert(is_weak_out<int*>());
  static_assert(is_weak_in<int*>());
  static_assert(dispatch<int*>());
}

with error:

~/concept-gcc/bin/g++ -std=gnu++1z -I ~/cmcstl2/include -I ~/cmcstl2/meta/include foo.cpp -c
foo.cpp: In function ‘int main()’:
foo.cpp:19:32: error: call of overloaded ‘dispatch()’ is ambiguous
   static_assert(dispatch<int*>());
                                ^
foo.cpp:5:16: note: candidate: constexpr bool dispatch() [with I = int*]
 constexpr bool dispatch() { return false; }
                ^
foo.cpp:8:16: note: candidate: constexpr bool dispatch() [with I = int*]
 constexpr bool dispatch() { return true; }
                ^

The compiler apparently considers both overloads of dispatch to be viable despite that the first overload's constraint !stl2::WeakInputIterator<I>() is clearly not satisfied when I is int*.