gcc/testsuite/g++.dg/modules/tmpl-part-req-1.h

template<typename _Iterator, typename>
struct Trait;

template<typename _Iterator>
struct Trait<_Iterator, void> {};

template<typename _Iterator>
requires  true
struct Trait<_Iterator, void>
{
  template<typename _Iter> struct __diff {};
  
  template<typename _Iter> requires true struct __diff<_Iter> {};
};