https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95310
--- Comment #3 from ensadc at mailnesia dot com --- When verifying the fix, I noticed a new bug: ==== template <class T> requires true using iter_reference_t = decltype(*T{}); template <typename F> struct result { using type = iter_reference_t<F>; }; template <class Out, class T> concept indirectly_writable = requires(Out&& o, T&& t) { iter_reference_t<Out>(*o) = 0; }; template<class In, class Out> requires indirectly_writable<Out, iter_reference_t<In>> void copy(In in, Out out) {} void test(const int* p, int* q) { copy(q, p); } ==== <source>: In function 'void test(const int*, int*)': <source>:19:14: error: no matching function for call to 'copy(int*&, const int*&)' 19 | copy(q, p); | ^ <source>:16:6: note: candidate: 'template<class In, class Out> requires indirectly_writable<Out, decltype(*{})> void copy(In, Out)' 16 | void copy(In in, Out out) {} | ^~~~ <source>:16:6: note: template argument deduction/substitution failed: <source>:16:6: note: constraints not satisfied <source>: In substitution of 'template<class In, class Out> requires indirectly_writable<Out, decltype(*{})> void copy(In, Out) [with In = int*; Out = const int*]': <source>:19:14: required from here <source>:10:9: required for the satisfaction of 'indirectly_writable<Out, iter_reference_t<F> >' [with Out = const int*; In = int*] <source>:10:31: in requirements with 'Out&& o', 'T&& t' [with T = int&; Out = const int*] <source>:11:29: note: the required expression 'decltype(*{})(*o)=0' is invalid 11 | iter_reference_t<Out>(*o) = 0; | ~~~~~~~~~~~~~~~~~~~~~~~~~~^~~ cc1plus: note: set '-fconcepts-diagnostics-depth=' to at least 2 for more detail ==== In <source>:10:9: required for the satisfaction of 'indirectly_writable<Out, iter_reference_t<F> >' [with Out = const int*; In = int*] Note the mismatch between `F` and `In`. GCC 10.2 prints `F = int*` instead of `In = int*`: <source>:10:9: required for the satisfaction of 'indirectly_writable<Out, iter_reference_t<F> >' [with Out = const int*; F = int*] The name `F` comes from a completely unrelated class template, but at least the names match in GCC 10.2. (Interestingly, the alias template is not expanded in this line.)