[Bug c++/104594] New: narrowing conversion of -1 to unsigned char at compile time not detected

["raffael at casagrande dot ch" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104594

            Bug ID: 104594
           Summary: narrowing conversion of -1 to unsigned char at compile
                    time not detected
           Product: gcc
           Version: 12.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: raffael at casagrande dot ch
  Target Milestone: ---

The current gcc trunk compiles the following piece of code:
------------------------------------------------------------
template <unsigned char DIM_FROM>
concept Geometry = (DIM_FROM == -1);

template <class INIT>
requires Geometry<INIT::n>
auto GaussNewton(const INIT& init) -> void {}

template<int N>
struct X {
  static constexpr int n = N;
};

int main() { GaussNewton(X<-1>{}); }
--------------------------------------

I think this should NOT compile since it entails a narrowing conversion of -1
to an unsigned char type at compile time. Clang as well as MSVC fail to compile
the code.
(In many other cases, gcc also fails to compile if such a narrowing conversion
happens at compile time.)