https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104823
--- Comment #2 from Patrick Palka <ppalka at gcc dot gnu.org> --- r12-6075 is exposing a latent bug with non-dependent decltype folding, namely we instantiate the expression only if it's potentially constant. In this case, id(v) is not potentially constant because id is not a constexpr function, so we discard the expression without actually instantiating it and therefore never issue the narrowing warning.
