https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108218
--- Comment #6 from Steven Sun <StevenSun2021 at hotmail dot com> --- g:4df7f8c79835d56928f51f9e674d326300936e8e c++: don't do constexpr folding in unevaluated context The implicit constexpr patch revealed that we were doing constant evaluation of arbitrary expressions in unevaluated contexts, leading to failure when we tried to evaluate e.g. a call to declval. This is wrong more generally; only manifestly-constant-evaluated expressions should be evaluated within an unevaluated operand. Making this change revealed a case we were failing to mark as manifestly constant-evaluated. gcc/cp/ChangeLog: * constexpr.c (maybe_constant_value): Don't evaluate in an unevaluated operand unless manifestly const-evaluated. (fold_non_dependent_expr_template): Likewise. * decl.c (compute_array_index_type_loc): This context is manifestly constant-evaluated.
