https://gcc.gnu.org/bugzilla/show_bug.cgi?id=80059
--- Comment #2 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
This seems to fix it:
--- a/gcc/cp/except.c
+++ b/gcc/cp/except.c
@@ -271,7 +271,7 @@ build_must_not_throw_expr (tree body, tree cond)
cond = perform_implicit_conversion_flags (boolean_type_node, cond,
tf_warning_or_error,
LOOKUP_NORMAL);
- cond = cxx_constant_value (cond);
+ cond = fold_non_dependent_expr (cond);
if (integer_zerop (cond))
return body;
else if (integer_onep (cond))
