[Bug c++/98142] fstrict-enums optimization applied only for unscoped enums with unfixed underlying type

["redi at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98142

--- Comment #4 from Jonathan Wakely <redi at gcc dot gnu.org> ---
(In reply to Barry Revzin from comment #2)
> I guess in general this is kind of a scary optimization, since it doesn't
> seem like it's really a global thing? Perhaps this calls for an attribute?
> 
> [[gnu::i_promise_on_penalty_of_ub_that_only_these_values_are_used]]
> enum class E : unsigned int { A, B, C, D };
> 
> Or, you know, [[gnu::strict_enum]] or something.

It couldn't be called "strict_enum". The -fstrict-enums flag means assume no
invalid values, i.e. nothing outside the valid values of the enum type. For any
enum type with a fixed underlying type, all values of the underlying type are
valid values of the enum type, so there are no invalid values. So "strict_enum"
as the attribute name would be contradicting the meaning of -fstrict-enums.

So purely on that basis, the better name would be
"i_promise_on_penalty_of_ub_that_only_these_values_are_used"