Re: [PATCH v2] c++: Accept elaborated-enum-base in system headers

[Jason Merrill <jason@redhat.com>]

On 6/14/23 09:31, Alex Coplan wrote:
> Hi,
> 
> This is a v2 patch addressing feedback for:
> https://gcc.gnu.org/pipermail/gcc-patches/2023-June/621050.html
> 
> macOS SDK headers using the CF_ENUM macro can expand to invalid C++ code
> of the form:
> 
> typedef enum T : BaseType T;
> 
> i.e. an elaborated-type-specifier with an additional enum-base.
> Upstream LLVM can be made to accept the above construct with
> -Wno-error=elaborated-enum-base.
> 
> This patch adds the -Welaborated-enum-base warning to GCC and adjusts
> the C++ parser to emit this warning instead of rejecting this code
> outright.
> 
> The macro expansion in the macOS headers occurs in the case that the
> compiler declares support for enums with underlying type using
> __has_feature, see
> https://gcc.gnu.org/pipermail/gcc-patches/2023-May/618450.html
> 
> GCC rejecting this construct outright means that GCC fails to bootstrap
> on Darwin in the case that it (correctly) implements __has_feature and
> declares support for C++ enums with underlying type.
> 
> With this patch, GCC can bootstrap on Darwin in combination with the
> (WIP) __has_feature patch posted at:
> https://gcc.gnu.org/pipermail/gcc-patches/2023-May/617878.html
> 
> Bootstrapped/regtested on aarch64-linux-gnu and x86_64-apple-darwin.
> OK for trunk?
> 
> Thanks,
> Alex
> 
> gcc/c-family/ChangeLog:
> 
>          * c.opt (Welaborated-enum-base): New.

> +Welaborated-enum-base
> +C++ ObjC++ Var(warn_elaborated_enum_base) Warning Init(1)
> +Warn if an additional enum-base is used in an elaborated-type-specifier.
> +That is, if an enum with given underlying type and no enumerator list
> +is used in a declaration other than just a standalone declaration of the
> +enum.

Just the first line of description here; the rest should go in 
doc/invoke.texi.

Jason