Re: [PATCH] Add gnu::diagnose_as attribute

[Jason Merrill <jason@redhat.com>]

On 7/7/21 4:23 AM, Matthias Kretz wrote:
> On Tuesday, 22 June 2021 21:52:16 CEST Jason Merrill wrote:
>>> 2. About the namespace aliases: IIUC an attribute would currently be
>>> rejected because of the C++ grammar. Do you want to make it valid before
>>> WG21 officially decides how to proceed? And if you have a pointer for me
>>> where I'd have to adjust the grammar rules, that'd help.
>>
>> You will want to adjust cp_parser_namespace_alias_definition to handle
>> attributes like cp_parser_namespace_definition.  The latter currently
>> accepts attributes both before and after the name, which seems like a
>> good pattern to follow so it doesn't matter which WG21 chooses.
>> Probably best to pedwarn about C++11 attributes in both locations for
>> now, not just after.
> 
> This introduces an ambiguity in cp_parser_declaration. The function has to
> decide whether to call cp_parser_namespace_definition or fall back to
> cp_parser_block_declaration (which calls
> cp_parser_namespace_alias_definition). But now the parser has to look ahead a
> lot farther:
> 
> namespace foo [[whatever]] {}
> namespace bar [[whatever]] = foo;
> 
> I.e. only at '{' vs. '=' can cp_parser_declaration decide to call
> cp_parser_namespace_definition.
> 
> Consequently, should I really modify cp_parser_namespace_definition to handle
> namespace aliases?

aliases can also appear at block scope, unlike namespace definitions, 
but you could factor out some of the alias handling to call from both 
places.

> Or can/should cp_parser_declaration look ahead behind the
> attribute(s)? How?

cp_parser_skip_attributes_opt

Jason