[Bug plugins/70757] New: Add plugin callbacks that run early enough to check for declarations using "bad" names

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=70757

            Bug ID: 70757
           Summary: Add plugin callbacks that run early enough to check
                    for declarations using "bad" names
           Product: gcc
           Version: 7.0
            Status: UNCONFIRMED
          Severity: enhancement
          Priority: P3
         Component: plugins
          Assignee: unassigned at gcc dot gnu.org
          Reporter: redi at gcc dot gnu.org
  Target Milestone: ---

I once tried to write a gcc plugin to check that libstdc++ was not defining any
non-standard names outside the reserved namespace, but quickly realised that
the plugin callbacks all happen too late, so that uninstantiated templates are
never seen by the plugin.

I also wanted it to check for the list of identifiers that are in the reserved
namespace, but need to be avoided in libstdc++ for other reasons, as
documetented at
https://gcc.gnu.org/onlinedocs/libstdc++/manual/source_code_style.html#coding_style.bad_identifiers

I would like a callback in the front-end so that names of decls could be
checked, and ideally the same thing during preprocessing so that macros can
also be checked.

This would help avoid/find bugs like PR 70727 and PR 64135, and would also
allow users to write the opposite plugin, to check that user code doesn't
define anything using reserved names, solving PR 51437.

See also https://gcc.gnu.org/ml/gcc/2016-04/msg00200.html