[Bug stdio/24998] New: #includes included inside extern C {} regions

["nathan at acm dot org" <sourceware-bugzilla at sourceware dot org>]

https://sourceware.org/bugzilla/show_bug.cgi?id=24998

            Bug ID: 24998
           Summary: #includes included inside extern C {} regions
           Product: glibc
           Version: unspecified
            Status: UNCONFIRMED
          Severity: normal
          Priority: P2
         Component: stdio
          Assignee: unassigned at sourceware dot org
          Reporter: nathan at acm dot org
  Target Milestone: ---

This is not just stdio, but where I fell over it.  stdio.h contains (near the
start):
__BEGIN_DECLS // extern C {

#define __need_size_t
#define __need_NULL
#include <stddef.h>
....

The stddef.h has its own extern C protection for C++ inclusion, so doesn't need
to be there.  At the moment that is harmless.

However C++20 will have modules, which includes the following 2 exciting
features:
1) header-units
2) include translation

#1 is a mechanism by which header-files can be compiled to a header-unit (there
are some restrictions on what's acceptable).  There's no reason why the C
headers could not be processed this way.

#2 turns #include directives into imports of header-units (via an
implementation-defined mechanism).  Imports are intended to be a top-level
construct, and cannot occur inside an extern "C" region.

So, <stddef.h> could be a header-unit, the #include could be translated to a
header unit, and then we find the interesting case that it's inside an extern C
region.

This is syntactically disallowed.  I do permit it as an extension, but there's
another problem, in that I need to record somewhere in the header-unit that it
only provides c-linkage entities.  I'd rather not do that :)

I can add a warning to G++ such that it tells you when a #include is inside an
extern "C" region, which will help locate the troublesome includes.   See gcc
BZ 91770

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