[Bug c++/57699] New: Disable empty parameter list misinterpretation in libstdc++ headers when !defined(NO_IMPLICIT_EXTERN_C)

[Bug c++/57699] New: Disable empty parameter list misinterpretation in libstdc++ headers when !defined(NO_IMPLICIT_EXTERN_C)

[redi at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=57699

            Bug ID: 57699
           Summary: Disable empty parameter list misinterpretation in
                    libstdc++ headers when !defined(NO_IMPLICIT_EXTERN_C)
           Product: gcc
           Version: 4.9.0
            Status: UNCONFIRMED
          Severity: enhancement
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: redi at gcc dot gnu.org

In cp/parser.c the function cp_parser_parameter_declaration_clause has:

  else if (token->type == CPP_CLOSE_PAREN)
    /* There are no parameters.  */
    {
#ifndef NO_IMPLICIT_EXTERN_C
      if (in_system_header && current_class_type == NULL
          && current_lang_name == lang_name_c)
        return NULL_TREE;
      else
#endif
        return void_list_node;
    }

This means that on "implicit extern C" systems (ones not known to have C++
compatible libc headers) we interpret void(*)() as void(*)(...)

This might be necessary to properly handle libc headers on those systems, but
should not be enabled for libstdc++ headers (or more generally, any headers
actually written in C++) because when we write void(*)() in C++ we damn well
mean what we wrote, we shouldn't have to say void(*)(void)

This has caused problems more than once in libstdc++, where correct C++ code in
an explicit extern "C" block is misinterpreted by the front end, because
libstdc++ headers are system headers (due to the #pragma we use) and we
sometimes have to declare functions in extern "C" blocks.  The problems are not
found immediately because they only happen on less-tested targets such as AIX
and eCos:
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=57691
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=50982#c40

It would be nice to have some way to mark libstdc++ headers to prevent the code
above returning NULL_TREE.

[Bug c++/57699] Disable empty parameter list misinterpretation in libstdc++ headers when !defined(NO_IMPLICIT_EXTERN_C)

[pinskia at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=57699

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
eCos  is open source so that should be fixed.

The easy fix for the libstdc++ headers is to use void as the argument.

[Bug c++/57699] Disable empty parameter list misinterpretation in libstdc++ headers when !defined(NO_IMPLICIT_EXTERN_C)

[bernd.edlinger at hotmail dot de]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=57699

Bernd Edlinger <bernd.edlinger at hotmail dot de> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |bernd.edlinger at hotmail dot de

--- Comment #3 from Bernd Edlinger <bernd.edlinger at hotmail dot de> ---
The eCos libc headers are 100% C++ compatible,
in fact the whole system is based on C++.

Is this NO_IMPLICIT_EXTERN_C define set by the
configure script? Should I make sure that the
configure script sets NO_IMPLICIT_EXTERN_C?

[Bug c++/57699] Disable empty parameter list misinterpretation in libstdc++ headers when !defined(NO_IMPLICIT_EXTERN_C)

[redi at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=57699

--- Comment #4 from Jonathan Wakely <redi at gcc dot gnu.org> ---
(In reply to Bernd Edlinger from comment #3)
> Is this NO_IMPLICIT_EXTERN_C define set by the
> configure script?

It's set by headers under gcc/config/

> Should I make sure that the
> configure script sets NO_IMPLICIT_EXTERN_C?

Yes, but whether eCos defines it is not really relevant to this PR, which is
about the general feature not whether it's used on a given platform or not.

[Bug c++/57699] Disable empty parameter list misinterpretation in libstdc++ headers when !defined(NO_IMPLICIT_EXTERN_C)

[bernd.edlinger at hotmail dot de]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=57699

--- Comment #5 from Bernd Edlinger <bernd.edlinger at hotmail dot de> ---
Well, if a portable O/S like eCos would need such special treatment,
the NO_IMPLICIT_EXTERN_C should not be bound to the target architecture,
it would be far more appropriate to define the NO_IMPLICIT_EXTERN_C
from the configure command line instead.

Actually I would have expected that some fixinclude scripts should
be able to fix this kind of coding errors in the header files.

Are you sure we still need that kind of hack ?

[Bug c++/57699] Disable empty parameter list misinterpretation in libstdc++ headers when !defined(NO_IMPLICIT_EXTERN_C)

[redi at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=57699

--- Comment #6 from Jonathan Wakely <redi at gcc dot gnu.org> ---
(In reply to Bernd Edlinger from comment #5)
> Well, if a portable O/S like eCos would need such special treatment,

eCos doesn't need it

[Bug c++/57699] Disable empty parameter list misinterpretation in libstdc++ headers when !defined(NO_IMPLICIT_EXTERN_C)

[bernd.edlinger at hotmail dot de]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=57699

--- Comment #7 from Bernd Edlinger <bernd.edlinger at hotmail dot de> ---
(In reply to Jonathan Wakely from comment #6)
> (In reply to Bernd Edlinger from comment #5)
> > Well, if a portable O/S like eCos would need such special treatment,
> 
> eCos doesn't need it

Of course. In that case, it would be much better, to be able to *disable*
the implicit extern "C" feature for X86 and ARM and whatever architecture,
just because I certainly know it when I call configure.

To my surprise a cross compiler for --target=arm-eabi has a completely
different syntax in the system headers than one for --target=i686-pc-linux-gnu.

In my eyes, configuring that somewhere in gcc/config is awkward in that
use case. Do you see my point?

[Bug c++/57699] Disable empty parameter list misinterpretation in libstdc++ headers when !defined(NO_IMPLICIT_EXTERN_C)

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=57699

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Target Milestone|---                         |9.0
         Resolution|---                         |FIXED
             Status|NEW                         |RESOLVED

--- Comment #9 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
This was removed in GCC 9 by r9-2724.