[RFA:] Correct handling of NO_IMPLICIT_EXTERN_C in c-lex.c:cb_enter_file

[Hans-Peter Nilsson <hans-peter dot nilsson at axis dot com>]

In cb_enter_file, the variable flags can have one of these
values for NO_IMPLICIT_EXTERN_C (see cpp_syshdr_flags):
" 3 4", " 3" or "".

Now spot the bug, given the patch below. :-) Yep, flags[2]
should not be dereferenced without knowing that flags[0] and
flags[1] are non-zero.  Specifically, flags[0] can be zero and
flags[1] and/or flags[2] nonzero.  In this case flags[2] was
'u', for "Multiple include guards may be useful for:\n", which
was the following string in memory.  The bug shows up for me
(YMMV) during compilation of libio/iomanip.cc, where it was
claimed that the declaration and definition of a function were
in different extern "C"/"C++" contexts with a linked-in cpplib.

It would right now be safe to omit the flags[1] check because of
the given domain of flags, but it doesn't *feel* safe.  The
whole "magic string contents" checking looks brittle IMHO.

This was bootstrapped on sparc-sun-solaris2.8, which looks like
it does not define NO_IMPLICIT_EXTERN_C, but which made no
difference there, perhaps due to alignment of constant strings.
I have no dejagnu installation on that system, so I couldn't
make check.  (BTW, I'm since long having trouble bootstrapping
on that system.  I had to delete the texinfo srcdir, because I
was running into some libz-related problem.  Sorry, I do not
intend to pursue this; it might be a local problem.)

The fix seems pretty obvious, but I'll leave it to CPP hackers
to judge that.  Ok to commit anyway?

	* c-lex.c [! NO_IMPLICIT_EXTERN_C] (cb_enter_file): Check
	in_system_header and flags[1] before dereferencing flags[2].

Index: c-lex.c
===================================================================
RCS file: /cvs/gcc/egcs/gcc/c-lex.c,v
retrieving revision 1.109
diff -c -p -c -p -4 -r1.109 c-lex.c
*** c-lex.c	2000/11/06 19:10:05	1.109
--- c-lex.c	2000/11/09 23:17:07
*************** cb_enter_file (pfile)
*** 713,721 ****
    in_system_header = (flags[0] != 0);
  #ifndef NO_IMPLICIT_EXTERN_C
    if (c_header_level)
      ++c_header_level;
!   else if (flags[2] != 0)
      {
        c_header_level = 1;
        ++pending_lang_change;
      }
--- 713,721 ----
    in_system_header = (flags[0] != 0);
  #ifndef NO_IMPLICIT_EXTERN_C
    if (c_header_level)
      ++c_header_level;
!   else if (in_system_header && flags[1] != 0 && flags[2] != 0)
      {
        c_header_level = 1;
        ++pending_lang_change;
      }

brgds, H-P