[Bug driver/99896] New: g++ drops -lc

["vries at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99896

            Bug ID: 99896
           Summary: g++ drops -lc
           Product: gcc
           Version: 10.2.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: driver
          Assignee: unassigned at gcc dot gnu.org
          Reporter: vries at gcc dot gnu.org
  Target Milestone: ---

[ Spinoff from gdb PR https://sourceware.org/bugzilla/show_bug.cgi?id=27681 . ]

Consider the following test-case, consisting of:
...
$ cat main.c 
#include <assert.h>
#include <stddef.h>

#ifndef _GNU_SOURCE
#define _GNU_SOURCE
#endif
#include <regex.h>

extern void foo (void);

int
main (void)
{
  regex_t re;

  int res = regcomp (&re, "bla", 0);
  assert (res == 0);

  int res2 = regexec (&re, "bla", 0, NULL, 0);
  assert (res2 == 0);

  regoff_t res3 = re_search (&re, "bla", 3, 0, 3, NULL);
  assert (res3 == 0);

  foo ();

  return 0;
} 
...
and:
...
$ cat foo.c 
#include <assert.h>
#include <stddef.h>

#include <pcre2posix.h>

extern void foo (void);

void
foo (void)
{
  regex_t re;

  int res = pcre2_regcomp (&re, "bla", 0);
  assert (res == 0);

  int res2 = pcre2_regexec (&re, "bla", 0, NULL, 0);
  assert (res2 == 0);
}
...

We can compile with gcc and run like this:
...
$ gcc main.c -lc foo.c -lpcre2-posix
$ ./a.out 
$
...

likewise, with clang:
...
$ clang main.c -lc foo.c -lpcre2-posix
$ ./a.out 
$ 
...

likewise, with clang++:
...
$ clang++ -x c++ main.c -lc foo.c -lpcre2-posix
$ ./a.out 
$
...

but with g++:
...
$ g++ -x c++ main.c -lc foo.c -lpcre2-posix
$ ./a.out 
Segmentation fault (core dumped)
$
...

Using -v, we can see what goes wrong.  With gcc, we have:
...
collect2 ... main.o -lc foo.o -lpcre2-posix ...
...

With g++, we have instead:
...
collect2 ... main.o foo.o -lpcre2-posix ...
...

Workaround: use -Wl:
...
$ g++ -x c++ main.c -Wl,-lc foo.c -lpcre2-posix
$ ./a.out 
$
...