[Bug c++/71973] New: c++ handles built-in functions inconsistently

["bernd.edlinger at hotmail dot de" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=71973

            Bug ID: 71973
           Summary: c++ handles built-in functions inconsistently
           Product: gcc
           Version: 7.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: bernd.edlinger at hotmail dot de
  Target Milestone: ---

I noticed that C++ does not emit a warning when
the header defines a built-in function with a
different/wrong signature.

For instance int fork(void) is a built-in EXT-LIB function:

but I define it differently:

cat test.c
void fork();

gcc -Wall -S test.c
test3.c:1:6: warning: conflicting types for built-in function 'fork'
 void fork();
      ^~~~
however with C++ the warning does not fire:

cat test.cc
extern "C" 
void fork();

g++ -O3 -Wall -S test.cc

zero warnings.

However the fact that there is a builtin has an effect:

cat test.cc
extern "C"
void fork() __attribute__((nothrow))

void bar() throw()
{
  fork();
}

g++ -O3 -Wall -S test.cc

emits eh code in bar

which does not happen when the builtin fork is not defined,
either by
g++ -O3 -Wall -ansi -S test.cc

no eh code.

... or changing the name of fork to zork for instance.

results also in no eh code.

It works of fork is declared as
extern "C" 
void fork() throw();


or if bar is declared as
__attribute__((nothrow))
void bar ()
{
  fork();
}