[Bug c/99972] New: missing -Wunused-result on a call to a locally redeclared warn_unused_result function

[Bug c/99972] New: missing -Wunused-result on a call to a locally redeclared warn_unused_result function

[msebor at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99972

            Bug ID: 99972
           Summary: missing -Wunused-result on a call to a locally
                    redeclared warn_unused_result function
           Product: gcc
           Version: 11.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: msebor at gcc dot gnu.org
  Target Milestone: ---

The test case below (related to pr99420) shows that GCC doesn't consistently
propagate function attributes across redeclarations of extern functions at
function scope.

The noreturn attribute on fnr() is copied from the declaration in gnr() to the
one in hnr() and successfully suppresses -Wreturn-type in both callers.  That's
how it's supposed to work.

But the warn_unused_result on fwur() is clearly not copied from the declaration
inb gwur() to the one in hwur() because only the call to the function in gwur()
triggers -Wunused-result but the call in hwur() doesn't.  That's a bug with the
same root cause as pr99420.

$ cat z.c && gcc -S -Wall -xc++ z.c
int gnr (void)
{ 
  __attribute__ ((noreturn)) int fnr (void);
  fnr ();
  // no return, no warning (good)
}

int hnr (void)
{
  int fnr (void);

  fnr ();
  // no return, no warning (good)
}


void gwur (void)
{
  __attribute__ ((warn_unused_result)) int fwur (void);
  fwur ();   // -Wunused-result (good)
}

void hwur (void)
{
  int fwur (void); 

  fwur ();   // missing -Wunused-result (bug)
}
z.c: In function ‘int hnr()’:
z.c:14:1: warning: no return statement in function returning non-void
[-Wreturn-type]
   14 | }
      | ^
z.c: In function ‘void gwur()’:
z.c:20:8: warning: ignoring return value of ‘int fwur()’ declared with
attribute ‘warn_unused_result’ [-Wunused-result]
   20 |   fwur ();   // -Wunused-result (good)
      |   ~~~~~^~

[Bug c/99972] missing -Wunused-result on a call to a locally redeclared warn_unused_result function

[msebor at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99972

Martin Sebor <msebor at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
      Known to fail|                            |10.2.0, 11.0, 4.7.0, 4.8.4,
                   |                            |4.9.4, 5.5.0, 6.4.0, 7.2.0,
                   |                            |8.3.0, 9.1.0
           Keywords|                            |diagnostic

--- Comment #1 from Martin Sebor <msebor at gcc dot gnu.org> ---
This never worked correctly so it's not a regression.

Clang does the right thing and warns where expected:

z.c:20:3: warning: ignoring return value of function declared with
      'warn_unused_result' attribute [-Wunused-result]
  fwur ();   // -Wunused-result (good)
  ^~~~
z.c:27:3: warning: ignoring return value of function declared with
      'warn_unused_result' attribute [-Wunused-result]
  fwur ();   // missing -Wunused-result (bug)
  ^~~~
2 warnings generated.

[Bug c/99972] missing -Wunused-result on a call to a locally redeclared warn_unused_result function

[msebor at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99972

--- Comment #2 from Martin Sebor <msebor at gcc dot gnu.org> ---
The code (obviously) needs to be compiled as C to show the C bug (the C++ front
end is also buggy but differently; pr99974 tracks that):

$ gcc -S -Wall pr99972.c
pr99972.c: In function ‘gwur’:
pr99972.c:20:3: warning: ignoring return value of ‘fwur’ declared with
attribute ‘warn_unused_result’ [-Wunused-result]
   20 |   fwur ();   // -Wunused-result (good)
      |   ^~~~~~~

[Bug c/99972] missing -Wunused-result on a call to a locally redeclared warn_unused_result function

[msebor at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99972

Martin Sebor <msebor at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
     Ever confirmed|0                           |1
           Assignee|unassigned at gcc dot gnu.org      |msebor at gcc dot gnu.org
   Last reconfirmed|                            |2021-04-08
           Keywords|                            |patch
             Status|UNCONFIRMED                 |ASSIGNED

--- Comment #3 from Martin Sebor <msebor at gcc dot gnu.org> ---
Patch: https://gcc.gnu.org/pipermail/gcc-patches/2021-April/567800.html

[Bug c/99972] missing -Wunused-result on a call to a locally redeclared warn_unused_result function

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99972

--- Comment #4 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The master branch has been updated by Martin Sebor <msebor@gcc.gnu.org>:

https://gcc.gnu.org/g:da879e01ecd35737c18be1da3324f4560aba1961

commit r11-8205-gda879e01ecd35737c18be1da3324f4560aba1961
Author: Martin Sebor <msebor@redhat.com>
Date:   Thu Apr 15 15:49:30 2021 -0600

    Propagate type attribute when merging extern declarations at local scope.

    Resolves:
    PR c/99420 - bogus -Warray-parameter on a function redeclaration in
function scope
    PR c/99972 - missing -Wunused-result on a call to a locally redeclared
warn_unused_result function

    gcc/c/ChangeLog:

            PR c/99420
            PR c/99972
            * c-decl.c (pushdecl): Always propagate type attribute.

    gcc/testsuite/ChangeLog:

            PR c/99420
            PR c/99972
            * gcc.dg/Warray-parameter-9.c: New test.
            * gcc.dg/Wnonnull-6.c: New test.
            * gcc.dg/Wreturn-type3.c: New test.
            * gcc.dg/Wunused-result.c: New test.
            * gcc.dg/attr-noreturn.c: New test.
            * gcc.dg/attr-returns-nonnull.c: New test.

[Bug c/99972] missing -Wunused-result on a call to a locally redeclared warn_unused_result function

[msebor at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99972

Martin Sebor <msebor at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Target Milestone|---                         |11.0
         Resolution|---                         |FIXED
             Status|ASSIGNED                    |RESOLVED

--- Comment #5 from Martin Sebor <msebor at gcc dot gnu.org> ---
Fix committed in r11-8205.