[Bug c/95445] New: diagnose incompatible calls to functions declared without prototype

[Bug c/95445] New: diagnose incompatible calls to functions declared without prototype

[msebor at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95445

            Bug ID: 95445
           Summary: diagnose incompatible calls to functions declared
                    without prototype
           Product: gcc
           Version: 10.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: msebor at gcc dot gnu.org
  Target Milestone: ---

GCC silently accepts calls to built-in functions declared without a prototype
as long as the arguments match the expected types (based on the prototype
hardcoded into GCC), but issues warnings for calls where the arguments don't
match.

But GCC makes no attempt to make sure that in calls to other (i.e.,
non-built-in) functions declared without prototype provided arguments have the
same type across all the calls.  Diagnosing mismatches as suggested in the
comments below would help detect bugs caused by their incompatibily.

This enhancement is different from pr92212 which is about calls to K&R
functions defined in the same translation unit.

$ cat x.c && gcc -O2 -S -Wall x.c
char* strchr ();

char* f0 (const char *s)
{
  return strchr (s, 'x');    // no warning (okay)
}

char* f1 (const char *s)
{
  return strchr (s, "y");    // warning (good)
}

void g ();

void h (void)
{
  g (1);                     // (presumably) okay, f's type is 'void (int)'
  g ("foo");                 // should be diagnosed
  g (1, "bar");              // ditto
}
x.c: In function ‘f1’:
x.c:10:21: warning: passing argument 2 of ‘strchr’ makes integer from pointer
without a cast [-Wint-conversion]
   10 |   return strchr (s, "y");    // warning (good)
      |                     ^~~
      |                     |
      |                     char *
x.c:1:7: note: expected ‘int’ but argument is of type ‘char *’
    1 | char* strchr ();
      |       ^~~~~~

[Bug c/95445] diagnose incompatible calls to functions declared without prototype

[msebor at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95445

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Blocks|                            |87403
           Severity|normal                      |enhancement
           See Also|                            |https://gcc.gnu.org/bugzill
                   |                            |a/show_bug.cgi?id=92212
           Keywords|                            |diagnostic


Referenced Bugs:

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=87403
[Bug 87403] [Meta-bug] Issues that suggest a new warning

[Bug c/95445] diagnose incompatible calls to functions declared without prototype

[egallager at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95445

Eric Gallager <egallager at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |egallager at gcc dot gnu.org

--- Comment #1 from Eric Gallager <egallager at gcc dot gnu.org> ---
Is this like clang's -Wknr-promoted-parameter?
https://clang.llvm.org/docs/DiagnosticsReference.html#wknr-promoted-parameter

[Bug c/95445] diagnose incompatible calls to functions declared without prototype

[fw at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95445

Florian Weimer <fw at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |fw at gcc dot gnu.org

--- Comment #2 from Florian Weimer <fw at gcc dot gnu.org> ---
Current Clang warns like this:

t.c:17:5: warning: passing arguments to 'g' without a prototype is deprecated
in all versions of C and is not supported in C2x [-Wdeprecated-non-prototype]
  g (1);                     // (presumably) okay, f's type is 'void (int)'
    ^
t.c:18:5: warning: passing arguments to 'g' without a prototype is deprecated
in all versions of C and is not supported in C2x [-Wdeprecated-non-prototype]
  g ("foo");                 // should be diagnosed
    ^
t.c:19:5: warning: passing arguments to 'g' without a prototype is deprecated
in all versions of C and is not supported in C2x [-Wdeprecated-non-prototype]
  g (1, "bar");              // ditto
    ^

Maybe that's sufficient?

Trying to infer the type of g from its uses seems to be rather questionable.
For K&R projects, GCC already has -flto -Wlto-type-mismatch, which can diagnose
violations across translation units.

[Bug c/95445] diagnose incompatible calls to functions declared without prototype

[sjames at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95445

Sam James <sjames at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           See Also|                            |https://gcc.gnu.org/bugzill
                   |                            |a/show_bug.cgi?id=108694

--- Comment #3 from Sam James <sjames at gcc dot gnu.org> ---
I don't think we need to go beyond that (I don't really care about the uses).

That'll make this a dupe of/depends on PR108694?