[Bug c/99317] New: Missed warning

[Bug c/99317] New: Missed warning

[pj at hugeone dot co.uk]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99317

            Bug ID: 99317
           Summary: Missed warning
           Product: gcc
           Version: 10.2.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: pj at hugeone dot co.uk
  Target Milestone: ---

The code:

int *foo(void *v, void *w, int x) {
    float * f = v;
    int * i = w;
    return (x ? f : i); 
}


int *foo1(void *v, void *w, int x) {
    float * f = v;
    int * i = w;
    return (1 ? f : (void *)i); 
}

int *bar(void *v, void *w, int x) {
    float * f = v;
    int * i = w;
    return (x ? f : (void *)i); 
}

Function foo correctly emits the warning:

source>: In function 'foo':
<source>:7:19: warning: pointer type mismatch in conditional expression
    7 |     return (x ? f : i);
      |                   ^

Casting removes that warming even in the trivial foo1 example.

https://godbolt.org/z/ozsPPY

[Bug c/99317] Missed warning

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99317

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
I dont think this is exactly a bug. The warning is a pedantic warning and with
void*, things are implicitly converted by standard c rules.

[Bug c/99317] Missed warning

[pj at hugeone dot co.uk]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99317

--- Comment #2 from Piotr <pj at hugeone dot co.uk> ---
@(In reply to Andrew Pinski from comment #1)
> I dont think this is exactly a bug. The warning is a pedantic warning and
> with void*, things are implicitly converted by standard c rules.

With not `void *` it is exactly the same.  https://godbolt.org/z/zM8Eqs

No -pedantic option too.

[Bug c/99317] Missed warning

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99317

--- Comment #3 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Right.
basically what I am saying is:
x ? void* : char*
implies an implict conversion of the second operand to void*.
Without the cast, there is no implict conversion in standard C, that is what
the warning is about.
With -pendatic-errors, the warning turns into an error.

[Bug c/99317] Missed warning

[msebor at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99317

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |msebor at gcc dot gnu.org
         Resolution|---                         |WONTFIX
             Status|UNCONFIRMED                 |RESOLVED
           See Also|                            |https://gcc.gnu.org/bugzill
                   |                            |a/show_bug.cgi?id=44209

--- Comment #4 from Martin Sebor <msebor at gcc dot gnu.org> ---
To expand on Andrew's comment: The C rules specify the type of the result of
operator ?: when either of the second and third operands is a pointer to void,
but they do no specify the result type when the operands are pointers to
incompatible types (like int* and float*).  The warning points out the latter.

The C++ rules, OTOH, are more restrictive and make the ?: expression valid only
when the types of the arguments can be implicitly converted to one another. 
Since in C++ a pointer to void isn't implicitly convertible to a pointer to an
object type all the expressions in the test case are invalid.

In C mode, the -Wc++-compat option helps detect C/C++ incompatibilities and
detects this problem (below).  So with that, since the problem is diagnosed
under the right option, I think this report can be resolved as WONTFIX.  (That
the warning in comment #0 is issued unconditionally and not under the control
of a specific option, is a separate problem tracked in in bug 44209.)

$ gcc -S -Wall -Wc++-compat pr99317.c
pr99317.c: In function ‘foo’:
pr99317.c:3:17: warning: request for implicit conversion from ‘void *’ to
‘float *’ not permitted in C++ [-Wc++-compat]
    3 |     float * f = v;
      |                 ^
pr99317.c:4:15: warning: request for implicit conversion from ‘void *’ to ‘int
*’ not permitted in C++ [-Wc++-compat]
    4 |     int * i = w;
      |               ^
pr99317.c:5:19: warning: pointer type mismatch in conditional expression
    5 |     return (x ? f : i);
      |                   ^
pr99317.c:5:19: warning: request for implicit conversion from ‘void *’ to ‘int
*’ not permitted in C++ [-Wc++-compat]
    5 |     return (x ? f : i);
      |            ~~~~~~~^~~~
pr99317.c: In function ‘foo1’:
pr99317.c:10:17: warning: request for implicit conversion from ‘void *’ to
‘float *’ not permitted in C++ [-Wc++-compat]
   10 |     float * f = v;
      |                 ^
pr99317.c:11:15: warning: request for implicit conversion from ‘void *’ to ‘int
*’ not permitted in C++ [-Wc++-compat]
   11 |     int * i = w;
      |               ^
pr99317.c:12:19: warning: request for implicit conversion from ‘void *’ to ‘int
*’ not permitted in C++ [-Wc++-compat]
   12 |     return (1 ? f : (void *)i);
      |            ~~~~~~~^~~~~~~~~~~~
pr99317.c: In function ‘bar’:
pr99317.c:16:17: warning: request for implicit conversion from ‘void *’ to
‘float *’ not permitted in C++ [-Wc++-compat]
   16 |     float * f = v;
      |                 ^
pr99317.c:17:15: warning: request for implicit conversion from ‘void *’ to ‘int
*’ not permitted in C++ [-Wc++-compat]
   17 |     int * i = w;
      |               ^
pr99317.c:18:19: warning: request for implicit conversion from ‘void *’ to ‘int
*’ not permitted in C++ [-Wc++-compat]
   18 |     return (x ? f : (void *)i);
      |            ~~~~~~~^~~~~~~~~~~~