[Bug c/110703] New: Incorrect "-Wfloat-conversion" diagnostic with _Generic

[Bug c/110703] New: Incorrect "-Wfloat-conversion" diagnostic with _Generic

[jepler at unpythonic dot net]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110703

            Bug ID: 110703
           Summary: Incorrect "-Wfloat-conversion" diagnostic with
                    _Generic
           Product: gcc
           Version: 12.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: jepler at unpythonic dot net
  Target Milestone: ---

When compiled with `gcc -Wfloat-conversion -c boo.c` the following code results
in a `-Wfloat-conversion` diagnostic across a range of gcc versions including
gcc version 12.2.0 (Debian 12.2.0-14) and godbolt "trunk" as of today
(https://godbolt.org/z/3K7sTTv1x)

```
#include <math.h>

#define tgsin(x) \
    _Generic((x), \
            double: sin((x)), \
            float: sinf((x)) \
            )

int f(double d) {
    return tgsin(d) > 0;
}
```

The result is
```
$ gcc -Wfloat-conversion -c boo.c
boo.c: In function ‘f’:
boo.c:10:18: warning: conversion from ‘double’ to ‘float’ may change value
[-Wfloat-conversion]
   10 |     return tgsin(d) > 0;
      |                  ^
boo.c:6:25: note: in definition of macro ‘tgsin’
    6 |             float: sinf(x) \
      |                         ^
```

As far as I can tell, the diagnostic is spurious, because it applies to the
"float" case of _Generic, which is not the one that is actually chosen. That
is, inspecting the resulting object file `boo.o` there is a call to sin but not
to sinf.

This is prompted by https://github.com/v923z/micropython-ulab/pull/636 and
earlier
https://github.com/micropython/micropython/commit/f31f9a8b70db03cbcbcf39b493f959d0e284962a
-- it at first appeared to be related to -Os size optimization, but that's just
related to how glibc chooses to implement fpclassify, whether via _Generic or
not.

This may be related to https://gcc.gnu.org/bugzilla/show_bug.cgi?id=68193

[Bug c/110703] Incorrect "-Wfloat-conversion" diagnostic with _Generic

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110703

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
     Ever confirmed|0                           |1
   Last reconfirmed|                            |2023-07-17

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
It is basically the same as PR 68193 really.

[Bug c/110703] Incorrect "-Wfloat-conversion" diagnostic with _Generic

[muecker at gwdg dot de]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110703

Martin Uecker <muecker at gwdg dot de> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |muecker at gwdg dot de

--- Comment #2 from Martin Uecker <muecker at gwdg dot de> ---
PATCH

https://gcc.gnu.org/pipermail/gcc-patches/2023-August/626319.html

[Bug c/110703] Incorrect "-Wfloat-conversion" diagnostic with _Generic

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110703

--- Comment #3 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The master branch has been updated by Martin Uecker <uecker@gcc.gnu.org>:

https://gcc.gnu.org/g:54be338589ea93ad4ff53d22adde476a0582537b

commit r14-3004-g54be338589ea93ad4ff53d22adde476a0582537b
Author: Martin Uecker <uecker@tugraz.at>
Date:   Fri Aug 4 07:48:21 2023 +0200

    c: _Generic should not warn in non-active branches
[PR68193,PR97100,PR110703]

    To avoid false diagnostics, use c_inhibit_evaluation_warnings when
    a generic association is known to not match during parsing.  We may
    still generate false positives if the default branch comes earler than
    a specific association that matches.

    PR c/68193
    PR c/97100
    PR c/110703

    gcc/c/:
            * c-parser.cc (c_parser_generic_selection): Inhibit evaluation
            warnings branches that are known not be taken during parsing.

    gcc/testsuite/ChangeLog:
            * gcc.dg/pr68193.c: New test.

[Bug c/110703] Incorrect "-Wfloat-conversion" diagnostic with _Generic

[uecker at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110703

uecker at gcc dot gnu.org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |RESOLVED
                 CC|                            |uecker at gcc dot gnu.org
      Known to fail|                            |4.9.0
   Target Milestone|---                         |14.0
         Resolution|---                         |FIXED

--- Comment #4 from uecker at gcc dot gnu.org ---
fixed on trunk.