[Bug c++/107388] New: GCC should diagnose unsigned to signed conversion with -Wconversion

[Bug c++/107388] New: GCC should diagnose unsigned to signed conversion with -Wconversion

[nikolasklauser at berlin dot de]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107388

            Bug ID: 107388
           Summary: GCC should diagnose unsigned to signed conversion
                    with -Wconversion
           Product: gcc
           Version: 13.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: nikolasklauser at berlin dot de
  Target Milestone: ---

It looks like GCC currently doesn't diagnose a conversion from unsigned int to
int, but as I understand it -Wconversion should diagnose any conversion that
might change the value.

Here is an example: (Godbolt: https://godbolt.org/z/Mr8KvTW66)

#include <cstddef>
#include <cstdlib>

void func(unsigned int size) {
  [[maybe_unused]] int v = size; // doesn't get diagnosed
}

[Bug c++/107388] GCC should diagnose unsigned to signed conversion with -Wconversion

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107388

Marek Polacek <mpolacek at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |RESOLVED
         Resolution|---                         |INVALID
                 CC|                            |mpolacek at gcc dot gnu.org

--- Comment #1 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
This is -Wsign-conversion:

$ ./cc1plus -quiet t.C -Wsign-conversion
t.C: In function ‘void func(unsigned int)’:
t.C:2:28: warning: conversion to ‘int’ from ‘unsigned int’ may change the sign
of the result [-Wsign-conversion]
    2 |   [[maybe_unused]] int v = size; // doesn't get diagnosed
      |                            ^~~~

[Bug c++/107388] GCC should diagnose unsigned to signed conversion with -Wconversion

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107388

Marek Polacek <mpolacek at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
         Resolution|INVALID                     |WORKSFORME

[Bug c++/107388] GCC should diagnose unsigned to signed conversion with -Wconversion

[nikolasklauser at berlin dot de]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107388

--- Comment #2 from Nikolas Klauser <nikolasklauser at berlin dot de> ---
Shouldn't -Wconversion also enable -Wsign-conversion? -Wconversion sounds to me
like it should diagnose all conversions, especially since it's the same kind of
bug I want to catch by enabling the flags. In which case would I be interested
in different sizes but not different signs?