[Bug c/92219] New: -Wconversion generates a false warning for modulo expression when the modulus has smaller type

[gcc at mailinator dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=92219

            Bug ID: 92219
           Summary: -Wconversion generates a false warning for modulo
                    expression when the modulus has smaller type
           Product: gcc
           Version: 9.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: gcc at mailinator dot com
  Target Milestone: ---

```
[]$ gcc -v
Using built-in specs.
COLLECT_GCC=/usr/bin/gcc
COLLECT_LTO_WRAPPER=/usr/lib/gcc/x86_64-pc-linux-gnu/9.2.0/lto-wrapper
Target: x86_64-pc-linux-gnu
Configured with: /build/gcc/src/gcc/configure --prefix=/usr --libdir=/usr/lib
--libexecdir=/usr/lib --mandir=/usr/share/man --infodir=/usr/share/info
--with-bugurl=https://bugs.archlinux.org/
--enable-languages=c,c++,ada,fortran,go,lto,objc,obj-c++ --enable-shared
--enable-threads=posix --with-system-zlib --with-isl --enable-__cxa_atexit
--disable-libunwind-exceptions --enable-clocale=gnu --disable-libstdcxx-pch
--disable-libssp --enable-gnu-unique-object --enable-linker-build-id
--enable-lto --enable-plugin --enable-install-libiberty
--with-linker-hash-style=gnu --enable-gnu-indirect-function --enable-multilib
--disable-werror --enable-checking=release --enable-default-pie
--enable-default-ssp --enable-cet=auto
Thread model: posix
gcc version 9.2.0 (GCC) 

[]$ cat a.c
int f(long long a,int b){
        return a%b;
}

[]$ gcc -Wall -Wconversion a.c -c
a.c: In function ‘f’:
a.c:2:10: warning: conversion from ‘long long int’ to ‘int’ may change value
[-Wconversion]
    2 |  return a%b;
      |         ~^~
```

The specification for `-Wconversion` is "Warn for implicit conversions that may
alter a value.". In this case the result is guaranteed to be convertible to the
type of `b` (`int`) without changing the value.