[Bug c++/93652] New: -Wconversion gives false warning with bitwise operations on reference types returned from function

[gcc at mailinator dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=93652

            Bug ID: 93652
           Summary: -Wconversion gives false warning with bitwise
                    operations on reference types returned from function
           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: ---

```
[]$ /bin/g++ -v
Using built-in specs.
COLLECT_GCC=/bin/g++
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++,d --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 gdc_include_dir=/usr/include/dlang/gdc
Thread model: posix
gcc version 9.2.0 (GCC) 
[]$ cat a.cpp
char x,y;
char& f(){ return x; }
char& g(){ return y; }
void h(){ f()^=g(); }
[]$ /bin/g++ -Wconversion -c a.cpp 
a.cpp: In function ‘void h()’:
a.cpp:4:18: warning: conversion from ‘int’ to ‘char’ may change value
[-Wconversion]
    4 | void h(){ f()^=g(); }
      |                  ^

```

Similar to https://gcc.gnu.org/bugzilla/show_bug.cgi?id=38522, but this only
happens with reference returned from functions. Local reference created in the
function or local variable does not raise the bug.