[Bug c++/87406] Implement -Wconstant-conversion and -Wbitfield-constant-conversion

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=87406

--- Comment #1 from Jonathan Wakely <redi at gcc dot gnu.org> ---
We do warn about this, but only with -Wconversion which is not in -Wall or
-Wextra

c.cc:1:24: warning: conversion to ‘char’ from ‘int’ may alter its value
[-Wconversion]
 char f(int i) { return i; }
                        ^