[RFC] Warnings for cases where int promotion is unexpected and may cause bugs

["Aniruddh Agarwal" <me@anrddh.me>]

Hello,

A colleague patched a prod-critical bug today caused by an overlooked implicit int promotion when adding uint8_t's. g++  (v12.1) doesn't report any warnings for it with all combinations of warnings flags that I've tried, so I thought I'd ask if:

- there *is* already some combination of warning flags that *would* report a warning for this code

- if not, then if there's any interest in work (which of course I'd be happy to contribute to) on detecting and flagging this sort of problem. 

A (much simplified) example which illustrates the bug:
#+BEGIN_SRC cpp
#include <cstdint>

using std::uint8_t;

bool foo(uint8_t a, uint8_t b, uint8_t c) {
    return (a + b) == c;
}
#+END_SRC

Here's the problem: the expectation here is that "a + b" will have type uint8_t. So, for example it expects "foo(200, 200, 144)" to return "true".

In reality, "a + b" implicitly promotes to an "int" and so we end up comparing 400 and 144, which returns false.

(Side note, not immediately relevant: I'm not sure if this ends up being equivalent to calling something like a "bool operator==(int, uint8_t)" or if the RHS is also implicitly promoted to an int before the comparison. This is irrelevant for the immediate example because the end result is the same in either case, but I would appreciate it if someone can shed light on what the standard has to say on this for future reference.)

A correct implementation of the expected behavior is instead therefore:
#+BEGIN_SRC cpp
#include <cstdint>

using std::uint8_t;

bool foo(uint8_t a, uint8_t b, uint8_t c) {
    return static_cast<uint8_t>(a + b) == c;
}
#+END_SRC

Does anyone else find this very surprising, and as I asked above, does it seem worthwhile to try to flag code like in the first snippet? I don't know what gcc's general policy on trying to warn about code like this is. The new theoretical  warning would be in the spirit of -Wconversion.

-Ani