clang should warn when tautologically comparing a smaller-type numeric value against a larger-typed constant outside the smaller type's range

[llvmbot]

Bugzilla Link	12687
Version	unspecified
OS	All
Reporter	LLVM Bugzilla Contributor
CC	@gribozavr

Extended Description

I recently wrote code that looked roughly like this (the isfinite and copysign expressions were spelled in other terms that didn't make the code look so obviously wrong):

#include <math.h>
#include <stdlib.h>

inline double TimeClip(double time)
{
if (!isfinite(time) || abs(time) > 8.64e15)
return NAN;
return copysign(floor(fabs(time)), time + 0.0);
}

int main()
{
TimeClip(17.0);
return 0;
}

This code's buggy in that it uses abs (int -> int) when it should have used fabs (double -> double). The compiler can't be expected to figure that out, of course. But it should be able to figure out that no |int| value will ever be greater than 8.64e15, and it should emit a tautological-compare warning just as it would if it saw any other always-true or always-false comparison.

[gribozavr]

Another good example is:

bool f(uint16_t a) {
return a == -1;
}

[YutongZhuu]

The original issue is hard to understand, but we now have -Wtautological-compare, so this issue should be closed, right? @gribozavr