[Bug c/88566] New: -Wconversion not using value range information

[hoganmeier at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=88566

            Bug ID: 88566
           Summary: -Wconversion not using value range information
           Product: gcc
           Version: 9.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: hoganmeier at gmail dot com
  Target Milestone: ---

https://godbolt.org/z/p0RMde

unsigned char foo(uint8_t pin)
{
        if (pin >= 3 && pin <= 6) return pin - 2;
        if (pin >= 9 && pin <= 10) return pin - 4;
        if (pin >= 20 && pin <= 23) return pin - 13;
        return 0;
}

$ gcc -O3 -Wconversion -S
<source>:5:39: warning: conversion from 'int' to 'uint8_t' {aka 'unsigned
char'} may change value [-Wconversion]

    5 |  if (pin >= 3 && pin <= 6) return pin - 2;

      |                                   ~~~~^~~


gcc should be aware that the value is well within the uint8_t range.