[llvm-bugs] [Bug 31705] New: warn when loop condition comparison uses different size operands

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https://llvm.org/bugs/show_bug.cgi?id=31705

            Bug ID: 31705
           Summary: warn when loop condition comparison uses different
                    size operands
           Product: clang
           Version: unspecified
          Hardware: Macintosh
                OS: MacOS X
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: Static Analyzer
          Assignee: kremenek at apple.com
          Reporter: nivek.research at gmail.com
                CC: llvm-bugs at lists.llvm.org
    Classification: Unclassified

I recently encountered a infinite loop where the variable in a for loop
overflowed before reaching the termination value. For example, the 16-bit
unsigned int below is compared to a 64-bit unsigned long (on macOS) and since
the 16-bit value will overflow after 65535 it never reaches 65536 and the loop
will never terminates. I think it would be a great check for either the
compiler or more probably the static analyzer to warn that loop may not
terminate because of comparison may always be false due to overflow. I believe
the check would be good for for, while and do until loops.

#include <cstddef>
#include <cstdint>
#include <cstdio>

int main() {
    size_t limit = 65536;
    for (uint16_t index = 0; index < limit; index++) {
    }
    printf("completedi\n");
}

I believe similar warnings could be reported for

    uint16_t index = 0;
    while (index < limit) {
        index++;
    }

and

    do {
        index++;
    } while (index < limit);

The compiler does produce a warning if limit is replaced by a constant value:

main.cpp:11:15: warning: comparison of constant 65536 with expression of type
      'uint16_t' (aka 'unsigned short') is always true
      [-Wtautological-constant-out-of-range-compare]
        while (index < 65536) {
               ~~~~~ ^ ~~~~~

So perhaps this can just be enhanced.

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