[LLVMbugs] [Bug 12687] New: clang should warn when tautologically comparing a smaller-type numeric value against a larger-typed constant outside the smaller type's range

[bugzilla-daemon at llvm.org]

http://llvm.org/bugs/show_bug.cgi?id=12687

             Bug #: 12687
           Summary: clang should warn when tautologically comparing a
                    smaller-type numeric value against a larger-typed
                    constant outside the smaller type's range
           Product: clang
           Version: unspecified
          Platform: All
        OS/Version: All
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: Frontend
        AssignedTo: unassignedclangbugs at nondot.org
        ReportedBy: jwalden+llvm at mit.edu
                CC: llvmbugs at cs.uiuc.edu
    Classification: Unclassified


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.

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