[LLVMbugs] [Bug 22062] New: In C -Wtautological-constant-out-of-range-compare should not trigger for out of range enums

bugzilla-daemon at llvm.org bugzilla-daemon at llvm.org
Tue Dec 30 04:05:24 PST 2014


            Bug ID: 22062
           Summary: In C -Wtautological-constant-out-of-range-compare
                    should not trigger for out of range enums
           Product: clang
           Version: unspecified
          Hardware: Macintosh
                OS: MacOS X
            Status: NEW
          Severity: normal
          Priority: P
         Component: -New Bugs
          Assignee: unassignedclangbugs at nondot.org
          Reporter: fredm at spamcop.net
                CC: llvmbugs at cs.uiuc.edu
    Classification: Unclassified

Given this code:

  #include <stdio.h>

  typedef enum o {
      O_1 = 0,
  } o;

  int main(int argc, char** argv) {
    enum o e1 = -1;

    if( e1 >= 2 ) {
      printf( "That's not right\n" );


    return 0;

Clang warns as follows:

  $ clang -Wall -std=c89 t1.c
  t1.c:11:10: warning: comparison of constant 2 with expression of type 'enum
        is always false [-Wtautological-constant-out-of-range-compare]
    if( e1 >= 2 ) {
        ~~ ^  ~
  1 warning generated.

That is demonstrably untrue:
  $ ./a.out 
  That's not right

I don't believe that it is correct to assert the range of an enum to be limited
to the declared values in C, while that is valid in C++.

In C an enumeration is defined as an enumeration constant and of the
draft C standard April 12, 2011 ISO/IEC 9899:201x I am looking at says:
An identifier declared as an enumeration constant has type int. says:
Each enumerated type shall be compatible with char, a signed integer type, or
an unsigned integer type. The choice of type is implementation-defined,128) but
shall be capable of representing the values of all the members of the

As long as it isn't forbidden to assign out-of-enumeration values to an
enumeration I think that it is invalid to say that the comparison of any out of
declared enumeration range value is guaranteed to be tautological.

A similar issue appeared to be raised and fixed in bug 16154.

I have raised this in the cfe-users mailing list (14-Dec-2013) where it was
suggested to raise the issue as a PR. I am not quite sure how a year passed in
the interim :)

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