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From: "tschwinge at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug middle-end/101862] New: [C, C++] Potential '?:' diagnostic for
 always-true expressions in boolean context
Date: Wed, 11 Aug 2021 09:19:00 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D101862

            Bug ID: 101862
           Summary: [C, C++] Potential '?:' diagnostic for always-true
                    expressions in boolean context
           Product: gcc
           Version: 12.0
            Status: UNCONFIRMED
          Keywords: diagnostic
          Severity: normal
          Priority: P3
         Component: middle-end
          Assignee: unassigned at gcc dot gnu.org
          Reporter: tschwinge at gcc dot gnu.org
  Target Milestone: ---

Created attachment 51287
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=3D51287&action=3Dedit
'c-c++-common/goacc/prN.c'

Would it be possible for GCC to diagnose the "'?:' misuse" in the attached
C/C++ test case?  No matter what gets stored in 'arr', the 'assert' never
triggers, because what's actually meant here, is:

       for (int i =3D 0; i < 32; i++)
    -    assert (arr[i] =3D=3D ((i % 2) !=3D 0) ? i + 1 : i + 2);
    +    assert (arr[i] =3D=3D (((i % 2) !=3D 0) ? i + 1 : i + 2));

This is going to be more complicated than
'gcc/c-family/c-common.c:c_common_truthvalue_conversion', 'case COND_EXPR:',
which diagnoses "'?:' using integer constants in boolean context" for
'INTEGER_CST's -- which these are not, of course.  I suppose we'd need some
value range analysis (so, at some later stage in the pass pipeline?) to see
that 'i + 1'/'i + 2' are always-true expressions in boolean context?

Is this feasible or not?  (..., and if yes, if anybody got any pointers abo=
ut
where/how to do this...)=