[Bug c++/108155] New: no warning for for (int i = 1; 1 <= 12; ++i)

[Bug c++/108155] New: no warning for for (int i = 1; 1 <= 12; ++i)

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108155

            Bug ID: 108155
           Summary: no warning for for (int i = 1; 1 <= 12; ++i)
           Product: gcc
           Version: 13.0
            Status: UNCONFIRMED
          Keywords: diagnostic
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: redi at gcc dot gnu.org
            Blocks: 87403
  Target Milestone: ---

Somehow I typed a 1 instead of i for the loop condition, and so the loop ran
forever.

It seems like we should warn for logical-or-expression that only involves
constant expressions. I don't see why anybody would write `1 != 12` or `x || 1
< y`.

void foo(int) { }

int main()
{
  for (int i = 1; 1 <= 12; ++i)
    foo(i);
}


Even better would be to warn for expressions like `1 < y` where y is not a
constant, but is known to be unchanged by the loop.


Referenced Bugs:

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=87403
[Bug 87403] [Meta-bug] Issues that suggest a new warning

[Bug c++/108155] no warning for for (int i = 1; 1 <= 12; ++i)

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108155

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Last reconfirmed|                            |2022-12-17
     Ever confirmed|0                           |1
           Severity|normal                      |enhancement
             Status|UNCONFIRMED                 |NEW

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Confirmed.

[Bug c++/108155] no warning for for (int i = 1; 1 <= 12; ++i)

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108155

--- Comment #2 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Clang and EDG don't warn for this either, so maybe I'm the only person dumb
enough to write this. It still seems useful, if we can define the right
semantics without false positives, but it's not a priority.

[Bug c++/108155] no warning for for (int i = 1; 1 <= 12; ++i)

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108155

--- Comment #3 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
(In reply to Jonathan Wakely from comment #2)
> Clang and EDG don't warn for this either, so maybe I'm the only person dumb
> enough to write this. It still seems useful, if we can define the right
> semantics without false positives, but it's not a priority.

No I have accidently written this too before. 
The only false positive I can think of is with macros really.

[Bug c++/108155] no warning for for (int i = 1; 1 <= 12; ++i)

[dmalcolm at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108155

David Malcolm <dmalcolm at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |dmalcolm at gcc dot gnu.org

--- Comment #4 from David Malcolm <dmalcolm at gcc dot gnu.org> ---
FWIW -fanalyzer detects the example as of GCC 14:
  https://godbolt.org/z/4Effzrrq9 (C, gcc 14.1)
  https://godbolt.org/z/j8avza7hr (C++, gcc trunk)