[Bug c++/100879] New: gcc is complaining of a signed compare when comparing enums of different types (same underlying type)

[Bug c++/100879] New: gcc is complaining of a signed compare when comparing enums of different types (same underlying type)

[andre at kostur dot net]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100879

            Bug ID: 100879
           Summary: gcc is complaining of a signed compare when comparing
                    enums of different types (same underlying type)
           Product: gcc
           Version: 10.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: andre at kostur dot net
  Target Milestone: ---

With the following test code:

enum e1 { e1val };

enum e2 { e3val };

int main( int, char * [] ) {
   if ( e1val == e3val ) return 1;
}

The compiler emits two warnings:
<source>: In function 'int main(int, char**)':
<source>:6:15: warning: comparison between 'enum e1' and 'enum e2'
[-Wenum-compare]
    6 |    if ( e1val == e3val ) return 1;
      |         ~~~~~~^~~~~~~~
<source>:6:15: warning: comparison between types 'e1' and 'e2' [-Wsign-compare]

The first is correct, but the second warning seems wrong.  There is no sign
mismatch as both enums would have the same underlying type.

gcc 9.3 does not seem to suffer this problem.  gcc 10.1 does, and appears every
version up to at least 11.1 does.  (Checked on compiler-explorer)

[Bug c++/100879] [10/11/12 Regression] gcc is complaining of a signed compare when comparing enums of different types (same underlying type)

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100879

Richard Biener <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
            Summary|gcc is complaining of a     |[10/11/12 Regression] gcc
                   |signed compare when         |is complaining of a signed
                   |comparing enums of          |compare when comparing
                   |different types (same       |enums of different types
                   |underlying type)            |(same underlying type)
           Keywords|                            |diagnostic
   Target Milestone|---                         |10.4

[Bug c++/100879] [10/11/12 Regression] gcc is complaining of a signed compare when comparing enums of different types (same underlying type)

[jason at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100879

Jason Merrill <jason at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Last reconfirmed|                            |2021-06-08
           Assignee|unassigned at gcc dot gnu.org      |jason at gcc dot gnu.org
             Status|UNCONFIRMED                 |ASSIGNED
                 CC|                            |jason at gcc dot gnu.org
     Ever confirmed|0                           |1

[Bug c++/100879] [10/11/12 Regression] gcc is complaining of a signed compare when comparing enums of different types (same underlying type)

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100879

--- Comment #1 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The master branch has been updated by Jason Merrill <jason@gcc.gnu.org>:

https://gcc.gnu.org/g:087253b9951766cbd93286b804ebb1ab59197aa8

commit r12-1314-g087253b9951766cbd93286b804ebb1ab59197aa8
Author: Jason Merrill <jason@redhat.com>
Date:   Tue Jun 8 17:48:49 2021 -0400

    c++: remove redundant warning [PR100879]

    Before my r277864, build_new_op promoted enums to int before passing them
on
    to cp_build_binary_op; after that commit, it doesn't, so
    warn_for_sign_compare sees the enum operands and gives a redundant warning.
    This warning dates back to 1995, and seems to have been dead code for a
long
    time--likely since build_new_op was added in 1997--so let's just remove it.

            PR c++/100879

    gcc/c-family/ChangeLog:

            * c-warn.c (warn_for_sign_compare): Remove C++ enum mismatch
            warning.

    gcc/testsuite/ChangeLog:

            * g++.dg/diagnostic/enum3.C: New test.

[Bug c++/100879] [10/11/12 Regression] gcc is complaining of a signed compare when comparing enums of different types (same underlying type)

[jason at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100879

Jason Merrill <jason at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Target Milestone|10.4                        |12.0
             Status|ASSIGNED                    |RESOLVED
         Resolution|---                         |FIXED

--- Comment #2 from Jason Merrill <jason at gcc dot gnu.org> ---
Fixed for GCC 12, thanks.

That the warning used -Wsign-compare seems to be because it was associated with
that option before -Wenum-compare was added, and never updated perhaps because
it was dead code for a long time.

[Bug c++/100879] [10/11/12 Regression] gcc is complaining of a signed compare when comparing enums of different types (same underlying type)

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100879

--- Comment #3 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The releases/gcc-11 branch has been updated by Jason Merrill
<jason@gcc.gnu.org>:

https://gcc.gnu.org/g:4f2819223873266b4cdfa7af54752a37d1ebd665

commit r11-8628-g4f2819223873266b4cdfa7af54752a37d1ebd665
Author: Jason Merrill <jason@redhat.com>
Date:   Tue Jun 8 17:48:49 2021 -0400

    c++: remove redundant warning [PR100879]

    Before my r277864, build_new_op promoted enums to int before passing them
on
    to cp_build_binary_op; after that commit, it doesn't, so
    warn_for_sign_compare sees the enum operands and gives a redundant warning.
    This warning dates back to 1995, and seems to have been dead code for a
long
    time--likely since build_new_op was added in 1997--so let's just remove it.

            PR c++/100879

    gcc/c-family/ChangeLog:

            * c-warn.c (warn_for_sign_compare): Remove C++ enum mismatch
            warning.

    gcc/testsuite/ChangeLog:

            * g++.dg/diagnostic/enum3.C: New test.