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From: "stephenheumann at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c/107164] New: No pedantic warning for declaration just
 referring to a previously-declared enum type
Date: Wed, 05 Oct 2022 21:57:42 +0000
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X-Bugzilla-Product: gcc
X-Bugzilla-Component: c
X-Bugzilla-Version: 13.0
X-Bugzilla-Keywords: accepts-invalid
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X-Bugzilla-Who: stephenheumann at gmail dot com
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D107164

            Bug ID: 107164
           Summary: No pedantic warning for declaration just referring to
                    a previously-declared enum type
           Product: gcc
           Version: 13.0
            Status: UNCONFIRMED
          Keywords: accepts-invalid
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: stephenheumann at gmail dot com
  Target Milestone: ---

gcc -std=3Dc17 -pedantic accepts this without any diagnostics:

enum E {a,b,c};
enum E;

C17 section 6.7.2.3 p9 says that an "enum identifier" type specifier using a
tag with an existing declaration visible "specifies the same type as that o=
ther
declaration, and does not redeclare the tag". Accordingly, the second line
above does not declare a declarator, a tag, or the members of an enumeratio=
n,
and so it violates the constraint in C17 section 6.7 p2.

(The relevant standard text is basically the same from C99 through draft C2=
3.
C90 is less explicit, but I think is intended to behave the same.)

This should at least give a pedantic warning. Perhaps it could be an
always-enabled warning, but I'm not sure if code like the above is supposed=
 to
be allowed as a GCC extension.=