From f5f1d92fe2e1d75c3fae34497929a1965af704ae Mon Sep 17 00:00:00 2001
From: Joseph Myers <joseph@codesourcery.com>
Date: Tue, 18 Oct 2022 23:25:47 +0000
Subject: [PATCH] c: Diagnose "enum tag;" after definition [PR107164]

As noted in bug 101764, a declaration "enum tag;" is invalid in
standard C after a definition, as well as when no definition is
visible; we had a pedwarn-if-pedantic for the forward declaration
case, but were missing one for the other case.  Add that missing
diagnostic (if pedantic only).

(These diagnostics will need to be appropriately conditioned when
support is added for C2x enums with fixed underlying type, since "enum
tag : type;" is OK both before and after a definition.)

Bootstrapped with no regressions for x86_64-pc-linux-gnu.

	PR c/107164

gcc/c/
	* c-decl.cc (shadow_tag_warned): If pedantic, diagnose "enum tag;"
	with previous declaration visible.

gcc/testsuite/
	* gcc.dg/c99-tag-4.c, gcc.dg/c99-tag-5.c, gcc.dg/c99-tag-6.c: New
	tests.
---
 gcc/c/c-decl.cc                  | 14 ++++++++++++++
 gcc/testsuite/gcc.dg/c99-tag-4.c |  8 ++++++++
 gcc/testsuite/gcc.dg/c99-tag-5.c |  8 ++++++++
 gcc/testsuite/gcc.dg/c99-tag-6.c |  9 +++++++++
 4 files changed, 39 insertions(+)
 create mode 100644 gcc/testsuite/gcc.dg/c99-tag-4.c
 create mode 100644 gcc/testsuite/gcc.dg/c99-tag-5.c
 create mode 100644 gcc/testsuite/gcc.dg/c99-tag-6.c

diff --git a/gcc/c/c-decl.cc b/gcc/c/c-decl.cc
index bcb4d7b..80f6e91 100644
--- a/gcc/c/c-decl.cc
+++ b/gcc/c/c-decl.cc
@@ -4814,6 +4814,20 @@ shadow_tag_warned (const struct c_declspecs *declspecs, int warned)
 	      warned = 1;
 	      pending_xref_error ();
 	    }
+	  else if (declspecs->typespec_kind != ctsk_tagdef
+		   && declspecs->typespec_kind != ctsk_tagfirstref
+		   && declspecs->typespec_kind != ctsk_tagfirstref_attrs
+		   && code == ENUMERAL_TYPE)
+	    {
+	      bool warned_enum = false;
+	      if (warned != 1)
+		warned_enum = pedwarn (input_location, OPT_Wpedantic,
+				       "empty declaration of %<enum%> type "
+				       "does not redeclare tag");
+	      if (warned_enum)
+		warned = 1;
+	      pending_xref_error ();
+	    }
 	  else
 	    {
 	      pending_invalid_xref = NULL_TREE;
diff --git a/gcc/testsuite/gcc.dg/c99-tag-4.c b/gcc/testsuite/gcc.dg/c99-tag-4.c
new file mode 100644
index 0000000..9ff3ccb
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/c99-tag-4.c
@@ -0,0 +1,8 @@
+/* Test for handling of tags.  "enum foo;" is invalid after an existing
+   declaration (does not redeclare the tag) as well as before: bug 107164.  */
+/* { dg-do compile } */
+/* { dg-options "-std=c99 -pedantic-errors" } */
+
+enum e1; /* { dg-error "ISO C forbids forward references to 'enum' types" } */
+enum e2 { E };
+enum e2; /* { dg-error "empty declaration of 'enum' type does not redeclare tag" } */
diff --git a/gcc/testsuite/gcc.dg/c99-tag-5.c b/gcc/testsuite/gcc.dg/c99-tag-5.c
new file mode 100644
index 0000000..97fcc75
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/c99-tag-5.c
@@ -0,0 +1,8 @@
+/* Test for handling of tags.  "enum foo;" is invalid after an existing
+   declaration (does not redeclare the tag) as well as before: bug 107164.  */
+/* { dg-do compile } */
+/* { dg-options "-std=c99 -pedantic" } */
+
+enum e1; /* { dg-warning "ISO C forbids forward references to 'enum' types" } */
+enum e2 { E };
+enum e2; /* { dg-warning "empty declaration of 'enum' type does not redeclare tag" } */
diff --git a/gcc/testsuite/gcc.dg/c99-tag-6.c b/gcc/testsuite/gcc.dg/c99-tag-6.c
new file mode 100644
index 0000000..8307217
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/c99-tag-6.c
@@ -0,0 +1,9 @@
+/* Test for handling of tags.  "enum foo;" is invalid after an existing
+   declaration (does not redeclare the tag) as well as before: bug 107164.
+   Test this is not diagnosed without -pedantic.  */
+/* { dg-do compile } */
+/* { dg-options "-std=c99" } */
+
+enum e1;
+enum e2 { E };
+enum e2;
-- 
2.7.4