[gcc r13-3366] c: Diagnose "enum tag;" after definition [PR107164]

[gcc r13-3366] c: Diagnose "enum tag;" after definition [PR107164]

[Joseph Myers]

https://gcc.gnu.org/g:f5f1d92fe2e1d75c3fae34497929a1965af704ae

commit r13-3366-gf5f1d92fe2e1d75c3fae34497929a1965af704ae
Author: Joseph Myers <joseph@codesourcery.com>
Date:   Tue Oct 18 23:25:47 2022 +0000

    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.

Diff:
---
 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(+)

diff --git a/gcc/c/c-decl.cc b/gcc/c/c-decl.cc
index bcb4d7b66fe..80f6e912187 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 00000000000..9ff3ccb8d4b
--- /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 00000000000..97fcc75bc26
--- /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 00000000000..8307217523c
--- /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;