[PATCH] c++: Don't accept multiple enum definitions within template class [PR115806]

[Simon Martin]

We have been accepting the following invalid code since revision 557831a91df

=== cut here ===
template <typename T> struct S {
  enum E { a };
  enum E { b };
};
S<int> s;
=== cut here ===

The problem is that start_enum will set OPAQUE_ENUM_P to true even if it
retrieves an existing definition for the enum, which causes the redefinition
check in cp_parser_enum_specifier to be bypassed.

This patch only sets OPAQUE_ENUM_P and ENUM_FIXED_UNDERLYING_TYPE_P when
actually pushing a new tag for the enum.

Successfully tested on x86_64-pc-linux-gnu.

        PR c++/115806

gcc/cp/ChangeLog:

        * decl.cc (start_enum): Only set OPAQUE_ENUM_P and
        ENUM_FIXED_UNDERLYING_TYPE_P when pushing a new tag.

gcc/testsuite/ChangeLog:

        * g++.dg/parse/enum15.C: New test.

---
 gcc/cp/decl.cc                      | 22 ++++++++++++----------
 gcc/testsuite/g++.dg/parse/enum15.C |  9 +++++++++
 2 files changed, 21 insertions(+), 10 deletions(-)
 create mode 100644 gcc/testsuite/g++.dg/parse/enum15.C

diff --git a/gcc/cp/decl.cc b/gcc/cp/decl.cc
index a468bfdb7b6..f23b635aec9 100644
--- a/gcc/cp/decl.cc
+++ b/gcc/cp/decl.cc
@@ -17059,22 +17059,24 @@ start_enum (tree name, tree enumtype, tree 
underlying_type,
          enumtype = cxx_make_type (ENUMERAL_TYPE);
          enumtype = pushtag (name, enumtype);
 
-         /* std::byte aliases anything.  */
-         if (enumtype != error_mark_node
-             && TYPE_CONTEXT (enumtype) == std_node
-             && !strcmp ("byte", TYPE_NAME_STRING (enumtype)))
-           TYPE_ALIAS_SET (enumtype) = 0;
+         if (enumtype != error_mark_node)
+           {
+             /* The enum is considered opaque until the opening '{' of the
+                enumerator list.  */
+             SET_OPAQUE_ENUM_P (enumtype, true);
+             ENUM_FIXED_UNDERLYING_TYPE_P (enumtype) = !! underlying_type;
+
+             /* std::byte aliases anything.  */
+             if (TYPE_CONTEXT (enumtype) == std_node
+                 && !strcmp ("byte", TYPE_NAME_STRING (enumtype)))
+               TYPE_ALIAS_SET (enumtype) = 0;
+           }
        }
       else
          enumtype = xref_tag (enum_type, name);
 
       if (enumtype == error_mark_node)
        return error_mark_node;
-
-      /* The enum is considered opaque until the opening '{' of the
-        enumerator list.  */
-      SET_OPAQUE_ENUM_P (enumtype, true);
-      ENUM_FIXED_UNDERLYING_TYPE_P (enumtype) = !! underlying_type;
     }
 
   SET_SCOPED_ENUM_P (enumtype, scoped_enum_p);
diff --git a/gcc/testsuite/g++.dg/parse/enum15.C 
b/gcc/testsuite/g++.dg/parse/enum15.C
new file mode 100644
index 00000000000..d19262156b9
--- /dev/null
+++ b/gcc/testsuite/g++.dg/parse/enum15.C
@@ -0,0 +1,9 @@
+// PR c++/115806
+// { dg-do compile }
+
+template <typename T>
+struct S {
+  enum E { a }; // { dg-note "previous definition" }
+  enum E { b }; // { dg-error "multiple definition" }
+};
+S<int> s;
-- 
2.44.0