Permit redeclaration of tags introduced by using decls

This valid construct appears in MSVC headers where it's used to provide a
definition for the '::type_info' compiler builtin type.

llvm-svn: 199490

clang/lib/Sema/SemaDecl.cpp

@@ -10681,7 +10681,8 @@ Decl *Sema::ActOnTag(Scope *S, unsigned TagSpec, TagUseKind TUK,
   }
 
   if (!Previous.empty()) {
-    NamedDecl *PrevDecl = (*Previous.begin())->getUnderlyingDecl();
+    NamedDecl *DirectPrevDecl = *Previous.begin();
+    NamedDecl *PrevDecl = DirectPrevDecl->getUnderlyingDecl();
 
     // It's okay to have a tag decl in the same scope as a typedef
     // which hides a tag decl in the same scope.  Finding this
@@ -10713,7 +10714,7 @@ Decl *Sema::ActOnTag(Scope *S, unsigned TagSpec, TagUseKind TUK,
       // in the same scope (so that the definition/declaration completes or
       // rementions the tag), reuse the decl.
       if (TUK == TUK_Reference || TUK == TUK_Friend ||
-          isDeclInScope(PrevDecl, SearchDC, S,
+          isDeclInScope(DirectPrevDecl, SearchDC, S,
                         SS.isNotEmpty() || isExplicitSpecialization)) {
         // Make sure that this wasn't declared as an enum and now used as a
         // struct or something similar.

clang/test/SemaCXX/using-decl-1.cpp

@@ -119,6 +119,27 @@ namespace foo
   };
 }
 
+namespace using_tag_redeclaration
+{
+  struct S;
+  namespace N {
+    using ::using_tag_redeclaration::S;
+    struct S {}; // expected-note {{previous definition is here}}
+  }
+  void f() {
+    N::S s1;
+    S s2;
+  }
+  void g() {
+    struct S; // expected-note {{forward declaration of 'S'}}
+    S s3; // expected-error {{variable has incomplete type 'S'}}
+  }
+  void h() {
+    using ::using_tag_redeclaration::S;
+    struct S {}; // expected-error {{redefinition of 'S'}}
+  }
+}
+
 // Don't suggest non-typenames for positions requiring typenames.
 namespace using_suggestion_tyname_val {
 namespace N { void FFF() {} }