1 files changed, 20 insertions, 1 deletions
diff --git a/clang/lib/Sema/SemaTemplate.cpp b/clang/lib/Sema/SemaTemplate.cpp
index 805709dab80..c642c0599b5 100644
--- a/clang/lib/Sema/SemaTemplate.cpp
+++ b/clang/lib/Sema/SemaTemplate.cpp
@@ -12,6 +12,7 @@
 #include "TreeTransform.h"
 #include "clang/AST/ASTConsumer.h"
 #include "clang/AST/ASTContext.h"
+#include "clang/AST/ASTMutationListener.h"
 #include "clang/AST/DeclFriend.h"
 #include "clang/AST/DeclTemplate.h"
 #include "clang/AST/Expr.h"
@@ -836,7 +837,8 @@ Sema::CheckClassTemplate(Scope *S, unsigned TagSpec, TagUseKind TUK,
                          AccessSpecifier AS, SourceLocation ModulePrivateLoc,
                          SourceLocation FriendLoc,
                          unsigned NumOuterTemplateParamLists,
-                         TemplateParameterList** OuterTemplateParamLists) {
+                         TemplateParameterList** OuterTemplateParamLists,
+                         bool *SkipBody) {
   assert(TemplateParams && TemplateParams->size() > 0 &&
          "No template parameters");
   assert(TUK != TUK_Reference && "Can only declare or define class templates");
@@ -993,6 +995,23 @@ Sema::CheckClassTemplate(Scope *S, unsigned TagSpec, TagUseKind TUK,
     // Check for redefinition of this class template.
     if (TUK == TUK_Definition) {
       if (TagDecl *Def = PrevRecordDecl->getDefinition()) {
+        // If we have a prior definition that is not visible, treat this as
+        // simply making that previous definition visible.
+        NamedDecl *Hidden = nullptr;
+        if (SkipBody && !hasVisibleDefinition(Def, &Hidden)) {
+          *SkipBody = true;
+          auto *Tmpl = cast<CXXRecordDecl>(Hidden)->getDescribedClassTemplate();
+          assert(Tmpl && "original definition of a class template is not a "
+                         "class template?");
+          if (auto *Listener = getASTMutationListener()) {
+            Listener->RedefinedHiddenDefinition(Hidden, KWLoc);
+            Listener->RedefinedHiddenDefinition(Tmpl, KWLoc);
+          }
+          Hidden->setHidden(false);
+          Tmpl->setHidden(false);
+          return Def;
+        }
+
         Diag(NameLoc, diag::err_redefinition) << Name;
         Diag(Def->getLocation(), diag::note_previous_definition);
         // FIXME: Would it make sense to try to "forget" the previous