1 files changed, 31 insertions, 2 deletions

diff --git a/clang/lib/Sema/SemaDecl.cpp b/clang/lib/Sema/SemaDecl.cpp
index e9c40aa2e5d..9bbfeaf19ef 100644
--- a/clang/lib/Sema/SemaDecl.cpp
+++ b/clang/lib/Sema/SemaDecl.cpp
@@ -1836,11 +1836,18 @@ static void filterNonConflictingPreviousTypedefDecls(ASTContext &Context,
 
     // Declarations of the same entity are not ignored, even if they have
     // different linkages.
-    if (auto *OldTD = dyn_cast<TypedefNameDecl>(Old))
+    if (auto *OldTD = dyn_cast<TypedefNameDecl>(Old)) {
       if (Context.hasSameType(OldTD->getUnderlyingType(),
                               Decl->getUnderlyingType()))
         continue;
 
+      // If both declarations give a tag declaration a typedef name for linkage
+      // purposes, then they declare the same entity.
+      if (OldTD->getAnonDeclWithTypedefName() &&
+          Decl->getAnonDeclWithTypedefName())
+        continue;
+    }
+
     if (!Old->isExternallyVisible())
       Filter.erase();
   }
@@ -1950,6 +1957,29 @@ void Sema::MergeTypedefNameDecl(TypedefNameDecl *New, LookupResult &OldDecls) {
   if (Old->isInvalidDecl())
     return New->setInvalidDecl();
 
+  if (auto *OldTD = dyn_cast<TypedefNameDecl>(Old)) {
+    auto *OldTag = OldTD->getAnonDeclWithTypedefName();
+    auto *NewTag = New->getAnonDeclWithTypedefName();
+    NamedDecl *Hidden = nullptr;
+    if (getLangOpts().CPlusPlus && OldTag && NewTag &&
+        OldTag->getCanonicalDecl() != NewTag->getCanonicalDecl() &&
+        !hasVisibleDefinition(OldTag, &Hidden)) {
+      // There is a definition of this tag, but it is not visible. Use it
+      // instead of our tag.
+      New->setTypeForDecl(OldTD->getTypeForDecl());
+      if (OldTD->isModed())
+        New->setModedTypeSourceInfo(OldTD->getTypeSourceInfo(),
+                                    OldTD->getUnderlyingType());
+      else
+        New->setTypeSourceInfo(OldTD->getTypeSourceInfo());
+
+      // Make the old tag definition visible.
+      if (auto *Listener = getASTMutationListener())
+        Listener->RedefinedHiddenDefinition(Hidden, NewTag->getLocation());
+      Hidden->setHidden(false);
+    }
+  }
+
   // If the typedef types are not identical, reject them in all languages and
   // with any extensions enabled.
   if (isIncompatibleTypedef(Old, New))
@@ -2019,7 +2049,6 @@ void Sema::MergeTypedefNameDecl(TypedefNameDecl *New, LookupResult &OldDecls) {
   Diag(New->getLocation(), diag::ext_redefinition_of_typedef)
     << New->getDeclName();
   Diag(Old->getLocation(), diag::note_previous_definition);
-  return;
 }
 
 /// DeclhasAttr - returns true if decl Declaration already has the target