1 files changed, 18 insertions, 9 deletions

diff --git a/clang/lib/Serialization/ASTReaderDecl.cpp b/clang/lib/Serialization/ASTReaderDecl.cpp
index 96c16ea10ee..83882a83693 100644
--- a/clang/lib/Serialization/ASTReaderDecl.cpp
+++ b/clang/lib/Serialization/ASTReaderDecl.cpp
@@ -1198,13 +1198,13 @@ void ASTDeclReader::VisitNamespaceDecl(NamespaceDecl *D) {
   D->LocStart = ReadSourceLocation(Record, Idx);
   D->RBraceLoc = ReadSourceLocation(Record, Idx);
 
+  // Defer loading the anonymous namespace until we've finished merging
+  // this namespace; loading it might load a later declaration of the
+  // same namespace, and we have an invariant that older declarations
+  // get merged before newer ones try to merge.
+  GlobalDeclID AnonNamespace = 0;
   if (Redecl.getFirstID() == ThisDeclID) {
-    // Each module has its own anonymous namespace, which is disjoint from
-    // any other module's anonymous namespaces, so don't attach the anonymous
-    // namespace at all.
-    NamespaceDecl *Anon = ReadDeclAs<NamespaceDecl>(Record, Idx);
-    if (F.Kind != MK_ImplicitModule && F.Kind != MK_ExplicitModule)
-      D->setAnonymousNamespace(Anon);
+    AnonNamespace = ReadDeclID(Record, Idx);
   } else {
     // Link this namespace back to the first declaration, which has already
     // been deserialized.
@@ -1212,6 +1212,15 @@ void ASTDeclReader::VisitNamespaceDecl(NamespaceDecl *D) {
   }
 
   mergeRedeclarable(D, Redecl);
+
+  if (AnonNamespace) {
+    // Each module has its own anonymous namespace, which is disjoint from
+    // any other module's anonymous namespaces, so don't attach the anonymous
+    // namespace at all.
+    NamespaceDecl *Anon = cast<NamespaceDecl>(Reader.GetDecl(AnonNamespace));
+    if (F.Kind != MK_ImplicitModule && F.Kind != MK_ExplicitModule)
+      D->setAnonymousNamespace(Anon);
+  }
 }
 
 void ASTDeclReader::VisitNamespaceAliasDecl(NamespaceAliasDecl *D) {
@@ -2196,6 +2205,8 @@ void ASTDeclReader::mergeRedeclarable(Redeclarable<T> *DBase, T *Existing,
     D->RedeclLink = Redeclarable<T>::PreviousDeclLink(ExistingCanon);
 
     // When we merge a namespace, update its pointer to the first namespace.
+    // We cannot have loaded any redeclarations of this declaration yet, so
+    // there's nothing else that needs to be updated.
     if (auto *Namespace = dyn_cast<NamespaceDecl>(D))
       Namespace->AnonOrFirstNamespaceAndInline.setPointer(
           assert_cast<NamespaceDecl*>(ExistingCanon));
@@ -2206,9 +2217,7 @@ void ASTDeclReader::mergeRedeclarable(Redeclarable<T> *DBase, T *Existing,
           DTemplate, assert_cast<RedeclarableTemplateDecl*>(ExistingCanon),
           TemplatePatternID);
 
-    // If this declaration was the canonical declaration, make a note of
-    // that. We accept the linear algorithm here because the number of
-    // unique canonical declarations of an entity should always be tiny.
+    // If this declaration was the canonical declaration, make a note of that.
     if (DCanon == D) {
       Reader.MergedDecls[ExistingCanon].push_back(Redecl.getFirstID());
       if (Reader.PendingDeclChainsKnown.insert(ExistingCanon).second)