diff --git a/clang/lib/AST/ASTImporter.cpp b/clang/lib/AST/ASTImporter.cpp
index 29a25338a725d..d271d82a3364f 100644
--- a/clang/lib/AST/ASTImporter.cpp
+++ b/clang/lib/AST/ASTImporter.cpp
@@ -1878,7 +1878,7 @@ ASTNodeImporter::ImportDeclContext(DeclContext *FromDC, bool ForceImport) {
       continue;
     }
 
-    if (Importer.hasLLDBRedeclCompletion()) {
+    if (!Importer.hasLLDBRedeclCompletion()) {
       FieldDecl *FieldFrom = dyn_cast_or_null<FieldDecl>(From);
       Decl *ImportedDecl = *ImportedOrErr;
       FieldDecl *FieldTo = dyn_cast_or_null<FieldDecl>(ImportedDecl);
diff --git a/lldb/source/Plugins/ExpressionParser/Clang/ClangASTImporter.cpp b/lldb/source/Plugins/ExpressionParser/Clang/ClangASTImporter.cpp
index 723a4d0cfcabe..eabe8a1776ee9 100644
--- a/lldb/source/Plugins/ExpressionParser/Clang/ClangASTImporter.cpp
+++ b/lldb/source/Plugins/ExpressionParser/Clang/ClangASTImporter.cpp
@@ -824,23 +824,17 @@ bool ClangASTImporter::CompleteTagDecl(clang::TagDecl *decl) {
   if (!decl_origin.Valid())
     return false;
 
-  ImporterDelegateSP delegate_sp(
-      GetDelegate(&decl->getASTContext(), decl_origin.ctx));
-
-  ASTImporterDelegate::CxxModuleScope std_scope(*delegate_sp,
-                                                &decl->getASTContext());
-
-  if (!TypeSystemClang::UseRedeclCompletion()) {
-    if (!TypeSystemClang::GetCompleteDecl(decl_origin.ctx, decl_origin.decl))
-      return false;
-
-    if (delegate_sp)
-      delegate_sp->ImportDefinitionTo(decl, decl_origin.decl);
-  } else {
+  if (TypeSystemClang::UseRedeclCompletion()) {
     auto *origin_def = llvm::cast<TagDecl>(decl_origin.decl)->getDefinition();
     if (!origin_def)
       return false;
 
+    ImporterDelegateSP delegate_sp(
+        GetDelegate(&decl->getASTContext(), decl_origin.ctx));
+
+    ASTImporterDelegate::CxxModuleScope std_scope(*delegate_sp,
+                                                  &decl->getASTContext());
+
     // This is expected to pull in a definition for result_decl (if in redecl
     // completion mode)
     llvm::Expected<Decl *> result = delegate_sp->Import(origin_def);
@@ -859,6 +853,18 @@ bool ClangASTImporter::CompleteTagDecl(clang::TagDecl *decl) {
     if (!decl->isThisDeclarationADefinition() && result_decl != decl)
       if (result_decl->getPreviousDecl() == nullptr)
         result_decl->setPreviousDecl(decl);
+  } else {
+    if (!TypeSystemClang::GetCompleteDecl(decl_origin.ctx, decl_origin.decl))
+      return false;
+
+    ImporterDelegateSP delegate_sp(
+        GetDelegate(&decl->getASTContext(), decl_origin.ctx));
+
+    ASTImporterDelegate::CxxModuleScope std_scope(*delegate_sp,
+                                                  &decl->getASTContext());
+
+    if (delegate_sp)
+      delegate_sp->ImportDefinitionTo(decl, decl_origin.decl);
   }
 
   return true;
@@ -880,19 +886,7 @@ bool ClangASTImporter::CompleteObjCInterfaceDecl(
   if (!decl_origin.Valid())
     return false;
 
-  ImporterDelegateSP delegate_sp(
-      GetDelegate(&interface_decl->getASTContext(), decl_origin.ctx));
-
-  if (!TypeSystemClang::UseRedeclCompletion()) {
-    if (!TypeSystemClang::GetCompleteDecl(decl_origin.ctx, decl_origin.decl))
-      return false;
-
-    if (delegate_sp)
-      delegate_sp->ImportDefinitionTo(interface_decl, decl_origin.decl);
-
-    if (ObjCInterfaceDecl *super_class = interface_decl->getSuperClass())
-      RequireCompleteType(clang::QualType(super_class->getTypeForDecl(), 0));
-  } else {
+  if (TypeSystemClang::UseRedeclCompletion()) {
     ObjCInterfaceDecl *origin_decl =
         llvm::cast<ObjCInterfaceDecl>(decl_origin.decl);
 
@@ -900,7 +894,7 @@ bool ClangASTImporter::CompleteObjCInterfaceDecl(
     if (!origin_decl)
       return false;
 
-    auto delegate_sp(
+    ImporterDelegateSP delegate_sp(
         GetDelegate(&interface_decl->getASTContext(), decl_origin.ctx));
 
     llvm::Expected<Decl *> result = delegate_sp->Import(origin_decl);
@@ -915,6 +909,18 @@ bool ClangASTImporter::CompleteObjCInterfaceDecl(
     return false;
   }
 
+  if (!TypeSystemClang::GetCompleteDecl(decl_origin.ctx, decl_origin.decl))
+    return false;
+
+  ImporterDelegateSP delegate_sp(
+      GetDelegate(&interface_decl->getASTContext(), decl_origin.ctx));
+
+  if (delegate_sp)
+    delegate_sp->ImportDefinitionTo(interface_decl, decl_origin.decl);
+
+  if (ObjCInterfaceDecl *super_class = interface_decl->getSuperClass())
+    RequireCompleteType(clang::QualType(super_class->getTypeForDecl(), 0));
+
   return true;
 }
 
@@ -1204,8 +1210,6 @@ ClangASTImporter::ASTImporterDelegate::ImportImpl(Decl *From) {
     DeclContext::lookup_result lr = dc->lookup(*dn_or_err);
     for (clang::Decl *candidate : lr) {
       if (candidate->getKind() == From->getKind()) {
-        RegisterImportedDecl(From, candidate);
-
         // If we're dealing with redecl chains. We want to find the definition,
         // so skip if the decl is actually just a forwad decl.
         if (TypeSystemClang::UseRedeclCompletion())
@@ -1213,6 +1217,7 @@ ClangASTImporter::ASTImporterDelegate::ImportImpl(Decl *From) {
               !tag_decl || !tag_decl->getDefinition())
             continue;
 
+        RegisterImportedDecl(From, candidate);
         m_decls_to_ignore.insert(candidate);
         return candidate;
       }
diff --git a/lldb/source/Plugins/ExpressionParser/Clang/ClangASTSource.h b/lldb/source/Plugins/ExpressionParser/Clang/ClangASTSource.h
index b37cc0a60b3dc..7128befb62293 100644
--- a/lldb/source/Plugins/ExpressionParser/Clang/ClangASTSource.h
+++ b/lldb/source/Plugins/ExpressionParser/Clang/ClangASTSource.h
@@ -214,7 +214,7 @@ class ClangASTSource : public ImporterBackedASTSource,
   ///
   /// Clang AST contexts like to own their AST sources, so this is a state-
   /// free proxy object.
-  class ClangASTSourceProxy : public clang::ExternalASTSource {
+  class ClangASTSourceProxy : public ImporterBackedASTSource {
   public:
     ClangASTSourceProxy(ClangASTSource &original) : m_original(original) {}
 
diff --git a/lldb/source/Plugins/TypeSystem/Clang/TypeSystemClang.cpp b/lldb/source/Plugins/TypeSystem/Clang/TypeSystemClang.cpp
index a83a0e0de7c41..b430b59a9f12e 100644
--- a/lldb/source/Plugins/TypeSystem/Clang/TypeSystemClang.cpp
+++ b/lldb/source/Plugins/TypeSystem/Clang/TypeSystemClang.cpp
@@ -1788,6 +1788,8 @@ TypeSystemClang::CreateClassTemplateSpecializationDecl(
       static_cast<TagDecl::TagKind>(kind));
   class_template_specialization_decl->setDeclContext(decl_ctx);
   class_template_specialization_decl->setInstantiationOf(class_template_decl);
+  if (TypeSystemClang::UseRedeclCompletion())
+    ast.getTypeDeclType(class_template_specialization_decl, nullptr);
   class_template_specialization_decl->setTemplateArgs(
       TemplateArgumentList::CreateCopy(ast, args));
   class_template_specialization_decl->setDeclName(