diff --git a/clang/include/clang/Serialization/ASTReader.h b/clang/include/clang/Serialization/ASTReader.h
index 63bad2663e0b..cc53b8ceb0cc 100644
--- a/clang/include/clang/Serialization/ASTReader.h
+++ b/clang/include/clang/Serialization/ASTReader.h
@@ -499,7 +499,8 @@ private:
   typedef ArrayRef<llvm::support::unaligned_uint32_t> LexicalContents;
 
   /// \brief Map from a DeclContext to its lexical contents.
-  llvm::DenseMap<const DeclContext*, LexicalContents> LexicalDecls;
+  llvm::DenseMap<const DeclContext*, std::pair<ModuleFile*, LexicalContents>>
+      LexicalDecls;
 
   /// \brief Map from the TU to its lexical contents from each module file.
   std::vector<std::pair<ModuleFile*, LexicalContents>> TULexicalDecls;
diff --git a/clang/lib/Serialization/ASTReader.cpp b/clang/lib/Serialization/ASTReader.cpp
index ad528b0213ab..91dc4bdbc90e 100644
--- a/clang/lib/Serialization/ASTReader.cpp
+++ b/clang/lib/Serialization/ASTReader.cpp
@@ -975,11 +975,18 @@ bool ASTReader::ReadLexicalDeclContextStorage(ModuleFile &M,
 
   assert(!isa<TranslationUnitDecl>(DC) &&
          "expected a TU_UPDATE_LEXICAL record for TU");
-  // FIXME: Once we remove RewriteDecl, assert that we didn't already have
-  // lexical decls for this context.
-  LexicalDecls[DC] = llvm::makeArrayRef(
-      reinterpret_cast<const llvm::support::unaligned_uint32_t *>(Blob.data()),
-      Blob.size() / 4);
+  // If we are handling a C++ class template instantiation, we can see multiple
+  // lexical updates for the same record. It's important that we select only one
+  // of them, so that field numbering works properly. Just pick the first one we
+  // see.
+  auto &Lex = LexicalDecls[DC];
+  if (!Lex.first) {
+    Lex = std::make_pair(
+        &M, llvm::makeArrayRef(
+                reinterpret_cast<const llvm::support::unaligned_uint32_t *>(
+                    Blob.data()),
+                Blob.size() / 4));
+  }
   DC->setHasExternalLexicalStorage(true);
   return false;
 }
@@ -6253,7 +6260,7 @@ void ASTReader::FindExternalLexicalDecls(
   } else {
     auto I = LexicalDecls.find(DC);
     if (I != LexicalDecls.end())
-      Visit(getOwningModuleFile(cast<Decl>(DC)), I->second);
+      Visit(I->second.first, I->second.second);
   }
 
   ++NumLexicalDeclContextsRead;
diff --git a/clang/test/Modules/Inputs/templates-right.h b/clang/test/Modules/Inputs/templates-right.h
index daea97b86b88..32487c60c269 100644
--- a/clang/test/Modules/Inputs/templates-right.h
+++ b/clang/test/Modules/Inputs/templates-right.h
@@ -38,6 +38,10 @@ int defineListDoubleRight() {
   return ld.size;
 }
 
+inline void defineListLongRight() {
+  List<long> ll;
+}
+
 template<typename T> struct MergePatternDecl;
 
 void outOfLineInlineUseRightF(void (OutOfLineInline<int>::*)() = &OutOfLineInline<int>::f);
diff --git a/clang/test/Modules/Inputs/templates-top.h b/clang/test/Modules/Inputs/templates-top.h
index 31f5e4199281..a08268352399 100644
--- a/clang/test/Modules/Inputs/templates-top.h
+++ b/clang/test/Modules/Inputs/templates-top.h
@@ -10,6 +10,7 @@ public:
 };
 
 extern List<double> *instantiateListDoubleDeclaration;
+extern List<long> *instantiateListLongDeclaration;
 
 namespace A {
   class Y {
diff --git a/clang/test/Modules/templates.mm b/clang/test/Modules/templates.mm
index 4c6e4723a3bb..190084ad2b74 100644
--- a/clang/test/Modules/templates.mm
+++ b/clang/test/Modules/templates.mm
@@ -28,6 +28,8 @@ void testTemplateClasses() {
   N::Set<char> set_char;
   set_char.insert('A');
 
+  static_assert(sizeof(List<long>) == sizeof(List<short>), "");
+
   List<double> list_double;
   list_double.push_back(0.0);
 }