Index: lib/Serialization/ASTCommon.h
===================================================================
--- lib/Serialization/ASTCommon.h
+++ lib/Serialization/ASTCommon.h
@@ -96,6 +96,7 @@
 template<typename Fn> void numberAnonymousDeclsWithin(const DeclContext *DC,
                                                       Fn Visit) {
   unsigned Index = 0;
+  SmallVector<NamedDecl *, 2> DeclsToVisit;
   for (Decl *LexicalD : DC->decls()) {
     // For a friend decl, we care about the declaration within it, if any.
     if (auto *FD = dyn_cast<FriendDecl>(LexicalD))
@@ -105,8 +106,17 @@
     if (!ND || !needsAnonymousDeclarationNumber(ND))
       continue;
 
-    Visit(ND, Index++);
+    DeclsToVisit.push_back(ND);
   }
+
+  // Decls have deterministic IDs, but they might show up in different order in
+  // a DeclContext. Make sure serialization gets the same order everytime by
+  // sorting the decls by ID.
+  llvm::sort(DeclsToVisit, [](const NamedDecl *L, const NamedDecl *R) {
+    return L->getID() < R->getID();
+  });
+  for (auto *ND : DeclsToVisit)
+    Visit(ND, Index++);
 }
 
 } // namespace serialization
Index: lib/Serialization/ASTWriter.cpp
===================================================================
--- lib/Serialization/ASTWriter.cpp
+++ lib/Serialization/ASTWriter.cpp
@@ -3182,7 +3182,16 @@
 
   uint64_t Offset = Stream.GetCurrentBitNo();
   SmallVector<uint32_t, 128> KindDeclPairs;
-  for (const auto *D : DC->decls()) {
+
+  // Decls have deterministic IDs, but they might show up in different order in
+  // a DeclContext. Make sure serialization gets the same order everytime by
+  // sorting the decls by ID.
+  SmallVector<Decl *, 64> SortedDecls(DC->decls());
+  llvm::sort(SortedDecls, [](const Decl *L, const Decl *R) {
+    return L->getID() < R->getID();
+  });
+
+  for (const auto *D : SortedDecls) {
     KindDeclPairs.push_back(D->getKind());
     KindDeclPairs.push_back(GetDeclRef(D));
   }