Index: lib/AST/ASTImporter.cpp
===================================================================
--- lib/AST/ASTImporter.cpp
+++ lib/AST/ASTImporter.cpp
@@ -1946,6 +1946,12 @@
 }
 
 bool ASTNodeImporter::IsStructuralMatch(EnumDecl *FromEnum, EnumDecl *ToEnum) {
+  // Eliminate a potential failure point where we attempt to re-import
+  // something we're trying to import while completin ToEnum
+  if (Decl *ToOrigin = Importer.GetOriginalDecl(ToEnum))
+    if (auto *ToOriginEnum = dyn_cast<EnumDecl>(ToOrigin))
+        ToEnum = ToOriginEnum;
+
   StructuralEquivalenceContext Ctx(
       Importer.getFromContext(), Importer.getToContext(),
       Importer.getNonEquivalentDecls(), getStructuralEquivalenceKind(Importer));