diff --git a/lib/AST/DeclBase.cpp b/lib/AST/DeclBase.cpp
index 4eceda4fac0f616a3cfa4c1fa3adec794405ce0f..8e7373631dbbbea7437216defd76b3b15b02662e 100644
--- a/lib/AST/DeclBase.cpp
+++ b/lib/AST/DeclBase.cpp
@@ -85,6 +85,7 @@ const char *Decl::getDeclKindName() const {
 
 void Decl::setInvalidDecl(bool Invalid) {
   InvalidDecl = Invalid;
+  assert(!isa<TagDecl>(this) || !cast<TagDecl>(this)->isCompleteDefinition());
   if (Invalid && !isa<ParmVarDecl>(this)) {
     // Defensive maneuver for ill-formed code: we're likely not to make it to
     // a point where we set the access specifier, so default it to "public"