diff --git a/folly/FBString.h b/folly/FBString.h
index 4882aac..6c83046 100644
--- a/folly/FBString.h
+++ b/folly/FBString.h
@@ -1890,12 +1890,14 @@ inline basic_fbstring<E, T, A, S>& basic_fbstring<E, T, A, S>::operator=(
 
 template <typename E, class T, class A, class S>
 template <typename TP>
-inline typename std::enable_if<
-    std::is_same<
-        typename std::decay<TP>::type,
-        typename basic_fbstring<E, T, A, S>::value_type>::value,
-    basic_fbstring<E, T, A, S>&>::type
-basic_fbstring<E, T, A, S>::operator=(TP c) {
+inline auto
+basic_fbstring<E, T, A, S>::operator=(TP c)
+  -> typename std::enable_if<
+      std::is_same<
+          typename std::decay<TP>::type,
+          typename basic_fbstring<E, T, A, S>::value_type>::value,
+      basic_fbstring<E, T, A, S>&>::type
+{
   Invariant checker(*this);
 
   if (empty()) {