work around gcc < 7 in UniqueInstance

Summary: Some older versions of gcc cannot deduce the relevant types correctly, and they are still in use in corners of the codebase.

Differential Revision: D28111907

fbshipit-source-id: 0367cb102f450f0f601350308a5831c7fd68f775

folly/detail/UniqueInstance.h

@@ -26,6 +26,9 @@ namespace detail {
 
 class UniqueInstance {
  public:
+#if __GNUC__ && __GNUC__ < 7 && !__clang__
+  explicit UniqueInstance(...) noexcept {}
+#else
   template <template <typename...> class Z, typename... Key, typename... Mapped>
   FOLLY_EXPORT explicit UniqueInstance(
       tag_t<Z<Key..., Mapped...>>, tag_t<Key...>, tag_t<Mapped...>) noexcept {
@@ -34,6 +37,7 @@ class UniqueInstance {
     auto& global = createGlobal<Value, key_t<Z, Key...>>();
     enforce(tmpl, ptrs, sizeof...(Key), sizeof...(Mapped), global);
   }
+#endif
 
   UniqueInstance(UniqueInstance const&) = delete;
   UniqueInstance(UniqueInstance&&) = delete;