Revert D88284968: Slight cleanup of find_by_ordinal

Differential Revision:
D88284968

Original commit changeset: ac3a59f9418a

Original Phabricator Diff: D88284968

fbshipit-source-id: 2d2da05b4aeb248481906602d9b5e71afb00f6c8

Author: Wenbin Lin

third-party/thrift/src/thrift/lib/cpp2/op/Get.h

@@ -250,10 +250,7 @@ using ord_result_t =
     decltype(std::declval<F>()(type::detail::pos_to_ordinal<I>{}));
 
 template <size_t... I, typename F>
-constexpr ord_result_t<F> find_by_ordinal_impl(
-    F&& f, std::index_sequence<I...>);
-template <typename F>
-constexpr bool find_by_ordinal_impl(F&&, std::index_sequence<>);
+ord_result_t<F> find_by_ordinal_impl(F&& f, std::index_sequence<I...>);
 
 struct GetValueOrNull {
   template <typename T>
@@ -337,15 +334,22 @@ constexpr void for_each_ordinal(F&& f) {
       std::forward<F>(f), std::make_integer_sequence<size_t, num_fields<T>>{});
 }
 
-/// Calls the given function with with ordinal<1> to ordinal<N>, returning the
-/// first truthy result produced.
-/// Returns false when called on empty structs.
-template <typename T, typename F>
-constexpr decltype(auto) find_by_ordinal(F&& f) {
+/// Calls the given function with with ordinal<1> to ordinal<N>, returing the
+/// first 'true' result produced.
+template <
+    typename T,
+    typename F,
+    std::enable_if_t<num_fields<T> != 0>* = nullptr>
+decltype(auto) find_by_ordinal(F&& f) {
   return detail::find_by_ordinal_impl(
       std::forward<F>(f), std::make_integer_sequence<size_t, num_fields<T>>{});
 }
 
+template <typename T, typename F>
+std::enable_if_t<num_fields<T> == 0, bool> find_by_ordinal(F&&) {
+  return false;
+}
+
 template <typename T, typename Id>
 using get_field_id = type::field_id<
     get_ordinal<T, Id>::value == type::Ordinal()
@@ -473,8 +477,7 @@ constexpr void for_each_ordinal_impl(F&& f, std::index_sequence<I...>) {
 }
 
 template <size_t... I, typename F>
-constexpr ord_result_t<F> find_by_ordinal_impl(
-    F&& f, std::index_sequence<I...>) {
+ord_result_t<F> find_by_ordinal_impl(F&& f, std::index_sequence<I...>) {
   auto result = ord_result_t<F>();
   // TODO(afuller): Use a short circuting c++17 folding expression.
   for_each_ordinal_impl(
@@ -487,10 +490,6 @@ constexpr ord_result_t<F> find_by_ordinal_impl(
       std::index_sequence<I...>{});
   return result;
 }
-template <typename F>
-constexpr bool find_by_ordinal_impl(F&&, std::index_sequence<>) {
-  return false;
-}
 
 template <typename Id, typename T>
 struct Get {