Canonical SFINAE in fibers

Summary: [Folly] Canonical SFINAE in fibers, where the `enable_if` is on the outside.

Reviewed By: andriigrynenko

Differential Revision: D14294993

fbshipit-source-id: 498c9bfcd346edb5e4b5b65941e2a09c93e870b6

folly/fibers/WhenN-inl.h

@@ -22,15 +22,15 @@ namespace folly {
 namespace fibers {
 
 template <class InputIterator>
-typename std::vector<typename std::enable_if<
+typename std::enable_if<
     !std::is_same<
         invoke_result_t<
             typename std::iterator_traits<InputIterator>::value_type>,
         void>::value,
-    typename std::pair<
+    typename std::vector<typename std::pair<
         size_t,
         invoke_result_t<
-            typename std::iterator_traits<InputIterator>::value_type>>>::type>
+            typename std::iterator_traits<InputIterator>::value_type>>>>::type
 collectN(InputIterator first, InputIterator last, size_t n) {
   typedef invoke_result_t<
       typename std::iterator_traits<InputIterator>::value_type>

folly/fibers/WhenN.h

@@ -39,17 +39,16 @@ namespace fibers {
  * @return vector of pairs (task index, return value of task)
  */
 template <class InputIterator>
-typename std::vector<
-    typename std::enable_if<
-        !std::is_same<
-            invoke_result_t<
-                typename std::iterator_traits<InputIterator>::value_type>,
-            void>::value,
-        typename std::pair<
-            size_t,
-            invoke_result_t<
-                typename std::iterator_traits<InputIterator>::value_type>>>::
-        type> inline collectN(InputIterator first, InputIterator last, size_t n);
+typename std::enable_if<
+    !std::is_same<
+        invoke_result_t<
+            typename std::iterator_traits<InputIterator>::value_type>,
+        void>::value,
+    typename std::vector<typename std::pair<
+        size_t,
+        invoke_result_t<
+            typename std::iterator_traits<InputIterator>::value_type>>>>::
+    type inline collectN(InputIterator first, InputIterator last, size_t n);
 
 /**
  * collectN specialization for functions returning void