if constexpr shortcut for CancellationToken::merge(x)

Summary: This takes the `merge1` microbench (stacked) from 74ns to 24ns without regressing anything. This is helpful for generic programs, where the pack MIGHT have 1 element, and there's no reason to shortcut this on the client side.

Reviewed By: Orvid

Differential Revision: D65044520

fbshipit-source-id: e2f91bbc69439d5759d79d8a5391f5ba3e0239ed

folly/CancellationToken-inl.h

@@ -448,12 +448,17 @@ std::pair<CancellationSource, std::tuple<Data...>*> CancellationSource::create(
 
 template <typename... Ts>
 inline CancellationToken CancellationToken::merge(Ts&&... tokens) {
-  bool canBeCancelled = detail::variadicDisjunction(tokens.canBeCancelled()...);
-  return canBeCancelled
-      ? CancellationToken(
-            detail::FixedMergingCancellationState<sizeof...(Ts)>::create(
-                std::forward<Ts>(tokens)...))
-      : CancellationToken();
+  if constexpr (sizeof...(Ts) == 1) {
+    return (tokens, ...);
+  } else {
+    bool canBeCancelled =
+        detail::variadicDisjunction(tokens.canBeCancelled()...);
+    return canBeCancelled
+        ? CancellationToken(
+              detail::FixedMergingCancellationState<sizeof...(Ts)>::create(
+                  std::forward<Ts>(tokens)...))
+        : CancellationToken();
+  }
 }
 
 } // namespace folly

folly/test/CancellationTokenTest.cpp

@@ -293,6 +293,12 @@ TEST(CancellationTokenTest, MergedToken) {
   EXPECT_FALSE(token.canBeCancelled());
 }
 
+TEST(CancellationTokenTest, Merging1TokenIsEfficient) {
+  CancellationSource src;
+  CancellationToken tok = src.getToken();
+  EXPECT_TRUE(tok == CancellationToken::merge(tok));
+}
+
 TEST(CancellationTokenTest, TokenWithData) {
   struct Guard {
     int& counter;