PickTaskWrapper fallback for old compilers

Summary: On old compilers, we don't have `SafeTask`, so even though `coro::Future<int>` (e.g.) is safe, we have to derive `Task` instead of `SafeTask` from it in `collectAll()` and `timeout()`.

Reviewed By: ispeters

Differential Revision: D77839263

fbshipit-source-id: 54e514a543bc941c24755ca9ab92c9e748ba47f7

Author: snarkmaster

third-party/folly/src/folly/coro/detail/PickTaskWrapper.h

@@ -106,6 +106,18 @@ struct PickTaskWrapperImpl<Safety, /*await now*/ false> {
   using TaskWithExecutor = SafeTaskWithExecutor<Safety, T>;
 };
 
+#else // no FOLLY_HAS_IMMOVABLE_COROUTINES
+
+// This fallback is required because `coro::Future<SafeType>` is safe and is
+// available on earlier build systems.  We have no choice but to emit `Task`.
+template <safe_alias Safety>
+struct PickTaskWrapperImpl<Safety, /*await now*/ false> {
+  template <typename T>
+  using Task = Task<T>;
+  template <typename T>
+  using TaskWithExecutor = TaskWithExecutor<T>;
+};
+
 #endif // FOLLY_HAS_IMMOVABLE_COROUTINES
 
 // Pass this as `AddWrapperMetaFn` to `PickTaskWrapper` to add `AsNoexcept`.