Make Expected(EmptyTag) ctor quasi-public for coroutine promises

Summary: I'm making `folly::exception_wrapper`-centric short-circuiting coroutine on the basis of `Expected`, and the current `PromiseReturn` magic relies on having access to this `EmptyTag` ctor.

Reviewed By: yfeldblum

Differential Revision: D56957309

fbshipit-source-id: 315b4342f101225ff5932db4eb678faab1ecf2cb

folly/Expected.h

@@ -1372,13 +1372,23 @@ class Expected final : expected_detail::ExpectedStorage<Value, Error> {
         std::move(base()), static_cast<Yes&&>(yes), static_cast<No&&>(no)));
   }
 
+  // Quasi-private, exposed only for implementing efficient short-circuiting
+  // coroutines on top of `Expected`.  Do NOT use this instead of
+  // `optional<Expected<>>`, for these reasons:
+  //   - This public ctor again become private in the future.
+  //   - It is incompatible with upcoming `std::expected`.
+  //   - It violates `folly::Expected`'s almost-never-empty guarantee.
+  //   - There's no native `empty()` test -- `uninitializedByException()` is
+  //     always `false` for some value types, and `!hasValue() && !hasError()`
+  //     is less efficient.
+  explicit Expected(expected_detail::EmptyTag tag) noexcept : Base{tag} {}
+
  private:
   friend struct expected_detail::PromiseReturn<Value, Error>;
-  using EmptyTag = expected_detail::EmptyTag;
 
-  explicit Expected(EmptyTag tag) noexcept : Base{tag} {}
   // for when coroutine promise return-object conversion is eager
-  Expected(EmptyTag tag, Expected*& pointer) noexcept : Base{tag} {
+  Expected(expected_detail::EmptyTag tag, Expected*& pointer) noexcept
+      : Base{tag} {
     pointer = this;
   }