FTL: Downcast to Optional<T> implicitly

The expression `ftl::Optional(std::optional(T()))` should not have type
`ftl::Optional<std::optional<T>>`.

Bug: 185536303
Test: ftl_test
Change-Id: I931cc58b985e7c41037ed50bc68abdc8028c4bdd

1 files changed, 7 insertions, 1 deletions

diff --git a/include/ftl/optional.h b/include/ftl/optional.h
index a0a95c4b9a..626507fd8f 100644
--- a/include/ftl/optional.h
+++ b/include/ftl/optional.h
@@ -32,6 +32,9 @@ template <typename T>
 struct Optional final : std::optional<T> {
   using std::optional<T>::optional;
 
+  // Implicit downcast.
+  Optional(std::optional<T> other) : std::optional<T>(std::move(other)) {}
+
   using std::optional<T>::has_value;
   using std::optional<T>::value;
 
@@ -94,8 +97,11 @@ struct Optional final : std::optional<T> {
   }
 };
 
-// Deduction guide.
+// Deduction guides.
 template <typename T>
 Optional(T) -> Optional<T>;
 
+template <typename T>
+Optional(std::optional<T>) -> Optional<T>;
+
 }  // namespace android::ftl