Add folly::Optional::to_optional for easier std::optional conversion

Summary: see title. `static_cast<optional<T>>(foo)` is too much typing. I like thrift optional_field_ref<T>'s to_optional (though on a side note I thought operator* of optional_field_ref T would give me a std::optional but i was wrong)

Reviewed By: yfeldblum

Differential Revision: D57116214

fbshipit-source-id: 970f4de50ae2fe8e057b320d6c2c881e55aefac7

third-party/folly/src/folly/Optional.h

@@ -201,7 +201,7 @@ class Optional {
       construct(*newValue);
     }
   }
-  /// Allow implict cast to std::optional
+  /// Allow explicit cast to std::optional
   /// @methodset Migration
   explicit operator std::optional<Value>() && noexcept(
       std::is_nothrow_move_constructible<Value>::value) {
@@ -217,6 +217,14 @@ class Optional {
                              : std::nullopt;
   }
 
+  std::optional<Value> toStdOptional() && noexcept {
+    return static_cast<std::optional<Value>>(std::move(*this));
+  }
+
+  std::optional<Value> toStdOptional() const& noexcept {
+    return static_cast<std::optional<Value>>(*this);
+  }
+
   /// Set the Optional
   /// @methodset Modifiers
   void assign(const None&) { reset(); }

third-party/folly/src/folly/test/OptionalTest.cpp

@@ -840,11 +840,17 @@ TEST(Optional, StdOptionalConversions) {
   folly::Optional<int> f = 42;
   std::optional<int> s = static_cast<std::optional<int>>(f);
   EXPECT_EQ(*s, 42);
+  EXPECT_EQ(s, f.toStdOptional());
   EXPECT_TRUE(f);
+
   f = static_cast<folly::Optional<int>>(s);
   EXPECT_EQ(*f, 42);
   EXPECT_TRUE(s);
 
+  folly::Optional<int> e;
+  EXPECT_FALSE(static_cast<std::optional<int>>(e));
+  EXPECT_FALSE(e.toStdOptional());
+
   const folly::Optional<int> fc = 12;
   s = static_cast<std::optional<int>>(fc);
   EXPECT_EQ(*s, 12);
@@ -856,6 +862,11 @@ TEST(Optional, StdOptionalConversions) {
   fp = static_cast<folly::Optional<std::unique_ptr<int>>>(std::move(sp));
   EXPECT_EQ(**fp, 42);
   EXPECT_FALSE(sp);
+
+  folly::Optional<std::unique_ptr<int>> fp2 = std::make_unique<int>(42);
+  auto sp2 = std::move(fp2).toStdOptional();
+  EXPECT_EQ(**sp2, 42);
+  EXPECT_FALSE(fp2);
 }
 
 TEST(Optional, MovedFromOptionalIsEmpty) {