add DeprecatedOptionalField structure

Summary:
Currently if we have "optionals" in thrift_cpp2_options, we will use folly::Optional for optional fields. The motivation of this task is that we want to get rid of "optionals" in thrift_cpp2_options and migrate existing code to use optional_field_ref instead.

As Step #1, we will create new thrift::DeprecatedOptionalField that's compatible with folly::Optional, and can be implicitly casted to each other. After the first step it done, we can codemod the API of thrift::DeprecatedOptionalField to make it identical to optional_field_ref.

This diff created DeprecatedOptionalField struct. I will modify thrift compiler in following diff to generate optional field with this code.

Reviewed By: vitaut

Differential Revision: D19785312

fbshipit-source-id: b5fcaadc9aec8d0a6a223e69bbcb7b565a27210f

thrift/lib/cpp2/OptionalField.h

@@ -0,0 +1,116 @@
+/*
+ * Copyright (c) Facebook, Inc. and its affiliates.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#pragma once
+
+#include <folly/Optional.h>
+#include <folly/Portability.h>
+#include <type_traits>
+
+namespace apache {
+namespace thrift {
+
+/**
+ * DeprecatedOptionalField is used for thrift optional field with "optionals"
+ * turned on. It will be eventually replaced by thrift::optional_field_ref.
+ */
+template <typename T>
+class DeprecatedOptionalField : public folly::Optional<T> {
+ private:
+  const folly::Optional<T>& toFolly() const {
+    return *this;
+  }
+
+ public:
+  using folly::Optional<T>::Optional;
+  using folly::Optional<T>::operator=;
+
+  template <typename L, typename R>
+  friend bool operator==(
+      const DeprecatedOptionalField<L>& lhs,
+      const DeprecatedOptionalField<R>& rhs);
+  template <typename L, typename R>
+  friend bool operator!=(
+      const DeprecatedOptionalField<L>& lhs,
+      const DeprecatedOptionalField<R>& rhs);
+  template <typename U>
+  friend bool operator==(
+      const DeprecatedOptionalField<U>& lhs,
+      const folly::Optional<U>& rhs);
+  template <typename U>
+  friend bool operator==(
+      const folly::Optional<U>& lhs,
+      const DeprecatedOptionalField<U>& rhs);
+  template <typename U>
+  friend bool operator!=(
+      const DeprecatedOptionalField<U>& lhs,
+      const folly::Optional<U>& rhs);
+  template <typename U>
+  friend bool operator!=(
+      const folly::Optional<U>& lhs,
+      const DeprecatedOptionalField<U>& rhs);
+};
+
+template <typename L, typename R>
+bool operator==(
+    const DeprecatedOptionalField<L>& lhs,
+    const DeprecatedOptionalField<R>& rhs) {
+  return lhs.toFolly() == rhs.toFolly();
+}
+template <typename L, typename R>
+bool operator!=(
+    const DeprecatedOptionalField<L>& lhs,
+    const DeprecatedOptionalField<R>& rhs) {
+  return lhs.toFolly() != rhs.toFolly();
+}
+template <typename U>
+bool operator==(
+    const DeprecatedOptionalField<U>& lhs,
+    const folly::Optional<U>& rhs) {
+  return lhs.toFolly() == rhs;
+}
+template <typename U>
+bool operator==(
+    const folly::Optional<U>& lhs,
+    const DeprecatedOptionalField<U>& rhs) {
+  return lhs == rhs.toFolly();
+}
+template <typename U>
+bool operator!=(
+    const DeprecatedOptionalField<U>& lhs,
+    const folly::Optional<U>& rhs) {
+  return lhs.toFolly() != rhs;
+}
+template <typename U>
+bool operator!=(
+    const folly::Optional<U>& lhs,
+    const DeprecatedOptionalField<U>& rhs) {
+  return lhs != rhs.toFolly();
+}
+
+template <class T>
+folly::Optional<T> castToFolly(const DeprecatedOptionalField<T>& t) {
+  return t;
+}
+
+} // namespace thrift
+} // namespace apache
+
+FOLLY_NAMESPACE_STD_BEGIN
+template <class T>
+struct hash<apache::thrift::DeprecatedOptionalField<T>>
+    : hash<folly::Optional<T>> {};
+FOLLY_NAMESPACE_STD_END

thrift/lib/cpp2/Thrift.h

@@ -20,6 +20,7 @@
 #include <thrift/lib/cpp/Thrift.h>
 #include <thrift/lib/cpp/protocol/TType.h>
 #include <thrift/lib/cpp2/FieldRef.h>
+#include <thrift/lib/cpp2/OptionalField.h>
 #include <thrift/lib/cpp2/TypeClass.h>
 
 #include <initializer_list>