Move in_place and friends to Utility.h

Summary:
[Folly] Move `in_place` and friends to `Utility.h`.

Document that they are backports from C++17.

Reviewed By: Orvid

Differential Revision: D5362364

fbshipit-source-id: 495cb7f339fc558f20d89100c141fc7a243239d5

folly/ExceptionWrapper.h

@@ -36,6 +36,7 @@
 #include <folly/FBString.h>
 #include <folly/Portability.h>
 #include <folly/Traits.h>
+#include <folly/Utility.h>
 
 #ifdef __GNUC__
 #pragma GCC diagnostic push

folly/Expected.h

@@ -32,8 +32,9 @@
 #include <folly/Likely.h>
 #include <folly/Portability.h>
 #include <folly/Preprocessor.h>
-#include <folly/Traits.h> // for in_place_t
+#include <folly/Traits.h>
 #include <folly/Unit.h>
+#include <folly/Utility.h>
 
 #define FOLLY_EXPECTED_ID(X) FB_CONCATENATE(FB_CONCATENATE(Folly, X), __LINE__)
 

folly/Traits.h

@@ -606,92 +606,6 @@ bool greater_than(LHS const lhs) {
     RHS, rhs, typename std::remove_reference<LHS>::type
   >(lhs);
 }
-
-namespace traits_detail {
-struct InPlaceTag {};
-template <class>
-struct InPlaceTypeTag {};
-template <std::size_t>
-struct InPlaceIndexTag {};
-}
-
-/**
- * Like std::piecewise_construct, a tag type & instance used for in-place
- * construction of non-movable contained types, e.g. by Synchronized.
- * Follows the naming and design of std::in_place suggested in
- * http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2016/p0032r2.pdf
- */
-using in_place_t = traits_detail::InPlaceTag (&)(traits_detail::InPlaceTag);
-
-template <class T>
-using in_place_type_t =
-    traits_detail::InPlaceTypeTag<T> (&)(traits_detail::InPlaceTypeTag<T>);
-
-template <std::size_t I>
-using in_place_index_t =
-    traits_detail::InPlaceIndexTag<I> (&)(traits_detail::InPlaceIndexTag<I>);
-
-inline traits_detail::InPlaceTag in_place(traits_detail::InPlaceTag = {}) {
-  return {};
-}
-
-template <class T>
-inline traits_detail::InPlaceTypeTag<T> in_place_type(
-    traits_detail::InPlaceTypeTag<T> = {}) {
-  return {};
-}
-
-template <std::size_t I>
-inline traits_detail::InPlaceIndexTag<I> in_place_index(
-    traits_detail::InPlaceIndexTag<I> = {}) {
-  return {};
-}
-
-/**
- * Initializer lists are a powerful compile time syntax introduced in C++11
- * but due to their often conflicting syntax they are not used by APIs for
- * construction.
- *
- * Further standard conforming compilers *strongly* favor an
- * std::initalizer_list overload for construction if one exists.  The
- * following is a simple tag used to disambiguate construction with
- * initializer lists and regular uniform initialization.
- *
- * For example consider the following case
- *
- *  class Something {
- *  public:
- *    explicit Something(int);
- *    Something(std::intiializer_list<int>);
- *
- *    operator int();
- *  };
- *
- *  ...
- *  Something something{1}; // SURPRISE!!
- *
- * The last call to instantiate the Something object will go to the
- * initializer_list overload.  Which may be surprising to users.
- *
- * If however this tag was used to disambiguate such construction it would be
- * easy for users to see which construction overload their code was referring
- * to.  For example
- *
- *  class Something {
- *  public:
- *    explicit Something(int);
- *    Something(folly::initlist_construct_t, std::initializer_list<int>);
- *
- *    operator int();
- *  };
- *
- *  ...
- *  Something something_one{1}; // not the initializer_list overload
- *  Something something_two{folly::initlist_construct, {1}}; // correct
- */
-struct initlist_construct_t {};
-constexpr initlist_construct_t initlist_construct{};
-
 } // namespace folly
 
 // Assume nothing when compiling with MSVC.

folly/Utility.h

@@ -132,6 +132,85 @@ using make_index_sequence = detail::make_index_sequence<N>;
 
 #endif
 
+/**
+ *  Backports from C++17 of:
+ *    std::in_place_t
+ *    std::in_place_type_t
+ *    std::in_place_index_t
+ *    std::in_place
+ *    std::in_place_type
+ *    std::in_place_index
+ */
+
+struct in_place_tag {};
+template <class>
+struct in_place_type_tag {};
+template <std::size_t>
+struct in_place_index_tag {};
+
+using in_place_t = in_place_tag (&)(in_place_tag);
+template <class T>
+using in_place_type_t = in_place_type_tag<T> (&)(in_place_type_tag<T>);
+template <std::size_t I>
+using in_place_index_t = in_place_index_tag<I> (&)(in_place_index_tag<I>);
+
+inline in_place_tag in_place(in_place_tag = {}) {
+  return {};
+}
+template <class T>
+inline in_place_type_tag<T> in_place_type(in_place_type_tag<T> = {}) {
+  return {};
+}
+template <std::size_t I>
+inline in_place_index_tag<I> in_place_index(in_place_index_tag<I> = {}) {
+  return {};
+}
+
+/**
+ * Initializer lists are a powerful compile time syntax introduced in C++11
+ * but due to their often conflicting syntax they are not used by APIs for
+ * construction.
+ *
+ * Further standard conforming compilers *strongly* favor an
+ * std::initalizer_list overload for construction if one exists.  The
+ * following is a simple tag used to disambiguate construction with
+ * initializer lists and regular uniform initialization.
+ *
+ * For example consider the following case
+ *
+ *  class Something {
+ *  public:
+ *    explicit Something(int);
+ *    Something(std::intiializer_list<int>);
+ *
+ *    operator int();
+ *  };
+ *
+ *  ...
+ *  Something something{1}; // SURPRISE!!
+ *
+ * The last call to instantiate the Something object will go to the
+ * initializer_list overload.  Which may be surprising to users.
+ *
+ * If however this tag was used to disambiguate such construction it would be
+ * easy for users to see which construction overload their code was referring
+ * to.  For example
+ *
+ *  class Something {
+ *  public:
+ *    explicit Something(int);
+ *    Something(folly::initlist_construct_t, std::initializer_list<int>);
+ *
+ *    operator int();
+ *  };
+ *
+ *  ...
+ *  Something something_one{1}; // not the initializer_list overload
+ *  Something something_two{folly::initlist_construct, {1}}; // correct
+ */
+struct initlist_construct_t {};
+constexpr initlist_construct_t initlist_construct{};
+
 /**
  * A simple function object that passes its argument through unchanged.
  *