Re: [v3 PATCH] Implement LWG 2857, {variant,optional,any}::emplace should return the constructed value.

[Jonathan Wakely]

On 16/03/17 00:21 +0200, Ville Voutilainen wrote:

Tested on Linux-x64.

2017-03-16  Ville Voutilainen  <ville.voutilai...@gmail.com>

   Implement LWG 2857, {variant,optional,any}::emplace should
   return the constructed value.
   * include/std/any (any_cast(any*)): Forward-declare.
   (emplace(_Args&&...)): Change the return type and
   return a reference to the constructed value.
   (emplace(initializer_list<_Up>, _Args&&...)): Likewise.
   * include/std/optional (emplace(_Args&&...)): Likewise.
   (emplace(initializer_list<_Up>, _Args&&...)): Likewise.
   * include/std/variant (emplace<_Tp>(_Args&&...)): Likewise.
   (emplace<_Tp>(initializer_list<_Up>, _Args&&...)): Likewise.
   (emplace<_Np>(_Args&&...)): Likewise.
   (emplace<_Np>(initializer_list<_Up>, _Args&&...)): Likewise.
   * testsuite/20_util/any/assign/emplace.cc: Add tests for
   checking the return value of emplace.
   * testsuite/20_util/any/misc/any_cast_neg.cc: Adjust.
   * testsuite/20_util/optional/assignment/6.cc: Add tests for
   checking the return value of emplace.
   * testsuite/20_util/variant/run.cc: Likewise.

diff --git a/libstdc++-v3/include/std/any b/libstdc++-v3/include/std/any
index e807617..fff13d9 100644
--- a/libstdc++-v3/include/std/any
+++ b/libstdc++-v3/include/std/any
@@ -74,6 +74,10 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
   *  An @c any object's state is either empty or it stores a contained object
   *  of CopyConstructible type.
   */
+  class any;
+  template<typename _ValueType>
+    inline _ValueType* any_cast(any* __any) noexcept;
+
  class any
  {
    // Holds either pointer to a heap object or the contained object itself.
@@ -268,18 +272,19 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION

    /// Emplace with an object created from @p __args as the contained object.
    template <typename _ValueType, typename... _Args>
-      typename __any_constructible<void,
+      typename __any_constructible<_Decay<_ValueType>&,
                                   _Decay<_ValueType>, _Args&&...>::type
      emplace(_Args&&... __args)
      {
        __do_emplace<_Decay<_ValueType>>
          (std::forward<_Args>(__args)...);
+       return *(std::any_cast<_Decay<_ValueType>>(this));

Can we avoid the branch in any_cast to check the stored type?
We know it's the right type, because we just stored it.