[v3 PATCH] Do the operator= SFINAE in the return type for optional, not in the template parameters.

[Ville Voutilainen]

Tested on Linux-x64.

2016-10-19  Ville Voutilainen  <ville.voutilai...@gmail.com>

    Do the operator= SFINAE in the return type for optional,
    not in the template parameters.
    * include/std/optional (operator=(_Up&&)): Move SFINAE
    from template parameters to the return type.
    (operator=(const optional<_Up>&)): Likewise.
    (operator=(optional<_Up>&&)): Likewise.

diff --git a/libstdc++-v3/include/std/optional 
b/libstdc++-v3/include/std/optional
index 21210ab..f272876 100644
--- a/libstdc++-v3/include/std/optional
+++ b/libstdc++-v3/include/std/optional
@@ -568,15 +568,14 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
         return *this;
       }
 
-      template<typename _Up = _Tp,
-               enable_if_t<__and_<
-                            __not_<is_same<optional<_Tp>, decay_t<_Up>>>,
-                            is_constructible<_Tp, _Up>,
-                            __not_<__and_<is_scalar<_Tp>,
-                                          is_same<_Tp, decay_t<_Up>>>>,
-                            is_assignable<_Tp&, _Up>>::value,
-                          bool> = true>
-        optional&
+      template<typename _Up = _Tp>
+        enable_if_t<__and_<
+                     __not_<is_same<optional<_Tp>, decay_t<_Up>>>,
+                     is_constructible<_Tp, _Up>,
+                     __not_<__and_<is_scalar<_Tp>,
+                                   is_same<_Tp, decay_t<_Up>>>>,
+                     is_assignable<_Tp&, _Up>>::value,
+                   optional&>
         operator=(_Up&& __u)
         {
           if (this->_M_is_engaged())
@@ -587,16 +586,15 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
           return *this;
         }
 
-      template<typename _Up,
-               enable_if_t<__and_<
-                            __not_<is_same<_Tp, _Up>>,
-                            is_constructible<_Tp, const _Up&>,
-                            is_assignable<_Tp&, _Up>,
-                            __not_<__converts_from_optional<_Tp, _Up>>,
-                            __not_<__assigns_from_optional<_Tp, _Up>>
-                            >::value,
-                          bool> = true>
-        optional&
+      template<typename _Up>
+       enable_if_t<__and_<
+                     __not_<is_same<_Tp, _Up>>,
+                     is_constructible<_Tp, const _Up&>,
+                     is_assignable<_Tp&, _Up>,
+                     __not_<__converts_from_optional<_Tp, _Up>>,
+                     __not_<__assigns_from_optional<_Tp, _Up>>
+                     >::value,
+                   optional&>
         operator=(const optional<_Up>& __u)
         {
           if (__u)
@@ -613,16 +611,15 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
           return *this;
         }
 
-      template<typename _Up,
-              enable_if_t<__and_<
-                            __not_<is_same<_Tp, _Up>>,
-                            is_constructible<_Tp, _Up>,
-                            is_assignable<_Tp&, _Up>,
-                            __not_<__converts_from_optional<_Tp, _Up>>,
-                            __not_<__assigns_from_optional<_Tp, _Up>>
-                            >::value,
-                          bool> = true>
-        optional&
+      template<typename _Up>
+       enable_if_t<__and_<
+                     __not_<is_same<_Tp, _Up>>,
+                     is_constructible<_Tp, _Up>,
+                     is_assignable<_Tp&, _Up>,
+                     __not_<__converts_from_optional<_Tp, _Up>>,
+                     __not_<__assigns_from_optional<_Tp, _Up>>
+                     >::value,
+                   optional&>
         operator=(optional<_Up>&& __u)
         {
           if (__u)