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.

From-SVN: r241372

libstdc++-v3/ChangeLog

+2016-10-20  Ville Voutilainen  <ville.voutilainen@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.
+
 2016-10-20  Jonathan Wakely  <jwakely@redhat.com>
 
 	* include/bits/allocator.h: Remove trailing whitespace, tab-indent.

libstdc++-v3/include/std/optional

@@ -568,15 +568,14 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
         return *this;
       }
 
-      template<typename _Up = _Tp,
+      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&
+		    optional&>
         operator=(_Up&& __u)
         {
           if (this->_M_is_engaged())
@@ -587,7 +586,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
           return *this;
         }
 
-      template<typename _Up,
+      template<typename _Up>
 	enable_if_t<__and_<
 		      __not_<is_same<_Tp, _Up>>,
 		      is_constructible<_Tp, const _Up&>,
@@ -595,8 +594,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 		      __not_<__converts_from_optional<_Tp, _Up>>,
 		      __not_<__assigns_from_optional<_Tp, _Up>>
 		      >::value,
-			   bool> = true>
-        optional&
+		    optional&>
         operator=(const optional<_Up>& __u)
         {
           if (__u)
@@ -613,7 +611,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
           return *this;
         }
 
-      template<typename _Up,
+      template<typename _Up>
 	enable_if_t<__and_<
 		      __not_<is_same<_Tp, _Up>>,
 		      is_constructible<_Tp, _Up>,
@@ -621,8 +619,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 		      __not_<__converts_from_optional<_Tp, _Up>>,
 		      __not_<__assigns_from_optional<_Tp, _Up>>
 		      >::value,
-			   bool> = true>
-        optional&
+		    optional&>
         operator=(optional<_Up>&& __u)
         {
           if (__u)