[v3 PATCH] Constrain optional's __constexpr_addressof in its return type, Use a constexpr addressof for optional, if available.

[Ville Voutilainen]

The background for these is at
https://stackoverflow.com/questions/40983822/cannot-use-a-constant-reference-of-experimentaloptional-when-using-clang-lib

We use excessive trickery to constrain __constexpr_addressof in
experimental::optional.
That trickery is not agreed on by compilers, and we can cope with a much simpler
approach.

The first patch just simplifies that trickery by moving the
constraints from a template
parameter to the return type.

The second patch uses a constexpr std::__addressof if it's available.
I split these
out in case we want just the first patch but not the second. The second patch
allows C++17 users to benefit from a completely constexpr addressof even
for experimental::optional (std::optional already gives that).

Tested on Linux-x64. Ok for trunk, the gcc-6 branch and the gcc-5 branch?

2016-12-06  Ville Voutilainen  <ville.voutilai...@gmail.com>

    Constrain optional's __constexpr_addressof in its return type.
    * include/experimental/optional (__constexpr_addressof):
    Constrain in the return type instead of in a template parameter.

2016-12-06  Ville Voutilainen  <ville.voutilai...@gmail.com>

    Use a constexpr addressof for optional, if available.
    * include/experimental/optional (_Has_addressof_mem)
    (_Has_addressof_free, _Has_addressof, __constexpr_addressof):
    Guard with #ifndef __cpp_lib_addressof_constexpr.
    (operator->()): Use std::__addressof if it's constexpr.

diff --git a/libstdc++-v3/include/experimental/optional 
b/libstdc++-v3/include/experimental/optional
index a631158..8263639 100644
--- a/libstdc++-v3/include/experimental/optional
+++ b/libstdc++-v3/include/experimental/optional
@@ -170,15 +170,19 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     * overloaded addressof operator (unary operator&), in which case the call
     * will not be a constant expression.
     */
-  template<typename _Tp, enable_if_t<!_Has_addressof<_Tp>::value, int>...>
-    constexpr _Tp* __constexpr_addressof(_Tp& __t)
+  template<typename _Tp>
+    constexpr
+    enable_if_t<!_Has_addressof<_Tp>::value, _Tp*>
+    __constexpr_addressof(_Tp& __t)
     { return &__t; }
 
   /**
     * @brief Fallback overload that defers to __addressof.
     */
-  template<typename _Tp, enable_if_t<_Has_addressof<_Tp>::value, int>...>
-    inline _Tp* __constexpr_addressof(_Tp& __t)
+  template<typename _Tp>
+    inline
+    enable_if_t<_Has_addressof<_Tp>::value, _Tp*>
+    __constexpr_addressof(_Tp& __t)
     { return std::__addressof(__t); }
 
   /**

diff --git a/libstdc++-v3/include/experimental/optional 
b/libstdc++-v3/include/experimental/optional
index 8263639..6994e77 100644
--- a/libstdc++-v3/include/experimental/optional
+++ b/libstdc++-v3/include/experimental/optional
@@ -134,6 +134,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
   __throw_bad_optional_access(const char* __s)
   { _GLIBCXX_THROW_OR_ABORT(bad_optional_access(__s)); }
 
+#ifndef __cpp_lib_addressof_constexpr
   template<typename _Tp, typename = void>
     struct _Has_addressof_mem : std::false_type { };
 
@@ -184,6 +185,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     enable_if_t<_Has_addressof<_Tp>::value, _Tp*>
     __constexpr_addressof(_Tp& __t)
     { return std::__addressof(__t); }
+#endif // __cpp_lib_addressof_constexpr
 
   /**
     * @brief Class template that holds the necessary state for @ref optional
@@ -709,7 +711,13 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       // [X.Y.4.5] Observers.
       constexpr const _Tp*
       operator->() const
-      { return __constexpr_addressof(this->_M_get()); }
+      {
+#ifndef __cpp_lib_addressof_constexpr
+       return __constexpr_addressof(this->_M_get());
+#else
+       return std::__addressof(this->_M_get());
+#endif
+      }
 
       _Tp*
       operator->()