libstdc++: Prefer const T to std::add_const_t<T>

For any typedef-name or template parameter, T, add_const_t<T> is
equivalent to T const, so we can avoid instantiating the std::add_const
class template and just say T const (or const T).

This isn't true for a non-typedef like int&, where int& const would be
ill-formed, but we shouldn't be using add_const_t<int&> anyway, because
we know what that type is.

The only place we need to continue using std::add_const is in the
std::bind implementation where it's used as a template template
parameter to be applied as a metafunction elsewhere.

libstdc++-v3/ChangeLog:

	* include/bits/stl_iterator.h (__iter_to_alloc_t): Replace
	add_const_t with const-qualifier.
	* include/bits/utility.h (tuple_element<N, cv T>): Likewise for
	all cv-qualifiers.
	* include/std/type_traits (add_const, add_volatile): Replace
	typedef-declaration with using-declaration.
	(add_cv): Replace add_const and add_volatile with cv-qualifiers.
	* include/std/variant (variant_alternative<N, cv T>): Replace
	add_const_t, add_volatile_t and add_cv_t etc. with cv-qualifiers.

4 files changed, 14 insertions, 18 deletions

diff --git a/libstdc++-v3/include/bits/stl_iterator.h b/libstdc++-v3/include/bits/stl_iterator.h
index 12a89ab..049cb02 100644
--- a/libstdc++-v3/include/bits/stl_iterator.h
+++ b/libstdc++-v3/include/bits/stl_iterator.h
@@ -2536,19 +2536,18 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
   // of associative containers.
   template<typename _InputIterator>
     using __iter_key_t = remove_const_t<
-    typename iterator_traits<_InputIterator>::value_type::first_type>;
+      typename iterator_traits<_InputIterator>::value_type::first_type>;
 
   template<typename _InputIterator>
-    using __iter_val_t =
-    typename iterator_traits<_InputIterator>::value_type::second_type;
+    using __iter_val_t
+      = typename iterator_traits<_InputIterator>::value_type::second_type;
 
   template<typename _T1, typename _T2>
     struct pair;
 
   template<typename _InputIterator>
-    using __iter_to_alloc_t =
-    pair<add_const_t<__iter_key_t<_InputIterator>>,
-	 __iter_val_t<_InputIterator>>;
+    using __iter_to_alloc_t
+      = pair<const __iter_key_t<_InputIterator>, __iter_val_t<_InputIterator>>;
 #endif // __cpp_deduction_guides
 
 _GLIBCXX_END_NAMESPACE_VERSION
diff --git a/libstdc++-v3/include/bits/utility.h b/libstdc++-v3/include/bits/utility.h
index e0e4030..6a192e2 100644
--- a/libstdc++-v3/include/bits/utility.h
+++ b/libstdc++-v3/include/bits/utility.h
@@ -86,19 +86,19 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
   template<size_t __i, typename _Tp>
     struct tuple_element<__i, const _Tp>
     {
-      typedef typename add_const<__tuple_element_t<__i, _Tp>>::type type;
+      using type = const __tuple_element_t<__i, _Tp>;
     };
 
   template<size_t __i, typename _Tp>
     struct tuple_element<__i, volatile _Tp>
     {
-      typedef typename add_volatile<__tuple_element_t<__i, _Tp>>::type type;
+      using type = volatile __tuple_element_t<__i, _Tp>;
     };
 
   template<size_t __i, typename _Tp>
     struct tuple_element<__i, const volatile _Tp>
     {
-      typedef typename add_cv<__tuple_element_t<__i, _Tp>>::type type;
+      using type = const volatile __tuple_element_t<__i, _Tp>;
     };
 
 #if __cplusplus >= 201402L
diff --git a/libstdc++-v3/include/std/type_traits b/libstdc++-v3/include/std/type_traits
index 2572d8e..e5f58bc 100644
--- a/libstdc++-v3/include/std/type_traits
+++ b/libstdc++-v3/include/std/type_traits
@@ -1577,20 +1577,17 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
   /// add_const
   template<typename _Tp>
     struct add_const
-    { typedef _Tp const     type; };
+    { using type = _Tp const; };
 
   /// add_volatile
   template<typename _Tp>
     struct add_volatile
-    { typedef _Tp volatile     type; };
+    { using type = _Tp volatile; };
 
   /// add_cv
   template<typename _Tp>
     struct add_cv
-    {
-      typedef typename
-      add_const<typename add_volatile<_Tp>::type>::type     type;
-    };
+    { using type = _Tp const volatile; };
 
 #if __cplusplus > 201103L
 
diff --git a/libstdc++-v3/include/std/variant b/libstdc++-v3/include/std/variant
index 5ff1e3e..f8f1566 100644
--- a/libstdc++-v3/include/std/variant
+++ b/libstdc++-v3/include/std/variant
@@ -107,15 +107,15 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 
   template<size_t _Np, typename _Variant>
     struct variant_alternative<_Np, const _Variant>
-    { using type = add_const_t<variant_alternative_t<_Np, _Variant>>; };
+    { using type = const variant_alternative_t<_Np, _Variant>; };
 
   template<size_t _Np, typename _Variant>
     struct variant_alternative<_Np, volatile _Variant>
-    { using type = add_volatile_t<variant_alternative_t<_Np, _Variant>>; };
+    { using type = volatile variant_alternative_t<_Np, _Variant>; };
 
   template<size_t _Np, typename _Variant>
     struct variant_alternative<_Np, const volatile _Variant>
-    { using type = add_cv_t<variant_alternative_t<_Np, _Variant>>; };
+    { using type = const volatile variant_alternative_t<_Np, _Variant>; };
 
   inline constexpr size_t variant_npos = -1;