libstdc++: Make "implicit expression variants" more explicit (P2102R0)

	* include/bits/iterator_concepts.h (indirectly_copyable_storable): Add
	const-qualified expression variations.
	* include/std/concepts (copyable): Likewise.

libstdc++-v3/ChangeLog

 2020-02-17  Jonathan Wakely  <jwakely@redhat.com>
 
+	* include/bits/iterator_concepts.h (indirectly_copyable_storable): Add
+	const-qualified expression variations.
+	* include/std/concepts (copyable): Likewise.
+
 	* include/std/type_traits (__is_standard_integer): New helper trait.
 	* include/std/utility (cmp_equal, cmp_not_equal, cmp_less, cmp_greater)
 	(cmp_less_equal, cmp_greater_equal, in_range): Define for C++20.

libstdc++-v3/include/bits/iterator_concepts.h

@@ -693,7 +693,10 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 
   template<typename _In, typename _Out>
     concept indirectly_copyable_storable = indirectly_copyable<_In, _Out>
+      && indirectly_writable<_Out, iter_value_t<_In>&>
       && indirectly_writable<_Out, const iter_value_t<_In>&>
+      && indirectly_writable<_Out, iter_value_t<_In>&&>
+      && indirectly_writable<_Out, const iter_value_t<_In>&&>
       && copyable<iter_value_t<_In>>
       && constructible_from<iter_value_t<_In>, iter_reference_t<_In>>
       && assignable_from<iter_value_t<_In>&, iter_reference_t<_In>>;

libstdc++-v3/include/std/concepts

@@ -251,7 +251,8 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 
   template<typename _Tp>
     concept copyable = copy_constructible<_Tp> && movable<_Tp>
-      && assignable_from<_Tp&, const _Tp&>;
+      && assignable_from<_Tp&, _Tp&> && assignable_from<_Tp&, const _Tp&>
+      && assignable_from<_Tp&, const _Tp>;
 
   template<typename _Tp>
     concept semiregular = copyable<_Tp> && default_initializable<_Tp>;