fix is_copy_assignable

bind_map used std::is_copy_assignable which suffers from the same problems
as std::is_copy_constructible, therefore the same fix has been applied.

Author: sgsaenger

include/pybind11/cast.h

@@ -803,6 +803,15 @@ template <typename Container> struct is_copy_constructible<Container, enable_if_
 template <typename T1, typename T2> struct is_copy_constructible<std::pair<T1, T2>>
     : all_of<is_copy_constructible<T1>, is_copy_constructible<T2>> {};
 
+// The same problems arise with std::is_copy_assignable, so we use the same workaround.
+template <typename T, typename SFINAE = void> struct is_copy_assignable : std::is_copy_assignable<T> {};
+template <typename Container> struct is_copy_assignable<Container, enable_if_t<all_of<
+        std::is_copy_assignable<Container>,
+        std::is_same<typename Container::value_type &, typename Container::reference>
+    >::value>> : is_copy_assignable<typename Container::value_type> {};
+template <typename T1, typename T2> struct is_copy_assignable<std::pair<T1, T2>>
+    : all_of<is_copy_assignable<T1>, is_copy_assignable<T2>> {};
+
 NAMESPACE_END(detail)
 
 // polymorphic_type_hook<itype>::get(src, tinfo) determines whether the object pointed

include/pybind11/stl_bind.h

@@ -512,7 +512,7 @@ template <typename, typename, typename... Args> void map_assignment(const Args &
 
 // Map assignment when copy-assignable: just copy the value
 template <typename Map, typename Class_>
-void map_assignment(enable_if_t<std::is_copy_assignable<typename Map::mapped_type>::value, Class_> &cl) {
+void map_assignment(enable_if_t<is_copy_assignable<typename Map::mapped_type>::value, Class_> &cl) {
     using KeyType = typename Map::key_type;
     using MappedType = typename Map::mapped_type;
 
@@ -528,7 +528,7 @@ void map_assignment(enable_if_t<std::is_copy_assignable<typename Map::mapped_typ
 // Not copy-assignable, but still copy-constructible: we can update the value by erasing and reinserting
 template<typename Map, typename Class_>
 void map_assignment(enable_if_t<
-        !std::is_copy_assignable<typename Map::mapped_type>::value &&
+        !is_copy_assignable<typename Map::mapped_type>::value &&
         is_copy_constructible<typename Map::mapped_type>::value,
         Class_> &cl) {
     using KeyType = typename Map::key_type;