Allow comparing wxList::compatibility_iterator with nullptr

This doesn't cost anything but allows the old code which relied on
compatibility_iterator being implicitly convertible to a pointer (and
hence comparable with nullptr) continue to compile and work.

Add a unit test checking that this works as expected.

include/wx/list.h

@@ -36,6 +36,7 @@
 #if wxUSE_STD_CONTAINERS
     #include "wx/beforestd.h"
     #include <algorithm>
+    #include <cstddef>
     #include <iterator>
     #include <list>
     #include "wx/afterstd.h"
@@ -121,6 +122,11 @@ public:
         bool operator !() const
             { return !( operator bool() ); }
 
+        bool operator==(std::nullptr_t) const
+            { return !*this; }
+        bool operator!=(std::nullptr_t) const
+            { return !(*this == nullptr); }
+
         elT GetData() const
             { return *m_iter; }
         void SetData( elT e )

tests/lists/lists.cpp

@@ -194,6 +194,29 @@ TEST_CASE("wxList::ctor", "[list]")
     CHECK( Baz::GetNumber() == 0 );
 }
 
+TEST_CASE("wxList::iterator::cmp", "[list]")
+{
+    int dummy[2];
+
+    wxListInt list;
+    list.push_back(dummy);
+    list.push_back(dummy + 1);
+
+    wxListInt::compatibility_iterator it = list.GetFirst();
+    CHECK( it == it );
+    CHECK( it != nullptr );
+
+    const wxListInt::compatibility_iterator last = list.GetLast();
+    CHECK( last != nullptr );
+    CHECK( it != last );
+
+    it = list.Item(1);
+    CHECK( it == last );
+
+    it = list.Find(dummy + 2);
+    CHECK( it == nullptr );
+}
+
 // Check for WX_DECLARE_LIST_3 which is used to define wxWindowList: we can't
 // use this class itself here, as it's in the GUI library, so declare something
 // similar.