Update error message for incompatible args

Updated the error message that is displayed when the argument types
of a method in a subclass are incompatible with the arguments of
the same method in the superclass.

Author: matys18

mypy/checker.py

@@ -1531,8 +1531,16 @@ def erase_override(t: Type) -> Type:
                 for i in range(len(override.arg_types)):
                     if not is_subtype(original.arg_types[i],
                                       erase_override(override.arg_types[i])):
+                        arg_type_in_super = original.arg_types[i]
                         self.msg.argument_incompatible_with_supertype(
-                            i + 1, name, type_name, name_in_super, supertype, node)
+                            i + 1,
+                            name,
+                            type_name,
+                            name_in_super,
+                            arg_type_in_super,
+                            supertype,
+                            node
+                        )
                         emitted_msg = True
 
                 if not is_subtype(erase_override(override.ret_type),

mypy/messages.py

@@ -797,10 +797,13 @@ def signature_incompatible_with_supertype(
 
     def argument_incompatible_with_supertype(
             self, arg_num: int, name: str, type_name: Optional[str],
-            name_in_supertype: str, supertype: str, context: Context) -> None:
+            name_in_supertype: str, arg_type_in_supertype: Type, supertype: str,
+            context: Context) -> None:
         target = self.override_target(name, name_in_supertype, supertype)
-        self.fail('Argument {} of "{}" incompatible with {}'
-                  .format(arg_num, name, target), context)
+        arg_type_in_supertype_f = self.format_bare(arg_type_in_supertype)
+        self.fail('Argument {} of "{}" is incompatible with {}; '
+                  'supertype defines the argument type as "{}"'
+                  .format(arg_num, name, target, arg_type_in_supertype_f), context)
 
         if name == "__eq__" and type_name:
             multiline_msg = self.comparison_method_example_msg(class_name=type_name)