Detect non-covariant return types in overriding cypclass methods

Cython/Compiler/Symtab.py

@@ -554,8 +554,14 @@ class Scope(object):
                                 cpp_override_allowed = True
                                 continue
 
-                            # in a cypclass, if the arguments are compatible
-                            # then the whole signature must be identical (return type excluded)
+                        # Any inherited method is visible
+                        # until overloaded by a method with the same signature
+                        if alt_entry.is_inherited:
+
+                            if self.is_cyp_class_scope:
+
+                                # in a cypclass, if the arguments are compatible, then the new method must actually
+                                # override the inherited method: the whole signature must be identical
                                 old_declarator = alt_entry.type.declarator_code(name, for_display = 1).strip()
                                 new_declarator = type.declarator_code(name, for_display = 1).strip()
                                 if not new_declarator == old_declarator:
@@ -565,9 +571,12 @@ class Scope(object):
                                     if alt_entry.pos is not None:
                                             error(alt_entry.pos, "Conflicting method is defined here")
 
-                        # Any inherited method is visible
-                        # until overloaded by a method with the same signature
-                        if alt_entry.is_inherited:
+                                # also, the return type must be covariant
+                                elif not type.return_type.subtype_of_resolved_type(alt_entry.type.return_type):
+                                    error(pos, "Cypclass method overrides another with incompatible return type")
+                                    if alt_entry.pos is not None:
+                                            error(alt_entry.pos, "Conflicting method is defined here")
+
                             previous_alternative_indices.append(index)
                             cpp_override_allowed = True
                             continue