New CAT call

Introduction

The ECMA standard introduces a new solution to the CAT call problem. Covariant redefinition of a formal argument is only possible to a detachable type:

class A
feature
   f (a: ANY) do end
end

class B1
inherit A redefine f end
feature
   f (a: STRING) do end   -- not valid
end

class B2
inherit A redefine f end
feature
   f (a: ?STRING) do end   -- valid
end

ECMA-2 is not very clear about what happens when an INTEGER_REF is passed to feature f of B2 (this is possible through a reference of class A). There are two possibilities:

• a is attached to an object of dynamic type INTEGER_REF.
• as is detached.

Will a be attached to void or will it be attached to the INTEGER_REF This wiki discusses some of concerns related to this solution.

Detachable type

According to the new approach a detachable type can be attached to an object of "any" type.