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This is not type safe. By up-casting a {{java|CatShelter}} to an {{java|AnimalShelter}}, one can try to place a dog in a cat shelter. That does not meet {{java|CatShelter}} parameter restrictions, and will result in a runtime error. The lack of type safety (known as the "catcall problem" in the Eiffel community, where "cat" or "CAT" is a Changed Availability or Type) has been a long-standing issue. Over the years, various combinations of global static analysis, local static analysis, and new language features have been proposed to remedy it,<ref>{{cite conference|title=Static Typing |author=Bertrand Meyer |book-title=OOPSLA 95 (Object-Oriented Programming, Systems, Languages and Applications), Atlanta, 1995. |date=October 1995 |url=http://se.ethz.ch/~meyer/publications/acm/typing.pdf}}</ref><ref name="competentCompilers">{{cite web|title=Type-safe covariance: Competent compilers can catch all catcalls |first1=Mark |last1=Howard |first2=Eric |last2=Bezault |first3=Bertrand |last3=Meyer |first4=Dominique |last4=Colnet |first5=Emmanuel |last5=Stapf |first6=Karine |last6=Arnout |first7=Markus |last7=Keller |date=April 2003 |access-date=23 May 2013 |url=http://se.ethz.ch/~meyer/ongoing/covariance/recast.pdf}}</ref> and these have been implemented in some Eiffel compilers.
Despite the type safety problem, the Eiffel designers consider covariant parameter types crucial for modeling real world requirements.<ref name="competentCompilers"/> The cat shelter illustrates a common phenomenon: it is ''a kind of'' animal shelter but has ''additional restrictions'', and it seems reasonable to use inheritance and restricted parameter types to model this. In proposing this use of inheritance, the Eiffel designers reject the [[Liskov substitution principle]], which states that objects of subclasses should always be less restricted than objects of their superclass.
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