Historically, wildcards are a direct descendant of the work by Atsushi Igarashi and Mirko
Viroli. Readers interested in a more comprehensive discussion should refer to On Variance-
Based Subtyping for Parametric Types by Atsushi Igarashi and Mirko Viroli, in the
Proceedings of the 16th European Conference on Object Oriented Programming (ECOOP
2002). This work itself builds upon earlier work by Kresten Thorup and Mads Torgersen
(Unifying Genericity, ECOOP 99), as well as a long tradition of work on declaration based
variance that goes back to Pierre America's work on POOL (OOPSLA 89).