4.5.4 Languages Based on Instantiation-Time Checking

Some languages, including C++ [Stroustrup 86] and Modula-3 [SRC], dispense with specifying constraints on type variables entirely, relying instead on checking each instantiation separately. These languages are very flexible in what sort of parameterized declarations and clients can be written, as the only constraints that need be met are that the individual instantiations made in some program typecheck, and they are simple for programmers to use. (C++ also allows constant values as parameters in addition to types.) However, dropping explicit constraints on instantiating type variables loses the ability to check a parameterized declaration for type correctness once and for all separately from its (potentially unknown) clients, loses the specification benefit to programmers about how parameterized declarations should be used, and forces the source code of parameterized entities to be made available to clients in order for them to typecheck instantiations.