You've got that completely the wrong way around. An erasing compiler has full access to the type (obviously - it's doing the erasing) and so can verify whether

You've got that completely the wrong way around. An erasing compiler has full access to the type (obviously - it's doing the erasing) and so can verify whether you're putting the correct types in or out of a collection. Where erasure can cause issues is at runtime. For example: X isa List<Int> Even if X is indeed a list of ints that information isn't held at runtime, so the test can't be supported. Having said that in some respects this is a C# / Java ecumenical matter. Haskell, for example, doesn't bake types into output code so I suppose it erases even more that java, but it supports very rich polymorphic behaviour.