Generic types are "erased" at compile time. The strong type checking in Kotlin can use those generic types at compile time, but at runtime, the types are no longer there. This has implications for cases where you need the actual type at runtime, such as for is checks.

One workaround is if you have an inline function that uses a generic type. You can add the reified keyword before the type, and that tells the Kotlin compiler to make that type available at runtime inside of that inline function. So, in the example above, if we skip the reified keyword, we will get a compile error like "Cannot check for instance of erased type: T". But with the reified keyword, the code compiles and works. filterByType() works the same way that filterIsInstance() does in the Kotlin standard library, returning a subset of items that match the desired type.