The compiler/REPL normally give an unchecked warning with .isInstanceOf[GenericType[SomeType]], which makes sense since it gets type-erased to GenericType[Object].

However, .isInstanceOf[GenericType[AnotherGenericType[...]]] does not give any warning, even though it is equally affected by type erasure.

I asked about this on Stack Overflow (some more examples there): http://stackoverflow.com/questions/11554323/why-does-some123-isinstanceofoptionliststring-not-give-an-unchecked

The answer http://stackoverflow.com/a/11555642/319931 shows where in the compiler code the issue is; in particular "[t]he check function returns unit without performing any test" for generic types.