const class Example {
    const NonConstClass[] oops  // this line compiles quite happily!
	
    new make(|This| f) { f(this) }
}

class NonConstClass { }

I notice the Fantom compiler lets you define const fields of Lists and Maps with non-const types.

I understand that the actual type is sys::List or sys::Map which may or may not be immutable, but I'd have thought the compiler would be able to inspect the defined parametrised type to make a judgement call.

Could this not be added?

We allow it because you are allowed to have an const field for an empty list or empty map of any type. For example:

fansh> Buf[,].toImmutable.isImmutable
true

So the rule is that list and map types are checked at runtime by calling toImmutable on them.