Presumably the issue is that a Readonly<Foo> shouldn't be a subtype of Foo
I should note that I haven't yet had the pleasure of using a language that handles const-ness properly, as Readonly<T> should be neither a subtype nor a supertype of T
As an aside, I'm on mobile and tried to visit the TypeScript playground to play around with this, but weirdly, the default code is an implementation of FizzBuzz! There was not an obvious way to clear it to get a blank editor. Even "select all" context menu was hijacked. So I gave up. I'll have to file an issue.
