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Maybe I'm off the mark here, but in my mind the definition of subtype is as follows: t1 is a subtype of t2 if any instance of t1 is an instance of t2. Surely with this definition String is a subtype of String?. Every instance of String (i.e. every string) is an instance of String?, after all.

Post-submit clarity edit: Ah, I think I understand. You're objecting to OP's definition of subtyping, as it breaks down for certain language implementations.

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String <: String? is true (Any non-null string is a possibly null string), but String? <: String isn’t true (Any possibly null string may not be a non-null string).

We're all in agreement there it seems.

If you just state it as a logical quantified proposition, it’s pretty obvious unless the type is complicated (then you need to break it down into multiple logical quantified propositions).

Yes of course, but again, I don't think anyone disagrees with you on this.

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