> The language can just have an implicit conversion rule for convenience
the presence of an implicit conversion rule `T -> T?` amounts to the observation that `T <: T?`, where <: is the subtyping relation
> make an unboxed integer nullable ...
I don't think any language allows this, in any case disallowing nullability for unboxed types amounts to the observation that `P !<: P?`, where !<: is "does not subtype"
I believe (unless I have misunderstood you) that both your examples are subsumed (heh) by subtyping
the presence of an implicit conversion rule `T -> T?` amounts to the observation that `T <: T?`, where <: is the subtyping relation
> make an unboxed integer nullable ...
I don't think any language allows this, in any case disallowing nullability for unboxed types amounts to the observation that `P !<: P?`, where !<: is "does not subtype"
I believe (unless I have misunderstood you) that both your examples are subsumed (heh) by subtyping