Fix Schema's clone function typing

[StefanoA1]

Summary
The Schema's clone function does not preserve the typing (and on its best case, it loses the name of the type and just types the cloned schema as a nameless object)

Examples
Before:
clone returns an untyped schema (or at best it returns an object but the type name/interface/class is lost)

After:
clone returns a typed schema which can be the original type or a type that extends the base type (would work for type extending it with partial params)

[vkarpov15]

I'm supportive of adding a generic param to clone(), but I'm not sure making clone() return something other than this is the right call. For one thing, you're missing a bunch of generic parameters to Schema. And I can confirm that this respects generics

Can't we do clone<T = this>(): T?

[StefanoA1]

I'm supportive of adding a generic param to clone(), but I'm not sure making clone() return something other than this is the right call. For one thing, you're missing a bunch of generic parameters to Schema. And I can confirm that this respects generics

Can't we do clone<T = this>(): T?

I agree, but then T should ensure to extend Schema<DocType> at least (to ensure that not any type is taken as a generic here), what are your thoughts on:
clone<T extends Schema<DocType> = this>(): T;
then ?

[vkarpov15]

Unfortunately not. 2 reasons: 1) T extends Schema<DocType> is missing a bunch of generic parameters, and 2) extends sometimes has some serious performance issues, see #10349 .

[StefanoA1]

@vkarpov15 ah! interesting, do you happen to have any docs on why extends causes such perf issues? thanks for the link

[vkarpov15]

Wish I knew, but I'm not experienced enough with TS internals to know why sometimes extends in generics is such a perf issue.