luau/rfcs/none-and-unknown-types.md

Add none and unknown types

Summary

Add top and bottom types that ane inhabited by everything and nothing respectively.

Motivation

There are lots of cases in local type inference, semantic subtyping, and type normalization, where it would be useful to have top and bottom types. Currently, any is filling that role, but it has special "switch off the type system" superpowers.

Design

Add:

• a type none, inhabited by nothing, and
• a type unknown, inhabited everything.

Use them, rather than any where appropriate. Ideally, we would then never infer any.

Drawbacks

Another bit of complexity budget spent.

These types will be visible to creators, so yay bikeshedding!

Alternatives

Stick with the current use of any for these cases.