Support for thread and buffer Types in User-Defined Type Functions

Summary

Add support for ‘thread’ and ‘buffer’ primitive types, omitted from original user-defined type function RFC.

Motivation

While more rarely used than ‘string’ or ‘number’, ‘thread’ and ‘buffer’ are perfectly valid primitive types already supported by Luau typechecking engine, so should be included in user-defined type function API.

Developers are already hitting this limitation and there’s no specific reason to exclude these types.

Design

We propose two new library properties and two new type tags.

No methods are added for types with these new tags.

Update types Library

Library Properties	Type	Description
thread	type	an immutable instance of the built-in type thread
buffer	type	an immutable instance of the built-in type buffer

Update type Instance

Instance Properties	Type	Description
tag	"nil" \| "unknown" \| "never" \| "any" \| "boolean" \| "number" \| "string" \| "singleton" \| "negation" \| "union" \| "intersection" \| "table" \| "function" \| "class" \| "thread" \| "buffer"	an immutable property holding this type’s tag

Drawback

Unclear if this proposal has any drawbacks.