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

docs/support-for-thread-and-buffer-types-in-user-defined-type-functions.md

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+# 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.