Create type-metamethod-support

docs/type-metamethod-support

@@ -0,0 +1,57 @@
+# Type Metamethod Support
+
+## Summary
+
+Add support for the `__type` metamethod on Luau defined tables.
+
+## Motivation
+
+The motivation of allowing developers to use the `__type` metamethod is to provide a better runtime mechanism for comparing between table types.
+
+Right now, the type checker can compare table types, however, using typeof on these tables will still return `table`, as to the runtime, these are the same type. Support for the `__type` metamethod is already supported for host-defined userdatas, so it would be trivial to extend this to tables.
+
+## Design
+
+The design would simply involve adding a new metamethod `__type`, that can be set in the metatable of another table:
+
+```lua
+local Cat_MT = {
+  __index = Cat,
+  __type = "Cat"
+}
+```
+
+When calling the `Cat` object with `typeof`, it should return the value of the defined `__type` metamethod if it exists:
+
+```lua
+local cat = Cat.new()
+print(typeof(cat)) --> "Cat"
+```
+  
+## Drawbacks
+
+The main drawback is that this allows spoofing of host-defined userdatas, lets take the Roblox datum `Color3`. This object contains a `__type` flag of Color3. If we allow __type on userdatas, there's no easy way to check between a host-defined userdata, and a spoofed Color3 Luau userdata/table at the Luau level.
+
+The easiest fix is to include a hardcoded prefix on the `__type` field to indicate its a Lua-defined table.
+
+## Alternatives
+
+This can already be implemented using a field exposed on the table:
+
+```lua
+local t1 = {__type = "Table1"}
+local t2 = {__type = "Table2"}
+
+local function useTable(t)
+  if t.__type == "Table1" then
+    print("table is of type 1")
+  elseif t.__type = "Table2" then
+    print("table is of type 2")
+  end
+end
+```
+
+The issue here is that it requires each table passed to the function to implement a `__type` field, which has its own problems
+
+* If the table is not frozen (or cant be frozen), someone can easily change the `__type` field and make the code take a path other than expected.
+* It creates a contract that every object table has a key of the same name to check types.