Added Sep-Oct 2022 Luau Recap (#726)

docs/_posts/2022-11-01-luau-recap-september-october-2022.md

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+---
+layout: single
+title:  "Luau Recap: September & October 2022"
+---
+
+Luau is our new language that you can read more about at [https://luau-lang.org](https://luau-lang.org).
+
+[Cross-posted to the [Roblox Developer Forum](https://devforum.roblox.com/t/luau-recap-september-october-2022/).]
+
+## Semantic subtyping
+
+One of the most important goals for Luau is to avoid *false
+positives*, that is cases where Script Analysis reports a type error,
+but in fact the code is correct. This is very frustrating, especially
+for beginners. Spending time chasing down a gnarly type error only to
+discover that it was the type system that's wrong is nobody's idea of fun!
+
+We are pleased to announce that a major component of minimizing false
+positives has landed, *semantic subtyping*, which removes a class of false positives caused
+by failures of subtyping.  For example, in the program
+
+```lua
+  local x : CFrame = CFrame.new()
+  local y : Vector3 | CFrame
+  if (math.random()) then
+    y = CFrame.new()
+  else
+    y = Vector3.new()
+  end
+  local z : Vector3 | CFrame = x * y -- Type Error!
+```
+
+an error is reported, even though there is no problem at runtime. This
+is because `CFrame`'s multiplication has two overloads:
+
+```lua
+    ((CFrame, CFrame) -> CFrame)
+  & ((CFrame, Vector3) -> Vector3)
+```
+
+The current syntax-driven algorithm for subtyping is not sophisticated
+enough to realize that this is a subtype of the desired type:
+
+```lua
+  (CFrame, Vector3 | CFrame) -> (Vector3 | CFrame)
+```
+
+Our new algorithm is driven by the semantics of subtyping, not the syntax of types,
+and eliminates this class of false positives.
+
+If you want to know more about semantic subtyping in Luau, check out our
+[technical blog post](https://luau-lang.org/2022/10/31/luau-semantic-subtyping.html)
+on the subject.
+
+## Other analysis improvements
+
+* Improve stringification of function types.
+* Improve parse error warnings in the case of missing tokens after a comma.
+* Improve typechecking of expressions involving variadics such as `{ ... }`.
+* Make sure modules don't return unbound generic types. 
+* Improve cycle detection in stringifying types.
+* Improve type inference of combinations of intersections and generic functions.
+* Improve typechecking when calling a function which returns a variadic e.g. `() -> (number...)`.
+* Improve typechecking when passing a function expression as a parameter to a function.
+* Improve error reporting locations.
+* Remove some sources of memory corruption and crashes.
+
+## Other runtime and debugger improvements
+
+* Improve performance of accessing debug info.
+* Improve performance of `getmetatable` and `setmetatable`.
+* Remove a source of freezes in the debugger.
+* Improve GC accuracy and performance.
+
+## Thanks
+
+Thanks for all the contributions!
+
+* [AllanJeremy](https://github.com/AllanJeremy)
+* [JohnnyMorganz](https://github.com/JohnnyMorganz)
+* [jujhar16](https://github.com/jujhar16)
+* [petrihakkinen](https://github.com/petrihakkinen)