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+---
+layout: single
+title:  "Luau Recap: August 2021"
+---
+
+Luau is our new language that you can read more about at [https://roblox.github.io/luau](https://roblox.github.io/luau).
+
+[Cross-posted to the [Roblox Developer Forum](https://devforum.roblox.com/t/luau-recap-august-2021/).]
+
+## Editor features
+
+The Roblox Studio [Luau-Powered Autocomplete & Language Features Beta](https://devforum.roblox.com/t/script-editor-luau-powered-autocomplete-language-features-beta) that our team has been working on has finally been released!
+Be sure to check that out and leave your feedback for things we can improve.
+
+To support that, a lot of work went into:
+* Improving fault-tolerant parser recovery scenarios
+* Storing additional information in the AST, including comments, better location information and partial syntax data
+* Tracking additional information about types and their fields, including tracking definition locations, function argument names, deprecation state and custom Roblox-specific tags
+* Updating reflection information to provide more specific Instance types and previously missing/wrong type annotations
+* Hybrid typechecking mode to try and infer types even in scripts with no typechecking enabled
+* Types that are attached to the DataModel tree elements to provide member information
+* Placing limits to finish typechecking in a finite space/time.
+* Autocomplete API for the Roblox Studio to get entity information and appropriate suggestions
+* Additional type inference engine improvements and fixes
+
+While our work continues to respond to the feedback we receive, our team members are shifting focus to add generic functions, extend Parallel Luau, improve Lua VM performance, provide documentation and support typechecking of Lua metatable-based OOP constructs. 
+
+## Typechecking improvements
+
+Type constraint resolver now remembers constraints placed on individual table fields.
+This should fix false-positive errors reported after making sure the optional table field is present:
+```lua
+--!strict
+local t: {value: number?} = {value = 2}
+
+if t.value then
+    local v: number = t.value -- ok
+end
+```
+
+It can also refine field types:
+```lua
+--!strict
+local t: {value: string|number} = {value = 2}
+
+if type(t.value) == "number" then
+    return t.value * 2 -- ok
+end
+```
+
+Like before, combining multiple conditions using 'and' and 'not' are also supported.
+
+---
+
+Constructing arrays with different values for optional/union types are now also supported for individual table fields and in functions call arguments:
+```lua
+--!strict
+type Foo = {x: number | string, b: number?}
+
+local function foo(l: {Foo}) end
+
+foo({
+	{x = 1234567},
+	{x = "hello"}, -- now ok
+})
+
+type Bar = {a: {Foo}}
+
+local foos: Bar = {a = {
+	{x = 1234567},
+	{x = "hello", b = 2}, -- now ok
+}}
+```
+
+---
+
+Finally, we have fixed an issue with Roblox class field access using indexing like `part["Anchored"] = true`.
+
+```
+## Linter improvements
+
+We have added a new linter check for duplicate local variable definitions.
+
+It is created to find duplicate names cases like these:
+```lua
+local function foo(a1, a2, a2) -- Function argument 'a2' already defined on column 24
+local a1, a2, a2 = f() -- Variable 'a2' already defined on column 11
+
+local bar = {}
+function bar:test(self) -- Function argument 'self' already defined implicitly
+```
+
+Our UnknownType linter warning was extended to check for correct class names passed into `FindFirstChildOfClass`, `FindFirstChildWhichIsA`, `FindFirstAncestorOfClass` and `FindFirstAncestorWhichIsA`.
+
+## Performance improvements
+
+We have added an optimization to 'table.unpack' for 2x performance improvement.
+
+One extra optimization for tables is to predict required table size based on fields that are assigned to it in the code after construction (when table is initialized as empty).
+
+Another optimization this month changes the location and rate of garbage collection invocations.
+We now try to avoid calling GC during the script execution and perform all the work in the GcJob part of the frame (it could be seen in the performance profiler). When possible we can now skip that job in the frame if we have some memory budget available.
+
+## Other improvements
+
+For general stability improvements we fixed a crash when strange types like '`nil??`' are used and when users have their own global functions names '`require`'.
+
+Indexing table with an incompatible type will now show squiggly error lines under the index instead of the whole expression, which was a bit misleading.
+
+An issue with debug information that caused `repeat ... until` condition line to be skipped when stepping was fixed.
+
+Type output was changed to replace display of types like '`{<g405>(g405) -> g405}`' with '`{<a>(a) -> a}`'.