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docs/_posts/2022-06-01-luau-recap-may-2022.md

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+---
+layout: single
+title:  "Luau Recap: May 2022"
+---
+
+Read more about Luau at [https://luau-lang.org](https://luau-lang.org).
+
+[Cross-posted to the [Roblox Developer Forum](https://devforum.roblox.com/t/luau-recap-may-2022/).]
+
+This month Luau team has worked to bring you a new language feature together with more typechecking improvements and bugfixes!
+
+## Generalized iteration
+
+We have extended the semantics of standard Lua syntax for iterating through containers, `for vars in values` with support for generalized iteration.
+In Lua, to iterate over a table you need to use an iterator like `next` or a function that returns one like `pairs` or `ipairs`. In Luau, you can now simply iterate over a table:
+
+```lua
+for k, v in {1, 4, 9} do
+    assert(k * k == v)
+end
+```
+
+This works for tables but can also be customized for tables or userdata by implementing `__iter` metamethod. It is called before the iteration begins, and should return an iterator function like `next` (or a custom one):
+
+```lua
+local obj = { items = {1, 4, 9} }
+setmetatable(obj, { __iter = function(o) return next, o.items end })
+
+for k, v in obj do
+    assert(k * k == v)
+end
+```
+
+The default iteration order for tables is specified to be consecutive for elements `1..#t` and unordered after that, visiting every element.
+Similar to iteration using `pairs`, modifying the table entries for keys other than the current one results in unspecified behavior.
+
+## Typechecking improvements
+
+We have added a missing check to compare implicit table keys against the key type of the table indexer:
+
+```lua
+-- error is correctly reported, implicit keys (1,2,3) are not compatible with [string]
+local t: { [string]: boolean } = { true, true, false }
+```
+
+Rules for `==` and `~=` have been relaxed for union types, if any of the union parts can be compared, operation succeeds:
+
+```lua
+--!strict
+local function compare(v1: Vector3, v2: Vector3?)
+    return v1 == v2 -- no longer an error
+end
+```
+
+Table value type propagation now correctly works with `[any]` key type:
+
+```lua
+--!strict
+type X = {[any]: string | boolean}
+local x: X = { key = "str" } -- no longer gives an incorrect error
+```
+
+If a generic function doesn't provide type annotations for all arguments and the return value, additional generic type parameters might be added automatically:
+
+```lua
+-- previously it was foo<T>, now it's foo<T, b>, because second argument is also generic
+function foo<T>(x: T, y) end
+```
+
+We have also fixed various issues that have caused crashes, with many of them coming from your bug reports.
+
+## Autocomplete improvements
+
+Studio autocomplete has received a few improvements:
+
+* Function argument names have been added to the function signature display
+* `Instance:GetAttribute` and `Instance:SetAttribute` will now suggest attribute names if instance can be resolved
+* Inside the new if-then-else expression, incorrect suggestions following `else` have been fixed
+* Deprecated lowercase `Color3:toHSV` so it's no longer suggested
+* Deprecated lowercase `Instance:findFirstChild`, `Instance:clone`, `Instance:remove`, `Instance:getChildren` and old alias called `Instance:children`.
+
+## Linter improvements
+
+`global-used-as-local` lint warning has been extended to notice when global variable writes always happen before their use in a local scope, suggesting that they can be replaced with a local variable:
+
+```lua
+function bar()
+    foo = 6 -- Global 'foo' is never read before being written. Consider changing it to local
+    return foo
+end
+function baz()
+    foo = 10
+    return foo
+end
+```
+
+## Performance improvements
+
+Garbage collection CPU utilization has been tuned to further reduce frame time spikes of individual collection steps and to bring different GC stages to the same level of CPU utilization.
+
+In open-source release of Luau, when optimization level 2 is enabled, compiler will now perform function inlining.
+Loop unrolling in optimization level 2 has also been improved to handle all language constructs inside the loop body.