Luau Recap: May 2021 (#42)

* Added May 2021 recap post

* Added missing items and fixed confusing function declaration example

* Apply suggestions from code review

Co-authored-by: Arseny Kapoulkine <arseny.kapoulkine@gmail.com>

* Small fix

Co-authored-by: Arseny Kapoulkine <arseny.kapoulkine@gmail.com>

docs/_posts/2021-05-31-luau-recap-may-2021.md

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+---
+layout: single
+title:  "Luau Recap: May 2021"
+---
+
+Luau is our new language that you can read more about at [https://roblox.github.io/luau](https://roblox.github.io/luau). This month we have added a new small feature to the language and spent a lot of time improving our typehecker.
+
+[Cross-posted to the [Roblox Developer Forum](https://devforum.roblox.com/t/luau-recap-may-2021/).]
+
+## Named function type arguments
+
+We've updated Luau syntax to support optional names of arguments inside function types.
+The syntax follows the same format as regular function argument declarations: `(a: number, b: string)`
+
+Names can be provided in any place where function type is used, for example:
+
+* in type aliases:
+```
+type MyCallbackType = (cost: number, name: string) -> string
+```
+
+* for variables:
+```
+local cb: (amount: number) -> number
+local function foo(cb: (name: string) -> ())
+```
+
+Variadic arguments cannot have an extra name, they are already written as ...: number.
+
+These names are used for documentation purposes and we also plan to display them in Roblox Studio auto-complete and type hovers.
+They do not affect how the typechecking of Luau scripts is performed.
+
+## Typechecking improvements
+
+Speaking of typechecking, we've implemented many improvements this month:
+* Typechecker will now visit bodies of all member functions, previously it didn't check methods if the self type was unknown
+* Made improvements to cyclic module import detection and error reporting
+* Fixed incorrect error on modification of table intersection type fields
+* When using an 'or' between a nillable type and a value, the resulting type is now inferred to be non-nil
+* We have improved error messages that suggest to use ':' for a method call
+* Fixed order of types in type mismatch error that was sometimes reversed
+* Fixed an issue with `table.insert` function signature
+* Fixed a bug which caused spurious unknown global errors
+
+We've also added new checks to our linter:
+* A new check will report uses of deprecated Roblox APIs
+* Linter will now suggest replacing globals with locals in more cases
+* New warning is generated if array loop starts or ends on index '0', but the array is indexed from '1'
+* FormatString lint will now check string patterns for `find`/`match` calls via `:` when object type is known to be a string
+
+We also fixed one of the sources for "Free types leaked into this module's public interface" error message and we are working to fix the remaining ones.
+
+As usual, typechecking improvements will not break execution of your games even if new errors get reported.
+
+## Editor features
+
+We continue to improve our built-in support for auto-complete that will be used in future Roblox Studio updates and will make it easier to implement custom extensions for applications that support Language Server Protocol.
+
+As part of this work we will improve the type information provided by Roblox APIs to match actual arguments and results.
+
+## Behavior changes
+
+When a relational comparison fails at runtime, the error message now specifies the comparison direction (e.g. `attempt to compare nil <= number`)
+
+## Performance improvements
+
+* Improved performance of table lookup with an index operator and a literal string: `t["name"]`
+* Bytecode compilation is now ~5% faster which can improve server startup time for games with lots of scripts