From a934f742d81aefa53e8e45fa7eec0042d3fde7fc Mon Sep 17 00:00:00 2001
From: Andy Friesen <afriesen@roblox.com>
Date: Mon, 11 Jul 2022 13:21:23 -0700
Subject: [PATCH] June recap (#583)

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+---
+layout: single
+title:  "Luau Recap: June 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-june-2022/).]
+
+# Lower bounds calculation
+
+A common problem that Luau has is that it primarily works by inspecting expressions in your program and narrowing the _upper bounds_ of the values that can inhabit particular variables.  In other words, each time we see a variable used, we eliminate possible sets of values from that variable's domain.
+
+There are some important cases where this doesn't produce a helpful result.  Take this function for instance:
+
+```lua
+function find_first_if(vec, f)
+	for i, e in ipairs(vec) do
+		if f(e) then
+			return i
+		end
+	end
+
+	return nil
+end
+```
+
+Luau scans the function from top to bottom and first sees the line `return i`.  It draws from this the inference that `find_first_if` must return the type of `i`, namely `number`.
+
+This is fine, but things go sour when we see the line `return nil`.  Since we are always narrowing, we take from this line the judgement that the return type of the function is `nil`.  Since we have already concluded that the function must return `number`, Luau reports an error.
+
+What we actually want to do in this case is to take these `return` statements as inferences about the _lower_ bound of the function's return type.  Instead of saying "this function must return values of type `nil`," we should instead say "this function may _also_ return values of type `nil`."
+
+Lower bounds calculation does precisely this.  Moving forward, Luau will instead infer the type `number?` for the above function.
+
+This does have one unfortunate consequence: If a function has no return type annotation, we will no longer ever report a type error on a `return` statement.  We think this is the right balance but we'll be keeping an eye on things just to be sure.
+
+Lower-bounds calculation is larger and a little bit riskier than other things we've been working on so we've set up a beta feature in Roblox Studio to enable them.  It is called "Experimental Luau language features."
+
+Please try it out and let us know what you think!
+
+## Known bug
+
+We have a known bug with certain kinds of cyclic types when lower-bounds calculation is enabled.  The following, for instance, is known to be problematic.
+
+```lua
+type T = {T?}? -- spuriously reduces to {nil}?
+```
+
+We hope to have this fixed soon.
+
+# All table literals now result in unsealed tables
+
+Previously, the only way to create a sealed table was by with a literal empty table.  We have relaxed this somewhat: Any table created by a `{}` expression is considered to be unsealed within the scope where it was created:
+
+```lua
+local T = {}
+T.x = 5 -- OK
+
+local V = {x=5}
+V.y = 2 -- previously disallowed.  Now OK.
+
+function mkTable()
+    return {x = 5}
+end
+
+local U = mkTable()
+U.y = 2 -- Still disallowed: U is sealed
+```
+
+# Other fixes
+
+* Adjust indentation and whitespace when creating multiline string representations of types, resulting in types that are easier to read.
+* Some small bugfixes to autocomplete
+* Fix a case where accessing a nonexistent property of a table would not result in an error being reported.
+* Improve parser recovery for the incorrect code `function foo() -> ReturnType` (the correct syntax is `function foo(): ReturnType`)
+* Improve the parse error offered for code that improperly uses the `function` keyword to start a type eg `type T = function`
+* Some small crash fixes and performance improvements
+
+# Thanks!
+
+A very special thanks to all of our open source contributors:
+
+* [Allan N Jeremy](https://github.com/AllanJeremy)
+* [Daniel Nachun](https://github.com/danielnachun)
+* [JohnnyMorganz](https://github.com/JohnnyMorganz/)
+* [Petri Häkkinen](https://github.com/petrihakkinen)
+* [Qualadore](https://github.com/Qualadore)