Luau Recap: November 2022 (#764)

Co-authored-by: Andy Friesen <[email protected]>

docs/_posts/2022-11-30-luau-recap-november-2022.md

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+---
+layout: single
+title:  "Luau Recap: November 2022"
+---
+
+While the team is busy to bring some bigger things in the future, we have made some small improvements this month.
+
+[Cross-posted to the [Roblox Developer Forum](https://devforum.roblox.com/t/luau-recap-november-2022/).]
+
+## Analysis improvements
+
+We have improved tagged union type refinements to only include unhandled type cases in the `else` branch of the `if` statement:
+
+```lua
+type Ok<T> = { tag: "ok", value: T }
+type Err = { tag: "error", msg: string }
+type Result<T> = Ok<T> | Err
+
+function unwrap<T>(r: Result<T>): T?
+    if r.tag == "ok" then
+        return r.value
+    else
+        -- Luau now understands that 'r' here can only be the 'Err' part
+        print(r.msg)
+        return nil
+    end
+end
+```
+
+For better inference, we updated the definition of `Enum.SomeType:GetEnumItems()` to return `{Enum.SomeType}` instead of common `{EnumItem}` and the return type of `next` function now includes the possibility of key being `nil`.
+
+Finally, if you use `and` operator on non-boolean values, `boolean` type will no longer be added by the type inference:
+
+```lua
+local function f1(a: number?)
+    -- 'x' is still a 'number?' and doesn't become 'boolean | number'
+    local x = a and 5
+end
+```
+
+## Error message improvements
+
+We now give an error when built-in types are being redefined:
+
+```lua
+type string = number -- Now an error: Redefinition of type 'string'
+```
+
+We also had a parse error missing in case you forgot your default type pack parameter value. We accepted the following code silently without raising an issue:
+
+```lua
+type Foo<T... = > = nil -- Now an error: Expected type, got '>'
+```
+
+Error about function argument count mismatch no longer points at the last argument, but instead at the function in question.
+So, instead of:
+
+```lua
+function myfunction(a: number, b:number) end
+myfunction(123)
+           ~~~
+```
+
+We now highlight this:
+
+```lua
+function myfunction(a: number, b:number) end
+myfunction(123)
+~~~~~~~~~~
+```
+
+If you iterate over a table value that could also be `nil`, you get a better explanation in the error message:
+
+```lua
+local function f(t: {number}?)
+    for i,v in t do -- Value of type {number}? could be nil
+        --...
+    end
+end
+```
+Previously it was `Cannot call non-function {number}?` which was confusing.
+
+And speaking of confusing, some of you might have seen an error like `Type 'string' could not be converted into 'string'`.
+
+This was caused by Luau having both a primitive type `string` and a table type coming from `string` library. Since the way you can get the type of the `string` library table is by using `typeof(string)`, the updated error message will mirror that and report `Type 'string' could not be converted into 'typeof(string)'`.
+
+
+Parsing now recovers with a more precise error message if you forget a comma in table constructor spanning multiple lines:
+
+```lua
+local t = {
+    a = 1
+    b = 2 -- Expected ',' after table constructor element
+    c = 3 -- Expected ',' after table constructor element
+}
+```