Warning when using types with short-circuit evaluation

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1

I am using types where the first variable can either be of type Vector3 or nil. However when I create a new variable with type Vector3 and set it to the first variable using short-circuit evaluation, it gives me a warning. As you can see I do a check to see that v is a Vector3, but that still causes a warning with the message: “Type mismatch Vector3 and Vector3 | boolean | nil”.

local v: Vector3? = Vector3.new(1, 1, 1)
local u: Vector3 = typeof(v) == 'Vector3' and v -- Here I get: Type mismatch Vector3 and Vector3 | boolean | nil

-- Alternative way below with a different warning: Type mismatch Vector3 and Vector3 | boolean
-- local u: Vector3 = typeof(v) == 'Vector3' and Vector3.new(v.X, v.Y, v.Z)

It seems like it is complaining about that variable u can get the type of a boolean. I know that a different way of doing the same thing is to just use an if statement, where I don’t get any warnings, like this:

local v: Vector3? = Vector3.new(1, 1, 1)
if typeof(v) == 'Vector3' then
	local u: Vector3 = v
end

However I would like to hear from you what causes the warning when doing it the first way and if it is possible to fix it.

2

You get it here because there is a chance typeof(v) == 'Vector3' can result in false therefore v won’t be evaluated.

You could do what you want conditionally:

local u: Vector3 = typeof(v) == 'Vector3' and v or some_other_vector

so that it always evaluates to a Vector3.

3

Alright, I see what is happening.

When attempting doing the way you suggested it still didn’t work, since now i get a type mismatch between Vector3 and Vector3 | nil.

local v: Vector3? = Vector3.new(1, 1, 1)
local w: Vector3 = Vector3.new(1, 2, 3)
local u: Vector3 = typeof(v) == 'Vector3' and v or w
4

Looks like it may be a bug? I dont have any issues with the linter when transposing the statement as a function

local v: Vector3? = Vector3.new(1, 1, 1)
local w: Vector3 = Vector3.new(1, 2, 3)

local function foo(givenVector: Vector3?): Vector3
	return v or w
end

local result: Vector3 = foo(v)

That said, I DONT get a lint when I would expect one when giving the result value Vector2? Anything else seems to give me the expected lint though.

local v: Vector3? = Vector3.new(1, 1, 1)
local w: Vector3 = Vector3.new(1, 2, 3)

local function foo(givenVector: Vector3?): Vector3
	return v or w
end

local result: Vector2 = foo(v)
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5

Are you sure that you are using the --!strict flag? I get a warning around the return in the function with the flag, and no warnings without it.

6

Ah no, I was not. Strangely your previous example was occurring even without the strict flag. I’m having a hard time finding any documentation on the type system so I’m not sure if this is expected behavior. Sorry I’m not much help.

1 Like