Type error with function table parameters

Help and Feedback Scripting Support
#1

Minimal reproduction code:

--!strict
type Object<T> = {
	type: T
}

type Thing = {
	[string | Object<any>]: any
}

function ret<T>(t: T) 
	return t
end

local _: Thing = ret {
	[{ type = "hello" } :: Object<"hello">] = 2,
	[{ type = "foo" } :: Object<"foo">] = "hello!",  -- type error, but there shouldn't be
	hello = workspace
}

There are two TypeErrors here:

TypeError: Type
    'Object<"foo">'
could not be converted into
    'Object<"hello">'
caused by:
  Property 'type' is not compatible.
Type '"foo"' could not be converted into '"hello"' in an invariant context

TypeError: Type 'string' could not be converted into 'number'

How do I fix this?

#2

Update: the same thing happens if Thing is simply:

type Thing = {[any]: any}
#3

I have a feeling this is just one of those challenges that you get a prize for if you solve it…

#4

It appears so. I keep running into many related type checker errors because Luau infers the strictest type based on the first item in the table, so I have to add a load of ugly type casts everywhere :frowning:

I’ll just mark this as solved.

#5

Ah, this is solved in a future version of Luau with DebugLuauDeferredConstraintResolution. Unfortunately, my LSP server does not support it.