new solver doing weird union stuff when calling functions

[LiterallyWize]

this is the simplest repro i could do of this

script:

--!strict

local module = require(script.module)

local func = nil

function start()
	if func then return end
	func = module()
end

function stop()
	if not func then return end
	func()
end

script.module:

function func2() -- 2nd function which is returned
	
end

function func1() -- 1st function which is initially called
	return func2
end

return func1

on line 14 of script, when calling func() the following warning occurs:

(((...any) -> ())?) & (buffer 
| class
| function 
| number
| string
| table
| thread
| true)
Cannot call a value of type (((...any) -> ())?) & (buffer | class | function | number | string | table | thread | true)

apparently it doesn't see it as just a function and fails to call it unless i cast func to () -> () on line 9 of script:

func = module() :: () -> ()

[biseson]

The system is inferring a broader type for func, which is why it produces that union type in the error message when you try to call it. Your current workaround of casting func to () -> () is a valid way to resolve the issue; telling the type checker explicitly that func is a function.

You can declare func's type at the top of the script to avoid the ambiguity. So, explicitly stating that func is either nil or a function takes any number of args and returns nothing (a.k.a. () -> ()):

--!strict

local module = require(script.module)

local func: (() -> ())? = nil -- declare func's type explicitly

function start()
    if func then return end
    func = module()
end

function stop()
    if not func then return end
    func() -- no need to cast here anymore
end

[aatxe]

The underlying problem here is that type normalization does not seem to handle converting ((...any) -> ()) | nil intersected with ~(false | nil) (the negation expands to that big union you're seeing, which is sort of an orthogonal DX concern) into the particular function type.

[LiterallyWize]

The system is inferring a broader type for func, which is why it produces that union type in the error message when you try to call it. Your current workaround of casting func to () -> () is a valid way to resolve the issue; telling the type checker explicitly that func is a function.

You can declare func's type at the top of the script to avoid the ambiguity. So, explicitly stating that func is either nil or a function takes any number of args and returns nothing (a.k.a. () -> ()):

--!strict

local module = require(script.module)

local func: (() -> ())? = nil -- declare func's type explicitly

function start()
    if func then return end
    func = module()
end

function stop()
    if not func then return end
    func() -- no need to cast here anymore
end

doing this was actually the first idea i had but oddly the same warning occurs, i should've mentioned that before