void can't seem to be an alternative return type in function retval annotations

[adriweb]

Environment

name	version
IDEA version	2020.3.1 (IC-203.6682.168)
Luanalysis version	1.2.2 and also repro'd in latest master (now 5018fdb14e170ccfcdaa1359a4db2e662a8dbd27)
OS	Linux Ubuntu 20.10

What are the steps to reproduce this issue?

---@alias Handler fun(): (void | nil)

---@type Handler
local myFunc3 = function() end

---@type Handler
local myFunc4 = function() return nil end

What happens?

Error on the function definition: Type mismatch. Required: 'fun(): nil' Found: 'fun(): void'

What were you expecting to happen?

For void to be able to be accepted as an alternative return type.

[Benjamin-Dobell]

This is a bit of a weird one. The syntax is functioning as is (currently) intended. However, it's certainly not what you want.

Each time you see someType | someOtherType you're creating a set. void is itself the absence of a type i.e. the empty set. So when you create a union of an empty set and some other set, the result is always whatever the other type/set was.

I'll have to have a think about this one. The reason I'm reluctant to treat void like a regular type is that Lua has fairly complicated semantics surrounding the absence of a value i.e. most of the time it's coerced to nil, but certainly not all the time.

[adriweb]

Oh, I see. The error makes sense now. And indeed, void isn't the same as nil all the time :)

Looking forward to what you'll come up with!