`never` doesn't do what it's supposed to do

[idanarye]

Using version 0.18 downloaded from the GH release:

$ ./emmylua_check --version
emmylua_check 0.18.0

I wrote this file:

---@return integer?
local function create_num()
    return 1
end

---@param _num integer
local function accept_num(_num)
end

---@return never
local function panic()
    error("panic!")
end

local num1 = create_num()
if num1 == nil then
    panic()
end
accept_num(num1)

local num2 = create_num()
if num2 == nil then
    error()
end
accept_num(num2)

And ran:

$ ./emmylua_check .
--- main.lua [1 warning]
warning: expected `integer` but found `integer?`.  [param-type-mismatch]
  -->: main.lua:19:12

  18 │ end
  19 │ accept_num(num1)
  20 │ 



Summary
  1 warning

Check completed with warnings
Check finished

num2 worked as expected - using the builtin error when its nil convinced the type checker that it cannot be nil. But why does it think num1 is still an integer?? I've annotated my panic() as ---@return never, which - in any language that has a never type - means it can never return (either by looping forever, by throwing an error, or by crashing the program itself via whatever means), and therefore the type checker should consider it equivalent to calling error() and conclude the num1 is integer and not integer? after that check. But it doesn't.

[CppCXY]

you can config like:

{
    "runtime": {
        "special": {
            "panic" : "error"
        },
        "version": "LuaLatest"
    }
}

In the current control-flow analysis we assume our type analysis is not yet complete, so we can't make use of that information.

[idanarye]

What's the purpose of the never type, then?

[xuhuanzy]

never is currently equivalent to nil