local a = Dec.State(3)
local b = Dec.State(4)
local sum = a + b
print(sum:Current()) -- 7
a:Set(4)
print(sum:Current()) -- 8

--!strict
type BaseClass = typeof(setmetatable(
    {},
    ({} :: any) :: {__add: (BaseClass, BaseClass) -> BaseClass})
)
type SubClass = BaseClass & {extraField: string}

-- OK
local function add1(x: BaseClass, y: BaseClass): BaseClass
    return x + y
end

-- OK
local function add2(x: SubClass, y: BaseClass): BaseClass
    return x + y
end

-- OK
local function add3(x: BaseClass, y: SubClass): BaseClass
    return x + y
end

-- Emits an Error: "Type Error: (22,9) Type 't1 & {| extraField: string |} where
-- t1 = { @metatable {| __add: (t1, t1) -> t1 |}, {  } }' could not be converted
-- into 'number'; none of the intersection parts are compatible
local function add4(x: SubClass, y: SubClass): BaseClass
    return x + y
end