The type of `self` in metamethods is not checked

[purplesyringa]

local record Test
    metamethod __len: function(Test): number
end

local test_mt: metatable<Test> = {
    __len = function(self: number): number -- somehow this typechecks?
        return self
    end
}

local test: Test = setmetatable({}, test_mt)
print(#test)