cast doesn't work with method call

[fperrad]

local record Object
end

local type Fn = function(self: Object)
local type Alt = {string:Fn}

local obj: Object = {}
(obj as Alt).foo(obj)  -- OK
(obj as Alt):foo()     -- error: cannot index key 'foo' in type Alt

On plain Lua, obj:foo() is equivalent to obj.foo(obj)