[Feature Request] Interfaces and Implementors

The current class system is alright, but imagine this: I have a Stream class which is extended by ReadStream, WriteStream, SeekableStream, etc.

---@class Stream
---@field close fun() Deallocates or hangs up a stream.

---@class SeekableStream: Stream
---@field seek fun(whence: SeekWhence, offset: integer): integer

---@class ReadStream: Stream
---@field read fun(self: ReadStream, length: integer): string|nil

---@class WriteStream: Stream
---@field write fun(data: string): integer

---@class StringStream: ReadStream, SeekableStream Streams data out of a string.

Now I have a function which takes any Seekable, Readable string. How do I specify this? Either we need a conjunction type (ReadStream & SeekableStream, which has keys from both), or we need (preferably) interfaces, which are generic and can be extended by classes. The reason I suggest interfaces is because if I write this:

---@param length integer
---@return string|nil
function StringStream:read(length)
	if self.buffer == nil then
		error("Stream is closed", 2)
	end
	if self.cursor > #self.buffer then
		return nil
	end
	local out = string.sub(self.buffer, self.cursor, math.min(#self.buffer, self.cursor + length))
	self.cursor = self.cursor + #out
	return out
end

and I omit the type annotation, it assumes read is already defined because StringStream extends ReadStream, and it doesn't validate types (arguments and return type are any).

Issue で説明されているクラス継承と型アノテーションの動作から始め、Stream、ReadStream、SeekableStream、StringStream の例も確認してください。提案されている conjunction type と interface のアプローチを比較し、その後、実装側で独自のアノテーションを省略した場合でも、継承されたメソッドのアノテーションによって引数と戻り値の型が引き続き検証されることを確認してください。