[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).

从 issue 中描述的类继承和类型注解行为开始，包括 Stream、ReadStream、SeekableStream 和 StringStream 的示例。比较所提出的合取类型和接口方法，然后验证当实现省略自身的注解时，继承的方法注解仍会验证参数和返回类型。