parser: implement thread returns result and multi_returns(fix #19281)

[shove70]

1. Implement Can't have thread type as result, option, or multi-return. #19281
2. Add tests.

struct Foo {
mut:
	field1 thread !int
	field2 thread ?int
	field3 thread (int, int)
}

fn Foo.new() Foo {
	mut foo := Foo{
		field1: spawn get_error()
		field2: spawn get_none()
		field3: spawn get_multi_returns()
	}
	return foo
}

fn (mut foo Foo) bar() {
	if _ := foo.field1.wait() {
		assert false
	}
	if _ := foo.field2.wait() {
		assert false
	}
	a, b := foo.field3.wait()
	assert a == 1 && b == 2
}

fn get_error() !int {
	return error('error')
}

fn get_none() ?int {
	return none
}

fn get_multi_returns() (int, int) {
	return 1, 2
}

fn test_main() {
	mut foo := Foo.new()
	foo.bar()
}

[Delta456]

I don't think that result and multi return type threads should even be allowed.

[shove70]

I don't think that result and multi return type threads should even be allowed.

Well, there may be some reasons I haven't thought of. Can you explain them in detail :)

[Delta456]

We generally don't allow result types and multi return types as struct fields. I don't see a reason why to have an exception.

[shove70]

We generally don't allow result types and multi return types as struct fields. I don't see a reason why to have an exception.

hmm, I see what you mean.

But I think the struct fields type here is "thread"

[spytheman]

Yes, in this case, the fields contain threads. The types after the thread keyword, are the types of what the thread results will be.

[spytheman]

Excellent work.