[RFC] Disjunctive Normal Form Types

Looks very interesting, and most importantly, useful.

Hello internals,

I would like to open the discussion on the Disjunctive Normal Form (DNF)
types RFC:
https://wiki.php.net/rfc/dnf_types

The implementation for it is available on my fork, [1] but currently relies
on a pending PR [2] to make the implementation simpler.

DNF types are a way to mix intersection and union types together in a
single type.
For example:

• (A&B)|null
• (Traversable&Countable)|array

Best regards,

George P. Banyard

[1] https://github.com/Girgias/php-src/pull/10
[2] https://github.com/php/php-src/pull/7309

DNF types are a way to mix intersection and union types together in a
single type.
For example:
 - (A&B)|null
 - (Traversable&Countable)|array

Hello George,

I like your proposal. I would like to offer 2 suggestions however:

1. I think that syntax would be cleaner without the parentheses

2. [Future Scope] I would go against non-DNF types. DNF forms a nice
   two level structure that's easy to comprehend. Arbitrary expressions
   may be too much for runtime checks (correct me if I'm wrong) and will
   definitely have high cognitive complexity. And CNF would allow to form
   invalid types too easily like (Traversable|null)&Countable

Best,
Anton

DNF types are a way to mix intersection and union types together in a
single type.
For example:
[...]

• (Traversable&Countable)|array

I have yet to read the fine print, but this example was a motivating
case for me to start down the union-types road years ago. I would be
very happy to finally see this in the language!