[RFC] Type Aliases

[withinboredom]

https://wiki.php.net/rfc/typed-aliases

Known issues:

• including types just includes the file then tosses it away -- could be cached
• Path must be literal string: include types 'types.php' works, but these don't:
  include types $path; Variable - won't work
  include types __DIR__ . '/types.php'; Expression - won't work
• Opcache interaction: Need to verify include types works correctly with opcache. The types file itself won't be opcached in the traditional sense.

Todos:

• Caching mechanism for include types: Store compiled type ASTs to avoid re-parsing
• Test coverage needed:
• Namespace scoping (curly brace, multiple unscoped, etc.)
• Nested include types (types file includes another types file?)
• Circular include detection
• Complex DNF types
• Interaction with class/function/const imports of same name
• Validate implementation matches RFC scoping: Verify FC(imports_type) is reset at namespace boundaries like other imports
• What happens with include types inside a function? (Should error?)
• Self-referential aliases: use type Foo|int as Foo; (Should error)
• Alias referencing undefined alias