Add way to annotate BackedEnum backing type

[AndrolGenhald]

I want to be able to take a BackedEnum<string> $enum so that $enum->value is definitely a string. Currently I have false positives because BackedEnum::$value is int|string.

• Add template to BackedEnum interface
• Add annotation to specify backing type when declaring enum (@enum<int> maybe?)
• Check annotation against PHP backing type (can only be string or int, but you can't use @enum<positive-int> with enum Foo: string)
• Check annotation against cases (case Bar = 0 should show an issue for @enum<positive-int>)
• The BackedEnum template needs to be filled in from the @enum<[type]> annotation so that BackedEnum::from, ::tryFrom, and ::cases use the correct type

[weirdan]

@implements BackedEnum<Type> is probably easier to implement.

[AndrolGenhald]

I would think @extends probably? Aren't backed enums implemented as classes extending BackedEnum with a bunch of additional constraints? Looking at this again though you're right that a new annotation for that shouldn't be necessary.

[weirdan]

Both BackedEnum and UnitEnum are interfaces, so it would be @implements