Make Enum an abstract struct

[straight-shoota]

The Enum is not intended to be instantiated and the compiler won't even let us. This type just serves as a common ancestor for all enum types.

That fits exactly the defintion of abstract types, but for some reason this keyword is missing on Enum. Instead all protections against instantiations are implemented explicitly in the compiler (and they go even further than for normal abstract types).
Adding the abstract modifier to the type def is still a useful addition, at least for documentation purposes.
This shouldn't change any behaviour as it's already impossible to instantiate Enum.

[ysbaddaden]

I guess the Enum type was implemented before abstract keyword?

Could that lead to eventually simplify the compiler?

[straight-shoota]

Could that lead to eventually simplify the compiler?

Probably not. Most of the restrictions are applied to several types (via Type#can_be_stored?), Enum is just one of them.

And the few restrictions that are specific to Enum and do not overlap with abstract (e.g. that you cannot inherit from it).

[HertzDevil]

This is where the compiler makes Enum abstract:

crystal/src/compiler/crystal/program.cr

Lines 213 to 214 in 5915e3b

	types["Enum"] = enum_t = @enum = NonGenericClassType.new self, self, "Enum", value
	abstract_value_type(enum_t)

What happens here is that Crystal allows you to reopen an abstract type without the abstract keyword.