Auto-generating equals and hash based on annotated attributes

[@lucaswerkmeister] I thought I’d open a dedicated issue for @gavinking’s suggestion here:

I would like to have a system based on annotations that produces equals() and hash at once.

If I understand that correctly, you would annotate some attributes identifying (or a different name), and you would get equals() and hash that used equals() / hash of all attributes annotated identifying.

Open questions:

1. Is this implemented by the compiler? It’s probably possible to just make this the behavior of Object.equals() and Object.hash using the metamodel.
2. If it’s generated by the compiler: What if there are identifying attributes, but equals() or hash was also overridden?
   1. Always disallow that. Possibly inconvenient when you inherit a class with identifying attributes, but want to implement your own equals().
   2. Only allow it if both equals() and hash are overridden, make it an error to override only one of them.
   3. Allow it. If only one is overridden, keep the “clever” version of the other.
   4. Allow it. If only one is overridden, fall back to the “dumb” version of the other.

To make any sense, this probably requires identifying to be an inherited annotation. But in #4020 @FroMage said that actual members can’t inherit annotations 😦

[Migrated from ceylon/ceylon-spec#971]

[gavinking]

Sorry, I mean to say:

identifying => [x, y, label];

Is a good approach, I think, generating equals() and hash from that.

[ePaul]

@xkr47 I guess just overwriting the hash attribute as hash => [v1].hash would be enough here?