[swift-evolution] [discussion] Change the behavior of @objc on a class?

[Jordan Rose]

Hey, all. An engineer at Apple noticed the following behavior:

1. class Foo: NSObject → exposed to Objective-C, Swift-style (mangled) runtime name
2. @objc class Foo: NSObject → exposed to Objective-C, Swift-style (mangled) runtime name
3. @objc(Foo) class Foo: NSObject → exposed to Objective-C, unmangled runtime name

(and 4. @objc class Foo → illegal, classes must have ObjC heritage to be @objc.)

They specifically observed that (1) and (2) have the same behavior, and suggested that maybe (2) should be shorthand for (3).

Pros:
- There aren't two ways to spell (1).
- Removing the mangling (and module uniquing) from the runtime name is probably one of the most common uses of @objc on a class.

Cons:
- It's a source-breaking change, for all that the "@objc" in (2) is redundant.
- For protocols, (1) and (2) are not equivalent, because @objc isn't inherited there.
- Mangling is used to namespace class names at run time; if you drop that, the ObjC name should probably have a prefix. (This applies more to frameworks than apps, though.)

There are probably people who say that last con applies to the generated header as well: we shouldn't put (1) or (2) into the generated header because the ObjC name might conflict with another class at compile time. This is valid, but probably too drastic a change at this point.

So, what does everyone think? I'm leaning towards "no change" because it's a bit subtle and not a big enough win, but if there's widespread support for this I'll pull it into a proposal.

Jordan

P.S. This is rdar://problem/22296436 for anyone else at Apple.
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