It looks like we just synthesize this wrong. @objc enums get an extra secret conformance that overrides the default domain, but does so incorrectly. The relevant code is in deriveBodyBridgedNSError_enum_NSErrorDomain in DerivedConformanceErrorType.cpp.

(We don't actually need them to override the default domain, but the _BridgedNSError protocol is used for imported enums as well.)

The derived implementation here is equivalent to

@derived
static var _NSErrorDomain: String {
    return "\(self)"
}

So what's really going on here is printing the type itself is omitting the module name. Although this code should probably be changed to explicitly do the ModuleName.TypeName format to match PrintObjC.

Side note: The following code, while a bad idea, is also relevant:

class Foo: NSObject {
    @objc enum Bar: Int, ErrorType {
        case A, B
    }
}

With this type, the enum will still end up in the generated Obj-C header with the domain "ModuleName.Bar" but the qualified name for the type is actually ModuleName.Foo.Bar. Switching the domain generation to always being "ModuleName.UnqualifiedTypeName" instead of relying on the value of "(self)" should fix this case as well.

Submitted as PR #1495.

Merged in f189f8d.