• `@objc` is now inferred on non-`final` members of @objcImplementation extensions
• Diagnostics now suggest adding `private` to ObjC helper members, not `@objc`, in line with currently proposed behavior
• Better diagnostic for members implemented in the wrong extension
Part of rdar://103150189.
Previously, Swift would reject an `override public init(…)` in an `@_objcImplementation` because ClangImporter would have already synthesized inherited initializers that conflicted with the overrides. Ignore these spurious conflicts, and also move a check out of IsObjCRequest and into the conflict-handling code.
Additional work towards rdar://70730077.
Without this change, an `@_objcImplementation` cannot override parent class methods, because the special access control behavior breaks the access control checks for overrides.
Treating this as forbidden was incorrect; an `@objc final` method is simply one that isn’t a member implementation but does have an ObjC entry point.
Formalize and centralize the definition of what a member implementation is, tweak it so that it’s basically “non-final and internal or greater”, and permit `@objc final`. Also remove the inference of `final` on `let`s in @_objcImpl extensions.
Stored properties are only allowed in the extension implementing the class's main interface, not its categories. This also means banning `@objc final`, which is unenforceable anyway when ObjC subclasses are allowed, and therefore allowing `@objc let` and `@objc static` properties to be overridden if they're declared in objcImplementations.
