Xcode 9.3 beta 1: overly aggressive disallowing implementation of method declared with NS_UNAVAILABLE

Number:rdar://37011288 Date Originated:2018.01.29
Status:Closed Resolved:fixed 9.3 beta 2
Product:Xcode Product Version:9.3 beta 1
Classification: Reproducible:always
Summary: we have a macro (TFS_UNRECOGNIZED_SELECTOR) which we use to "implement" methods that have been declared with NS_UNAVAILABLE on them for those circumstances when a method that had been so marked still manages to get invoked unintentionally at runtime.  the macro would produce code as follows:


  - (instancetype)initWithIdentifier:(NSString *)identifier
      [self doesNotRecognizeSelector:cmd];
      abort(); // will never be reached, but prevents compiler warning

however, in Xcode 9.3 beta, the compiler now complains about this in a few ways, most notably with an error:

  Code.m:115:1: Implementing unavailable method

the application of this is not entirely consistent.  there are places in our code where it's possible to place the declaration in

 @interface Code (Unavailable)

and the definition in

 @implementation Code (Unavailable)

however, this is not always guaranteed to workaround the problem.

further, there are places where either the SDK or our own code uses NS_DESIGNATED_INITIALIZER in a case where, for a subclass, NS_UNAVAILABLE is used; in such cases, it can be the case that a diagnostic is produced for either the missing designated initializer override (which we no longer wish to provide because we've designated a different NS_DESIGNATED_INITIALIZER) or the implementation of the unavailable method.  thus, in these circumstances, we cannot build.

the same ends up being true in some cases for @protocol implementors for which an NS_UNAVAILABLE has been provided.

Steps to Reproduce:
1. declare an init method with NS_UNAVAILABLE
2. define the method as follows

- (instancetype)init
    [self doesNotRecognizeSelector:cmd];
    abort(); // will never be reached, but prevents compiler warning
3. build

Expected Results: as with Xcode 9.2 and many previous version, compile without diagnostic issues.
Actual Results: Code.m:115:1: Implementing unavailable method


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