KVO observation of a NSProgress variable on a NSManagedObject crashes on iPhone XS

Originator:mail
Number:rdar://45729460 Date Originated:01-Nov-2018 03:59 PM
Status:Open Resolved:
Product:iOS + SDK Product Version:12.1
Classification:Crash/Hang/Data Loss Reproducible:Always
 
Summary:
When trying to observe a NSProgress that is stored on a NSManagedObject the app crashes at runtime. This only happens when linked against iOS 12.1 / compiled with Xcode 10.1 and when running on an iPhone XS.

Steps to Reproduce:
- Define a NSMangedObject subclass and add a NSProgress property.
- Create an instance of the subclass.
- Observe the NSProgress property via KVO.
- Run on iPhone XS.

Example project is attached.

Expected Results:
No crash

Actual Results:
The app/process crashes at runtime. A crashlog is attached.

Version:
12.1

Notes:
The crash only occurs when running on an iPhone XS. I could not reproduce on any other device or in the Simulator. The crash only occurs when the NSProgress is stored on a NSManagedObject; storing the NSProgress on any other NSObject works fine.

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