Warnings on XCAbstractTest.h, XCTestCase.h, and XCTestSuite.h - objective-c

Question

I recently turned on stricter warnings for a project, and Xcode started throwing warnings on some of Apple's classes. I'm not sure if this is an Apple bug, or if I'm doing something wrong. I'm using Xcode 6.1.1 on Yosemite.
TL;DR: adding some strict warning flags to your project starts throwing warnings in various XC*Test*.h headers. The issue is intermittent, but the steps below have reproduced the issue on multiple computers.
Steps to reproduce:
Create a new Xcode project via File → New → Project.
Choose iOS → Application → Single View Application.
For Product Name, type MyApp. Choose Objective-C and Universal. Save the project.
Build the project with CommandB. Run the tests with CommandU. Observe that they both finish with no errors or warnings.
Click on the Project to go to the Project Settings.
Click on the blue MyApp Project (as opposed to MyApp or MyAppTests targets).
Click on the Build Settings tab.
Search for Other Warning Flags.
Enter the following string for the Other Warning Flags for the app:
-Weverything -Wno-objc-missing-property-synthesis -Wno-objc-property-synthesis -Wno-direct-ivar-access -Wno-assign-enum -Wno-float-equal -Wno-sign-compare -Wno-switch-enum -Wno-gnu -Wnewline-eof
Build the app again. You should see three or four warnings about treating #import as #import, which is expected given the warnings you just enabled.
Clean the project with CommandShiftK
Build again. You may see extra warnings. If not, then you need to…
Delete the DerivedData folder, at least for this project. It is located at /Library/Developer/Xcode/DerivedData/MyApp-*, where * is some random identifier string. You may also need to delete /Library/Developer/Xcode/DerivedData/ModuleCache.
Clean and build again. And again. It may take a few times. Eventually...
You will see code and documentation warnings on XCAbstractTest.h, XCTestCase.h, and XCTestSuite.h. Screenshot:
As you can see, these warnings would be quite valid on my own code, but this is Apple’s framework code. Is there any way to stop these weirdly intermittent warnings from appearing? Or is it an Apple bug that the file gets warnings at all? I’m leaning Apple bug, because plenty of other Apple headers use #import instead of #import, and none of them is throwing warnings.
Update: I filed a radar: http://www.openradar.me/20038246
Update 2: I was able to glean some more information that may be helpful:
The issue appears to be at least partly related to where Derived Data is stored. See this screen recording, courtesy of Sam Marshall for details: http://cl.samdmarshall.com/a4Hy
Note that, when I reproduce the same steps as in Sam's video on my computer, I am not able to get rid of the extra warnings. They always come back. Possible differences between my and Sam's setups:
Sam is on Xcode 6.1, while I am on 6.1.1.
Sam is on Mavericks, while I am on Yosemite (10.10.2).
When I build in Xcode 6.3b2, I get an additional warning about "building module 'UIKit'". See screenshot: http://cl.ly/a3sK
Update 4: Radar was closed as a duplicate of an issue.
Update 5: I have a workaround, which is to disable the problem warnings on the Tests target only. Here’s a partial diff. Xcode won’t add the $(inherited) automatically, so you’ll have to add it yourself.
+ WARNING_CFLAGS = (
+ "$(inherited)",
+ "-Wno-documentation-unknown-command",
+ "-Wno-auto-import",
+ "-Wno-incomplete-module",
+ );