Subclassing UICollectionViewController subclass in test targets causes compilation failure

Originator:me
Number:rdar://42326672 Date Originated:2018/07/18
Status:Dupe of 19245651 Resolved:
Product:Developer Tools Product Version:Xcode 9.3 ... Xcode 10.0
Classification: Reproducible:Always
 
There seems to be a problem when importing subclasses from Swift UICollectionViewController subclasses in Objective-C.

When importing the automatically generated Swift header from an Objective-C test case, Xcode will report that the main app target imported from this header could not be found.

An example project can be found here: https://github.com/alexaubry/radar-app-module-not-found

Steps to Reproduce:

1. Define a Swift UICollectionViewController subclass in the main app target (A)
2. Define a Swift subclass of the subclass A in the test target (B)
3. Import the Swift interface of the main app in Objective-C test target and define a property that uses the B subclass.
4. Launch the unit tests

Expected Results:

The Objective-C code in the test compiles.

Actual Results:

The Objective-C code in the test does not compile, saying the main app module does not exist.

Version/Build:

Reproduced on Xcode 9.3, 9.4 and 10.0 beta using Swift 4.1 and Swift 4.2.

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