Thank you! Yes, I am properly xcode-select'd and am pointing at the beta. The full failure from the build log is posted below, let me know if I can post anything else!
The project uses the open source Quick - https://www.github.com/quick/quick and Nimble - https://www.github.com/quick/nimble testing frameworks (via CocoaPods) and I think that may be related. Specifically, it does this - https://github.com/Quick/Quick/blob/0038bcbab4292f3b028632556507c124e5ba69f3/Quick.podspec#L35 'FRAMEWORK_SEARCH_PATHS' => '$(PLATFORM_DIR)/Developer/Library/Frameworks'. From some playing around I've done, removing this seems to help, but there's not a great way to actually do that permanently.
Failure - https://developer.apple.com/forums/content/attachment/be225a18-fec9-4f41-9e3e-95e463a87347
Cameron2019
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In regards to ENABLE_TESTING_SEARCH_PATHS, my test targets have this enabled, and I've gone ahead with some post_install hooks in my Podfile to enable this in the Debug configuration for all of my pods as well.
With this, I still get the original error.
I then additionally modified all the pod-generated xcconfig files to remove all instances of FRAMEWORK_SEARCH_PATHS = $(PLATFORM_DIR)/Developer/Library/Frameworks.
With both of these fixes (?) in place, I get a new error: cannot specify -o when generating multiple output files
I could take the symbolic linking approach as suggested by TungstenT but that feels like it's papering over a deeper issue.