Open Radar

 
Please describe the issue:
SwiftUI previews of WatchKit Extension never show up, attempts to refresh the preview fail with “Command CompileSwiftSources failed with a nonzero exit code” even though the product builds and runs on Apple Watch without any error.

This is a regression for latest Xcode 12.5.1 where a same preview works OK.

Please list the steps you took to reproduce the issue:
1. Create a SwiftUI component for WatchKit Extension with a Preview
2. Try to view the preview

What did you expect to happen?
Preview shows up

What actually happened?
Error “Command CompileSwiftSources failed with a nonzero exit code”, preview never shows up

The command from “Build target WatchKit Extension - Log”:

CompileSwiftSources normal arm64 com.apple.xcode.tools.swift.compiler (in target 'WeathergraphWatchKitExtension' from project 'Weathergraph Independent')
    cd /Users/kafkat/Dev/weathergraph/weathergraph-ios
    export DEVELOPER_DIR\=/Applications/Xcode.app/Contents/Developer
    export SDKROOT\=/Applications/Xcode.app/Contents/Developer/Platforms/WatchSimulator.platform/Developer/SDKs/WatchSimulator8.0.sdk
    /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/swiftc -incremental -module-name WeathergraphWatchKitExtension -Onone -enable-batch-mode -enforce-exclusivity\=checked @/Users/kafkat/Library/Developer/Xcode/DerivedData/Weathergraph_Independent-apgbjjwodocdpiaehfrqnhmyfmsi/Build/Intermediates.noindex/Previews/Weathergraph\ WatchKit\ App\ -\ Debug/Intermediates.noindex/Weathergraph\ Independent.build/Debug-watchsimulator/WeathergraphWatchKitExtension.build/Objects-normal/arm64/WeathergraphWatchKitExtension.SwiftFileList -DDEBUG -sdk /Applications/Xcode.app/Contents/Developer/Platforms/WatchSimulator.platform/Developer/SDKs/WatchSimulator8.0.sdk -target arm64-apple-watchos7.0-simulator -g -module-cache-path /Users/kafkat/Library/Developer/Xcode/DerivedData/ModuleCache.noindex -Xfrontend -serialize-debugging-options -application-extension -enable-testing -index-store-path /Users/kafkat/Library/Developer/Xcode/DerivedData/Weathergraph_Independent-apgbjjwodocdpiaehfrqnhmyfmsi/Index/DataStore -swift-version 5 -I /Users/kafkat/Library/Developer/Xcode/DerivedData/Weathergraph_Independent-apgbjjwodocdpiaehfrqnhmyfmsi/Build/Intermediates.noindex/Previews/Weathergraph\ WatchKit\ App\ -\ Debug/Products/Debug-watchsimulator -F /Users/kafkat/Library/Developer/Xcode/DerivedData/Weathergraph_Independent-apgbjjwodocdpiaehfrqnhmyfmsi/Build/Intermediates.noindex/Previews/Weathergraph\ WatchKit\ App\ -\ Debug/Products/Debug-watchsimulator/PackageFrameworks -F /Users/kafkat/Library/Developer/Xcode/DerivedData/Weathergraph_Independent-apgbjjwodocdpiaehfrqnhmyfmsi/Build/Intermediates.noindex/Previews/Weathergraph\ WatchKit\ App\ -\ Debug/Products/Debug-watchsimulator/PackageFrameworks -F /Users/kafkat/Library/Developer/Xcode/DerivedData/Weathergraph_Independent-apgbjjwodocdpiaehfrqnhmyfmsi/Build/Intermediates.noindex/Previews/Weathergraph\ WatchKit\ App\ -\ Debug/Products/Debug-watchsimulator/PackageFrameworks -F /Users/kafkat/Library/Developer/Xcode/DerivedData/Weathergraph_Independent-apgbjjwodocdpiaehfrqnhmyfmsi/Build/Intermediates.noindex/Previews/Weathergraph\ WatchKit\ App\ -\ Debug/Products/Debug-watchsimulator -c -j8 -output-file-map /Users/kafkat/Library/Developer/Xcode/DerivedData/Weathergraph_Independent-apgbjjwodocdpiaehfrqnhmyfmsi/Build/Intermediates.noindex/Previews/Weathergraph\ WatchKit\ App\ -\ Debug/Intermediates.noindex/Weathergraph\ Independent.build/Debug-watchsimulator/WeathergraphWatchKitExtension.build/Objects-normal/arm64/WeathergraphWatchKitExtension-OutputFileMap.json -parseable-output -serialize-diagnostics -emit-dependencies -emit-module -emit-module-path /Users/kafkat/Library/Developer/Xcode/DerivedData/Weathergraph_Independent-apgbjjwodocdpiaehfrqnhmyfmsi/Build/Intermediates.noindex/Previews/Weathergraph\ WatchKit\ App\ -\ Debug/Intermediates.noindex/Weathergraph\ Independent.build/Debug-watchsimulator/WeathergraphWatchKitExtension.build/Objects-normal/arm64/WeathergraphWatchKitExtension.swiftmodule -Xcc -I/Users/kafkat/Library/Developer/Xcode/DerivedData/Weathergraph_Independent-apgbjjwodocdpiaehfrqnhmyfmsi/Build/Intermediates.noindex/Previews/Weathergraph\ WatchKit\ App\ -\ Debug/Intermediates.noindex/Weathergraph\ Independent.build/Debug-watchsimulator/WeathergraphWatchKitExtension.build/swift-overrides.hmap -Xcc -iquote -Xcc /Users/kafkat/Library/Developer/Xcode/DerivedData/Weathergraph_Independent-apgbjjwodocdpiaehfrqnhmyfmsi/Build/Intermediates.noindex/Previews/Weathergraph\ WatchKit\ App\ -\ Debug/Intermediates.noindex/Weathergraph\ Independent.build/Debug-watchsimulator/WeathergraphWatchKitExtension.build/WeathergraphWatchKitExtension-generated-files.hmap -Xcc -I/Users/kafkat/Library/Developer/Xcode/DerivedData/Weathergraph_Independent-apgbjjwodocdpiaehfrqnhmyfmsi/Build/Intermediates.noindex/Previews/Weathergraph\ WatchKit\ App\ -\ Debug/Intermediates.noindex/Weathergraph\ Independent.build/Debug-watchsimulator/WeathergraphWatchKitExtension.build/WeathergraphWatchKitExtension-own-target-headers.hmap -Xcc -I/Users/kafkat/Library/Developer/Xcode/DerivedData/Weathergraph_Independent-apgbjjwodocdpiaehfrqnhmyfmsi/Build/Intermediates.noindex/Previews/Weathergraph\ WatchKit\ App\ -\ Debug/Intermediates.noindex/Weathergraph\ Independent.build/Debug-watchsimulator/WeathergraphWatchKitExtension.build/WeathergraphWatchKitExtension-all-non-framework-target-headers.hmap -Xcc -ivfsoverlay -Xcc /Users/kafkat/Library/Developer/Xcode/DerivedData/Weathergraph_Independent-apgbjjwodocdpiaehfrqnhmyfmsi/Build/Intermediates.noindex/Previews/Weathergraph\ WatchKit\ App\ -\ Debug/Intermediates.noindex/Weathergraph\ Independent.build/Debug-watchsimulator/WeathergraphWatchKitExtension.build/all-product-headers.yaml -Xcc -iquote -Xcc /Users/kafkat/Library/Developer/Xcode/DerivedData/Weathergraph_Independent-apgbjjwodocdpiaehfrqnhmyfmsi/Build/Intermediates.noindex/Previews/Weathergraph\ WatchKit\ App\ -\ Debug/Intermediates.noindex/Weathergraph\ Independent.build/Debug-watchsimulator/WeathergraphWatchKitExtension.build/WeathergraphWatchKitExtension-project-headers.hmap -Xcc -I/Users/kafkat/Library/Developer/Xcode/DerivedData/Weathergraph_Independent-apgbjjwodocdpiaehfrqnhmyfmsi/SourcePackages/checkouts/promises/Sources/FBLPromises/include -Xcc -I/Users/kafkat/Library/Developer/Xcode/DerivedData/Weathergraph_Independent-apgbjjwodocdpiaehfrqnhmyfmsi/Build/Intermediates.noindex/Previews/Weathergraph\ WatchKit\ App\ -\ Debug/Products/Debug-watchsimulator/include -Xcc -I/Users/kafkat/Library/Developer/Xcode/DerivedData/Weathergraph_Independent-apgbjjwodocdpiaehfrqnhmyfmsi/Build/Intermediates.noindex/Previews/Weathergraph\ WatchKit\ App\ -\ Debug/Intermediates.noindex/Weathergraph\ Independent.build/Debug-watchsimulator/WeathergraphWatchKitExtension.build/DerivedSources-normal/arm64 -Xcc -I/Users/kafkat/Library/Developer/Xcode/DerivedData/Weathergraph_Independent-apgbjjwodocdpiaehfrqnhmyfmsi/Build/Intermediates.noindex/Previews/Weathergraph\ WatchKit\ App\ -\ Debug/Intermediates.noindex/Weathergraph\ Independent.build/Debug-watchsimulator/WeathergraphWatchKitExtension.build/DerivedSources/arm64 -Xcc -I/Users/kafkat/Library/Developer/Xcode/DerivedData/Weathergraph_Independent-apgbjjwodocdpiaehfrqnhmyfmsi/Build/Intermediates.noindex/Previews/Weathergraph\ WatchKit\ App\ -\ Debug/Intermediates.noindex/Weathergraph\ Independent.build/Debug-watchsimulator/WeathergraphWatchKitExtension.build/DerivedSources -Xcc -DDEBUG\=1 -emit-objc-header -emit-objc-header-path /Users/kafkat/Library/Developer/Xcode/DerivedData/Weathergraph_Independent-apgbjjwodocdpiaehfrqnhmyfmsi/Build/Intermediates.noindex/Previews/Weathergraph\ WatchKit\ App\ -\ Debug/Intermediates.noindex/Weathergraph\ Independent.build/Debug-watchsimulator/WeathergraphWatchKitExtension.build/Objects-normal/arm64/WeathergraphWatchKitExtension-Swift.h -import-objc-header /Users/kafkat/Dev/weathergraph/weathergraph-ios/Shared/Weathergraph-Bridging-Header.h -pch-output-dir /Users/kafkat/Library/Developer/Xcode/DerivedData/Weathergraph_Independent-apgbjjwodocdpiaehfrqnhmyfmsi/Build/Intermediates.noindex/PrecompiledHeaders -working-directory /Users/kafkat/Dev/weathergraph/weathergraph-ios -Xfrontend -enable-implicit-dynamic -Xfrontend -enable-private-imports -Xfrontend -enable-dynamic-replacement-chaining -Xfrontend -disable-previous-implementation-calls-in-dynamic-replacements