Compilation fails with no helpful clues on Swift 6, with certain upcoming feature flags

[MahdiBM]

Description

Compilation fails with no helpful clues on Swift 6 when a previously-valid upcoming feature flag is used in Package.swift, which is now enabled by default in Swift 6.
I was able figure out the "why", because on Linux test CI it was just actually mentioning the problem and asking to remove those upcoming feature flags:

<unknown>:0: error: upcoming feature 'ConciseMagicFile' is already enabled as of Swift version 6
<unknown>:0: error: upcoming feature 'ForwardTrailingClosures' is already enabled as of Swift version 6
<unknown>:0: error: upcoming feature 'DisableOutwardActorInference' is already enabled as of Swift version 6
<unknown>:0: error: upcoming feature 'BareSlashRegexLiterals' is already enabled as of Swift version 6
<unknown>:0: error: upcoming feature 'InferSendableFromCaptures' is already enabled as of Swift version 6

Reproduction

Clone Vapor's PennyBot. Checkout b976b803a3867279abad4edba5d482a126fcd254.

then set these env vars:

PENNY_DIR="path/to/cloned/penny-bot"
TOOLCHAIN_DIR="/path/to/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin"
DERIVED_DATA_DIR="/path/to/DerivedData"
PENNY_DERIVED_DATA_DIR="$DERIVED_DATA_DIR/penny-bot-bgzmrsylptrxaregfntbbyweebqd" 
XCODE_DEVELOPER_DIR="/path/to/Xcode.app/Contents/Developer"

then invoke swift-frontend:

cd $PENNY_DIR/.swiftpm/xcode
    $TOOLCHAIN_DIR/swift-frontend -c $DERIVED_DATA_DIR/Build/Intermediates.noindex/Penny.build/Debug/GitHubAPI.build/DerivedSources/resource_bundle_accessor.swift $PENNY_DIR/Lambdas/GitHubAPI/+Client.swift -primary-file $PENNY_DIR/Lambdas/GitHubAPI/+Repository.swift -primary-file $PENNY_DIR/Lambdas/GitHubAPI/+User.swift $PENNY_DIR/Lambdas/GitHubAPI/Aliases.swift $PENNY_DIR/Lambdas/GitHubAPI/AuthorizationHeader.swift $PENNY_DIR/Lambdas/GitHubAPI/Changes.swift $PENNY_DIR/Lambdas/GitHubAPI/Events+Action.swift $PENNY_DIR/Lambdas/GitHubAPI/GHEvent.swift $PENNY_DIR/Lambdas/GitHubAPI/GHMiddleware.swift $PENNY_DIR/Lambdas/GitHubAPI/GeneratedSources/Client.swift $PENNY_DIR/Lambdas/GitHubAPI/GeneratedSources/Types.swift $PENNY_DIR/Lambdas/GitHubAPI/Verifier.swift $PENNY_DIR/Lambdas/GitHubAPI/exports.swift $PENNY_DIR/Lambdas/GitHubAPI/uiName+.swift -emit-dependencies-path $DERIVED_DATA_DIR/Build/Intermediates.noindex/Penny.build/Debug/GitHubAPI.build/Objects-normal/arm64/+Repository.d -emit-const-values-path $DERIVED_DATA_DIR/Build/Intermediates.noindex/Penny.build/Debug/GitHubAPI.build/Objects-normal/arm64/+Repository.swiftconstvalues -emit-reference-dependencies-path $DERIVED_DATA_DIR/Build/Intermediates.noindex/Penny.build/Debug/GitHubAPI.build/Objects-normal/arm64/+Repository.swiftdeps -serialize-diagnostics-path $DERIVED_DATA_DIR/Build/Intermediates.noindex/Penny.build/Debug/GitHubAPI.build/Objects-normal/arm64/+Repository.dia -emit-dependencies-path $DERIVED_DATA_DIR/Build/Intermediates.noindex/Penny.build/Debug/GitHubAPI.build/Objects-normal/arm64/+User.d -emit-const-values-path $DERIVED_DATA_DIR/Build/Intermediates.noindex/Penny.build/Debug/GitHubAPI.build/Objects-normal/arm64/+User.swiftconstvalues -emit-reference-dependencies-path $DERIVED_DATA_DIR/Build/Intermediates.noindex/Penny.build/Debug/GitHubAPI.build/Objects-normal/arm64/+User.swiftdeps -serialize-diagnostics-path $DERIVED_DATA_DIR/Build/Intermediates.noindex/Penny.build/Debug/GitHubAPI.build/Objects-normal/arm64/+User.dia -target arm64-apple-macos14.0 -Xllvm -aarch64-use-tbi -enable-objc-interop -stack-check -sdk $XCODE_DEVELOPER_DIR/Platforms/MacOSX.platform/Developer/SDKs/MacOSX15.1.sdk -I $DERIVED_DATA_DIR/Build/Products/Debug -I $XCODE_DEVELOPER_DIR/Platforms/MacOSX.platform/Developer/usr/lib -F $DERIVED_DATA_DIR/Build/Products/Debug/PackageFrameworks -F $DERIVED_DATA_DIR/Build/Products/Debug/PackageFrameworks -F $DERIVED_DATA_DIR/Build/Products/Debug/PackageFrameworks -F $DERIVED_DATA_DIR/Build/Products/Debug/PackageFrameworks -F $DERIVED_DATA_DIR/Build/Products/Debug/PackageFrameworks -F $DERIVED_DATA_DIR/Build/Products/Debug/PackageFrameworks -F $DERIVED_DATA_DIR/Build/Products/Debug/PackageFrameworks -F $DERIVED_DATA_DIR/Build/Products/Debug/PackageFrameworks -F $DERIVED_DATA_DIR/Build/Products/Debug/PackageFrameworks -F $DERIVED_DATA_DIR/Build/Products/Debug/PackageFrameworks -F $DERIVED_DATA_DIR/Build/Products/Debug/PackageFrameworks -F $DERIVED_DATA_DIR/Build/Products/Debug/PackageFrameworks -F $DERIVED_DATA_DIR/Build/Products/Debug/PackageFrameworks -F $DERIVED_DATA_DIR/Build/Products/Debug/PackageFrameworks -F $DERIVED_DATA_DIR/Build/Products/Debug/PackageFrameworks -F $DERIVED_DATA_DIR/Build/Products/Debug/PackageFrameworks -F $DERIVED_DATA_DIR/Build/Products/Debug/PackageFrameworks -F $DERIVED_DATA_DIR/Build/Products/Debug/PackageFrameworks -F $DERIVED_DATA_DIR/Build/Products/Debug/PackageFrameworks -F $DERIVED_DATA_DIR/Build/Products/Debug/PackageFrameworks -F $DERIVED_DATA_DIR/Build/Products/Debug/PackageFrameworks -F $DERIVED_DATA_DIR/Build/Products/Debug/PackageFrameworks -F $DERIVED_DATA_DIR/Build/Products/Debug/PackageFrameworks -F $DERIVED_DATA_DIR/Build/Products/Debug/PackageFrameworks -F $DERIVED_DATA_DIR/Build/Products/Debug/PackageFrameworks -F $DERIVED_DATA_DIR/Build/Products/Debug/PackageFrameworks -F $DERIVED_DATA_DIR/Build/Products/Debug/PackageFrameworks -F $DERIVED_DATA_DIR/Build/Products/Debug/PackageFrameworks -F $DERIVED_DATA_DIR/Build/Products/Debug/PackageFrameworks -F $DERIVED_DATA_DIR/Build/Products/Debug/PackageFrameworks -F $DERIVED_DATA_DIR/Build/Products/Debug/PackageFrameworks -F $DERIVED_DATA_DIR/Build/Products/Debug/PackageFrameworks -F $DERIVED_DATA_DIR/Build/Products/Debug/PackageFrameworks -F $DERIVED_DATA_DIR/Build/Products/Debug/PackageFrameworks -F $DERIVED_DATA_DIR/Build/Products/Debug/PackageFrameworks -F $DERIVED_DATA_DIR/Build/Products/Debug/PackageFrameworks -F $DERIVED_DATA_DIR/Build/Products/Debug/PackageFrameworks -F $DERIVED_DATA_DIR/Build/Products/Debug/PackageFrameworks -F $DERIVED_DATA_DIR/Build/Products/Debug/PackageFrameworks -F $DERIVED_DATA_DIR/Build/Products/Debug -F $XCODE_DEVELOPER_DIR/Platforms/MacOSX.platform/Developer/Library/Frameworks -no-color-diagnostics -enable-testing -g -debug-info-format\=dwarf -dwarf-version\=4 -module-cache-path $DERIVED_DATA_DIR/ModuleCache.noindex -profile-generate -profile-coverage-mapping -swift-version 6 -enforce-exclusivity\=checked -Onone -D SWIFT_PACKAGE -D DEBUG -D Xcode -serialize-debugging-options -const-gather-protocols-file $DERIVED_DATA_DIR/Build/Intermediates.noindex/Penny.build/Debug/GitHubAPI.build/Objects-normal/arm64/GitHubAPI_const_extract_protocols.json -enable-experimental-feature DebugDescriptionMacro -enable-experimental-feature OpaqueTypeErasure -enable-upcoming-feature ExistentialAny -enable-upcoming-feature FullTypedThrows -empty-abi-descriptor -plugin-path $XCODE_DEVELOPER_DIR/Toolchains/XcodeDefault.xctoolchain/usr/lib/swift/host/plugins/testing -validate-clang-modules-once -clang-build-session-file $DERIVED_DATA_DIR/ModuleCache.noindex/Session.modulevalidation -Xcc -working-directory -Xcc $PENNY_DIR/.swiftpm/xcode -resource-dir $XCODE_DEVELOPER_DIR/Toolchains/XcodeDefault.xctoolchain/usr/lib/swift -enable-anonymous-context-mangled-names -file-compilation-dir $PENNY_DIR/.swiftpm/xcode -Xcc -fmodule-map-file\=$DERIVED_DATA_DIR/Build/Intermediates.noindex/GeneratedModuleMaps/CNIOLLHTTP.modulemap -Xcc -fmodule-map-file\=$DERIVED_DATA_DIR/Build/Intermediates.noindex/GeneratedModuleMaps/CNIOBoringSSLShims.modulemap -Xcc -fmodule-map-file\=$DERIVED_DATA_DIR/Build/Intermediates.noindex/GeneratedModuleMaps/CNIOBoringSSL.modulemap -Xcc -fmodule-map-file\=$DERIVED_DATA_DIR/Build/Intermediates.noindex/GeneratedModuleMaps/CNIOExtrasZlib.modulemap -Xcc -fmodule-map-file\=$DERIVED_DATA_DIR/Build/Intermediates.noindex/GeneratedModuleMaps/CNIOWASI.modulemap -Xcc -fmodule-map-file\=$DERIVED_DATA_DIR/Build/Intermediates.noindex/GeneratedModuleMaps/CNIOLinux.modulemap -Xcc -fmodule-map-file\=$DERIVED_DATA_DIR/Build/Intermediates.noindex/GeneratedModuleMaps/CNIODarwin.modulemap -Xcc -fmodule-map-file\=$DERIVED_DATA_DIR/Build/Intermediates.noindex/GeneratedModuleMaps/CNIOAtomics.modulemap -Xcc -fmodule-map-file\=$DERIVED_DATA_DIR/Build/Intermediates.noindex/GeneratedModuleMaps/CAsyncHTTPClient.modulemap -Xcc -D_LIBCPP_HARDENING_MODE\=_LIBCPP_HARDENING_MODE_DEBUG -Xcc -ivfsstatcache -Xcc $DERIVED_DATA_DIR/SDKStatCaches.noindex/macosx15.1-24B5046f-cc0fef0e302a5c173b4160f869a79156.sdkstatcache -Xcc -I$DERIVED_DATA_DIR/Build/Intermediates.noindex/Penny.build/Debug/GitHubAPI.build/swift-overrides.hmap -Xcc -I$DERIVED_DATA_DIR/SourcePackages/checkouts/swift-nio/Sources/CNIOLLHTTP/include -Xcc -I$DERIVED_DATA_DIR/SourcePackages/checkouts/swift-nio-ssl/Sources/CNIOBoringSSLShims/include -Xcc -I$DERIVED_DATA_DIR/SourcePackages/checkouts/swift-nio-ssl/Sources/CNIOBoringSSL/include -Xcc -I$DERIVED_DATA_DIR/SourcePackages/checkouts/swift-nio-extras/Sources/CNIOExtrasZlib/include -Xcc -I$DERIVED_DATA_DIR/SourcePackages/checkouts/swift-nio/Sources/CNIOWASI/include -Xcc -I$DERIVED_DATA_DIR/SourcePackages/checkouts/swift-nio/Sources/CNIOWindows/include -Xcc -I$DERIVED_DATA_DIR/SourcePackages/checkouts/swift-nio/Sources/CNIOLinux/include -Xcc -I$DERIVED_DATA_DIR/SourcePackages/checkouts/swift-nio/Sources/CNIODarwin/include -Xcc -I$DERIVED_DATA_DIR/SourcePackages/checkouts/swift-nio/Sources/CNIOAtomics/include -Xcc -I$DERIVED_DATA_DIR/SourcePackages/checkouts/swift-numerics/Sources/_NumericsShims/include -Xcc -I$DERIVED_DATA_DIR/SourcePackages/checkouts/async-http-client/Sources/CAsyncHTTPClient/include -Xcc -I$DERIVED_DATA_DIR/SourcePackages/checkouts/swift-atomics/Sources/_AtomicsShims/include -Xcc -I$DERIVED_DATA_DIR/Build/Products/Debug/include -Xcc -I$DERIVED_DATA_DIR/Build/Intermediates.noindex/Penny.build/Debug/GitHubAPI.build/DerivedSources-normal/arm64 -Xcc -I$DERIVED_DATA_DIR/Build/Intermediates.noindex/Penny.build/Debug/GitHubAPI.build/DerivedSources/arm64 -Xcc -I$DERIVED_DATA_DIR/Build/Intermediates.noindex/Penny.build/Debug/GitHubAPI.build/DerivedSources -Xcc -DSWIFT_PACKAGE -Xcc -DDEBUG\=1 -module-name GitHubAPI -package-name penny_bot -frontend-parseable-output -disable-clang-spi -target-sdk-version 15.1 -target-sdk-name macosx15.1 -external-plugin-path $XCODE_DEVELOPER_DIR/Platforms/MacOSX.platform/Developer/usr/lib/swift/host/plugins\#$XCODE_DEVELOPER_DIR/Platforms/MacOSX.platform/Developer/usr/bin/swift-plugin-server -external-plugin-path $XCODE_DEVELOPER_DIR/Platforms/MacOSX.platform/Developer/usr/local/lib/swift/host/plugins\#$XCODE_DEVELOPER_DIR/Platforms/MacOSX.platform/Developer/usr/bin/swift-plugin-server -plugin-path $XCODE_DEVELOPER_DIR/Toolchains/XcodeDefault.xctoolchain/usr/lib/swift/host/plugins -plugin-path $XCODE_DEVELOPER_DIR/Toolchains/XcodeDefault.xctoolchain/usr/local/lib/swift/host/plugins -o $DERIVED_DATA_DIR/Build/Intermediates.noindex/Penny.build/Debug/GitHubAPI.build/Objects-normal/arm64/+Repository.o -o $DERIVED_DATA_DIR/Build/Intermediates.noindex/Penny.build/Debug/GitHubAPI.build/Objects-normal/arm64/+User.o -index-unit-output-path /Penny.build/Debug/GitHubAPI.build/Objects-normal/arm64/+Repository.o -index-unit-output-path /Penny.build/Debug/GitHubAPI.build/Objects-normal/arm64/+User.o -index-store-path $DERIVED_DATA_DIR/Index.noindex/DataStore -index-system-modules

This succeeds.

Now add -enable-upcoming-feature InferSendableFromCaptures to the command.
Observe that it fails with non-zero "exit-status": 1.

You could also just try to build the package in Xcode and see if it throws build errors for you as well or not.

Expected behavior

"exit-status": 0.

Environment

Xcode 16.1 beta 2

swift-driver version: 1.115 Apple Swift version 6.0 (swiftlang-6.0.0.9.11 clang-1600.0.26.2)
Target: arm64-apple-macosx15.0

Additional information

No response