[Parity] [Apple] error: 'main' attribute can only apply to one type in a module

[kcieplak]

Is it reproducible with SwiftPM command-line tools: swift build, swift test, swift package etc?

• Confirmed reproduction steps with SwiftPM CLI. The description text must include reproduction steps with either of command-line SwiftPM commands, swift build, swift test, swift package etc.

Description

https://github.com/swift-nest/clutch.git

MacOS

The following package does not build using the 'swiftbuild' backend but does with 'native'

swift build `--buildsystem=swiftbuild` fails to build package

info: Discovering Swift tasks after 'Compiling ClutchTestMain.swift'
error: /private/var/lib/jenkins/workspace/swift_oss/swift-package-index-metrics/spi-workdir/52C8FBFE-E9ED-4331-A88D-43B058052AE6/clutch/.build-swiftbuild/out/Intermediates.noindex/clutch.build/Debug/clutchTests.build/DerivedSources/test_entry_point.swift:253:8 type '__SwiftPMTestAttachment' does not conform to protocol 'Decodable'
info: /private/var/lib/jenkins/workspace/swift_oss/swift-package-index-metrics/spi-workdir/52C8FBFE-E9ED-4331-A88D-43B058052AE6/Swift.Decodable.init:2:1 protocol requires initializer 'init(from:)' with type 'Decodable'
info: /private/var/lib/jenkins/workspace/swift_oss/swift-package-index-metrics/spi-workdir/52C8FBFE-E9ED-4331-A88D-43B058052AE6/clutch/.build-swiftbuild/out/Intermediates.noindex/clutch.build/Debug/clutchTests.build/DerivedSources/test_entry_point.swift:257:9 cannot automatically synthesize 'Decodable' because 'Data?' does not conform to 'Decodable'
error: /private/var/lib/jenkins/workspace/swift_oss/swift-package-index-metrics/spi-workdir/52C8FBFE-E9ED-4331-A88D-43B058052AE6/clutch/.build-swiftbuild/out/Intermediates.noindex/clutch.build/Debug/clutchTests.build/DerivedSources/test_entry_point.swift:253:8 type '__SwiftPMTestAttachment' does not conform to protocol 'Encodable'
info: /private/var/lib/jenkins/workspace/swift_oss/swift-package-index-metrics/spi-workdir/52C8FBFE-E9ED-4331-A88D-43B058052AE6/Swift.Encodable.encode:2:6 protocol requires function 'encode(to:)' with type 'Encodable'
info: /private/var/lib/jenkins/workspace/swift_oss/swift-package-index-metrics/spi-workdir/52C8FBFE-E9ED-4331-A88D-43B058052AE6/clutch/.build-swiftbuild/out/Intermediates.noindex/clutch.build/Debug/clutchTests.build/DerivedSources/test_entry_point.swift:257:9 cannot automatically synthesize 'Encodable' because 'Data?' does not conform to 'Encodable'
error: /private/var/lib/jenkins/workspace/swift_oss/swift-package-index-metrics/spi-workdir/52C8FBFE-E9ED-4331-A88D-43B058052AE6/clutch/.build-swiftbuild/out/Intermediates.noindex/clutch.build/Debug/clutchTests.build/DerivedSources/test_entry_point.swift:505:1 'main' attribute can only apply to one type in a module
error: /private/var/lib/jenkins/workspace/swift_oss/swift-package-index-metrics/spi-workdir/52C8FBFE-E9ED-4331-A88D-43B058052AE6/clutch/Tests/clutchTests/Main/ClutchTestMain.swift:13:1 'main' attribute can only apply to one type in a module
info: /private/var/lib/jenkins/workspace/swift_oss/swift-package-index-metrics/spi-workdir/52C8FBFE-E9ED-4331-A88D-43B058052AE6/clutch/.build-swiftbuild/out/Intermediates.noindex/clutch.build/Debug/clutchTests.build/DerivedSources/test_entry_point.swift:253:8: error: type '__SwiftPMTestAttachment' does not conform to protocol 'Decodable'
struct __SwiftPMTestAttachment: Codable {
       ^
Swift.Decodable.init:2:1: note: protocol requires initializer 'init(from:)' with type 'Decodable'
init(from decoder: any Decoder) throws}
^
/private/var/lib/jenkins/workspace/swift_oss/swift-package-index-metrics/spi-workdir/52C8FBFE-E9ED-4331-A88D-43B058052AE6/clutch/.build-swiftbuild/out/Intermediates.noindex/clutch.build/Debug/clutchTests.build/DerivedSources/test_entry_point.swift:257:9: note: cannot automatically synthesize 'Decodable' because 'Data?' does not conform to 'Decodable'
    let payload: Data?
        ^
/private/var/lib/jenkins/workspace/swift_oss/swift-package-index-metrics/spi-workdir/52C8FBFE-E9ED-4331-A88D-43B058052AE6/clutch/.build-swiftbuild/out/Intermediates.noindex/clutch.build/Debug/clutchTests.build/DerivedSources/test_entry_point.swift:253:8: error: type '__SwiftPMTestAttachment' does not conform to protocol 'Encodable'
struct __SwiftPMTestAttachment: Codable {
       ^
Swift.Encodable.encode:2:6: note: protocol requires function 'encode(to:)' with type 'Encodable'
func encode(to encoder: any Encoder) throws}
     ^
/private/var/lib/jenkins/workspace/swift_oss/swift-package-index-metrics/spi-workdir/52C8FBFE-E9ED-4331-A88D-43B058052AE6/clutch/.build-swiftbuild/out/Intermediates.noindex/clutch.build/Debug/clutchTests.build/DerivedSources/test_entry_point.swift:257:9: note: cannot automatically synthesize 'Encodable' because 'Data?' does not conform to 'Encodable'
    let payload: Data?
        ^
/private/var/lib/jenkins/workspace/swift_oss/swift-package-index-metrics/spi-workdir/52C8FBFE-E9ED-4331-A88D-43B058052AE6/clutch/.build-swiftbuild/out/Intermediates.noindex/clutch.build/Debug/clutchTests.build/DerivedSources/test_entry_point.swift:505:1: error: 'main' attribute can only apply to one type in a module
@main
^
/private/var/lib/jenkins/workspace/swift_oss/swift-package-index-metrics/spi-workdir/52C8FBFE-E9ED-4331-A88D-43B058052AE6/clutch/Tests/clutchTests/Main/ClutchTestMain.swift:13:1: error: 'main' attribute can only apply to one type in a module
@main
^
error: EmitSwiftModule normal arm64 failed with a nonzero exit code. Command line:     cd /private/var/lib/jenkins/workspace/swift_oss/swift-package-index-metrics/spi-workdir/52C8FBFE-E9ED-4331-A88D-43B058052AE6
 

Expected behaviour

Builds

Actual behavior

Fails to build

Steps to reproduce

git clone https://github.com/swift-nest/clutch.git
swift build --build-system=swiftbuild

Swift Package Manager version/commit hash

No response

Swift & OS version (output of swift --version ; uname -a)

No response

[owenv]

The test target includes an @main declaration which conflicts with the one generated by the test runner. The Package tries to workaround this by renaming the _main symbol using unsafeFlags, which fails because the duplicate attributes are detected during the compile