avoid conformance error surfacing when SwiftUI Preview builds tests

[fabstu]

conflicting conformance of 'Optional<Wrapped>' to protocol 'CustomStringConvertible'; there cannot be more than one conformance, even with different conditional bounds

----------------------------------------

Failed to build the scheme "swiftgrpc": Compile /Volumes/DerivedData-ramdisk/DerivedData/aduu-swift-ccwvbioiqcuclleyjznkpzajipeb/SourcePackages/checkouts/grpc-swift/Sources/GRPCInteroperabilityTestsCLI/main.swift:
/Volumes/DerivedData-ramdisk/DerivedData/aduu-swift-ccwvbioiqcuclleyjznkpzajipeb/SourcePackages/checkouts/grpc-swift/Sources/GRPCInteroperabilityTestsCLI/main.swift:87:21: error: conflicting conformance of 'Optional<Wrapped>' to protocol 'CustomStringConvertible'; there cannot be more than one conformance, even with different conditional bounds
extension Optional: CustomStringConvertible where Wrapped: ArgumentConvertible {
                    ^
Commander.Optional<τ_0_0>:1:11: note: 'Optional<Wrapped>' declares conformance to protocol 'CustomStringConvertible' here
extension Optional : CustomStringConvertible where Wrapped : CustomStringConvertible {
          ^
/Volumes/DerivedData-ramdisk/DerivedData/aduu-swift-ccwvbioiqcuclleyjznkpzajipeb/SourcePackages/checkouts/grpc-swift/Sources/GRPCInteroperabilityTestsCLI/main.swift:96:21: warning: conformance of 'Optional<Wrapped>' to protocol 'ArgumentConvertible' was already stated in the protocol's module 'Commander'
extension Optional: ArgumentConvertible where Wrapped: ArgumentConvertible {
                    ^
/Volumes/DerivedData-ramdisk/DerivedData/aduu-swift-ccwvbioiqcuclleyjznkpzajipeb/SourcePackages/checkouts/grpc-swift/Sources/GRPCInteroperabilityTestsCLI/main.swift:97:10: note: initializer 'init(parser:)' will not be used to satisfy the conformance to 'ArgumentConvertible'
  public init(parser: ArgumentParser) throws {
         ^
Commander.Optional<τ_0_0>:1:11: note: 'Optional<Wrapped>' declares conformance to protocol 'ArgumentConvertible' here
extension Optional : Commander.ArgumentConvertible where Wrapped : Commander.ArgumentConvertible {
          ^

This conformance error comes up when using SwiftUI previews, clicking Continue/Retry to show the Preview. Removing the offending conformance makes the Preview work correctly.

I don't know whether more is required for this though.

This forum post suggests to use use wrapper types or some such to avoid this kind of issue. I don't know whether anyone would want to do that.

[MrMage]

@glbrntt

[glbrntt]

This is coming from the CLI for the interoperability test runner which you shouldn't need here; I'm not sure why it gets built.

As it turns out we no longer use optional CLI options so we can just delete the conformance entirely.

[glbrntt]

Can you delete this whole section from // MARK: - Optional extensions for Commander up to (but not including) // MARK: - Command line options and "main".?

[fabstu]

@glbrntt I removed the second section too, it's building now correctly in SwiftUI Preview, in swift build and with Xcode11.
With Xcode11 some tests were crashing before already, but the linux-tests now do too. Can I make it buildable without seeing the linux-tests fail, or would that be too much work / is this even related.

[glbrntt]

Thanks @fabstu!

The failing test on Linux are unrelated to this change; looks like flakey tests.