Can't use output: .string with body closure

[krzyzanowskim]

Describe the bug
Can't compile run(executable, output: .string, body: body)

To Reproduce

/// Global function 'run(_:arguments:environment:workingDirectory:platformOptions:output:isolation:body:)' requires the types 'BytesOutput.OutputType' (aka 'Array<UInt8>') and '()' be equivalent
let r3 = try await Subprocess.run(.path("/bin/true"), output: .bytes) { execution, input, output in
    [1] as [UInt8]
}

/// Global function 'run(_:arguments:environment:workingDirectory:platformOptions:output:isolation:body:)' requires the types 'StringOutput<Unicode.UTF8>.OutputType' (aka 'Optional<String>') and '()' be equivalent
let r4 = try await Subprocess.run(.path("/bin/true"), output: .string) { execution, input, output in
    "" as String?
}

Expected behavior
A clear and concise description of what you expected to happen.

Environment (please complete the following information):

• macOS 15
• swift-driver version: 1.120.5 Apple Swift version 6.1.2 (swiftlang-6.1.2.1.2 clang-1700.0.13.5)

Additional context
Add any other context about the problem here.

[iCharlesHu]

Hi @krzyzanowskim! This behavior is intentional. Conceptually, Subprocess treats streaming output through AsyncSequence as a form of output, so you can’t define an output type while also streaming it. If you’re looking to stream the output as String, consider trying the new LineSequence API, which allows you to stream the child process’s output line by line as String.

[iCharlesHu]

Closing because this is the expected behavior. Feel free to reopen if you wish to discuss API changes.