Cannot compile C++ with clang++ when swiftly is installed

[sbehnke]

I get the following error compiling a C++ test file with swiftly installed:

❯ clang++ -std=c++20 test.cpp
test.cpp:1:10: fatal error: 'string' file not found
    1 | #include <string>
      |          ^~~~~~~~
1 error generated.

test.cpp

#include <string>

int main() {

}

The C++ standard library header files should be available even with swiftly installed.

This version of clang++ cannot find the C++ standard library.

❯ which clang++
/Users/sbehnke/.swiftly/bin/clang++

This works as expected.

❯ /usr/bin/clang++ -std=c++20 test.cpp

[cmcgee1024]

Thanks for raising this. I believe that others have seen this kind of problem in the past and it turned out to be related to the toolchain, and not swiftly.

Can you try doing this to attempt to rule swiftly out?

$(swiftly use --print-location)/usr/bin/clang++ -std=c++20 test.cpp

Note that /usr/bin/clang++ could be a different toolchain version than what swiftly has installed itself and is using. On macOS it is likely to be the Xcode toolchain, and not the open source one that is downloaded from swift.org. On Linux, it could be a clang++ that is from another non-swift offering.

[sbehnke]

I agree that it is the toolchain and not necessarily swiftly, but I'm not sure where else to report this. Is it possible to disable the swift 6.1 toolchain from swiftly when I'm not writing code in swift?

❯ $(swiftly use --print-location)/usr/bin/clang++ -std=c++20 test.cpp
test.cpp:1:10: fatal error: 'string' file not found
    1 | #include <string>
      |          ^~~~~~~~
1 error generated.
❯ swiftly use --print-location
/Users/sbehnke/Library/Developer/Toolchains/swift-6.1-RELEASE.xctoolchain

Forgot to mention that I'm on macOS 15.4.

[cmcgee1024]

For libc++, the open source macOS toolchain will need to have the SDKROOT environment variable set. On my system, without Xcode installed, but with the CommandLineTools installed, my SDKROOT can be set to this:

export SDKROOT="/Library/Developer/CommandLineTools/SDKs/MacOSX15.3.sdk"

If you set this environment variable and try again (adjusting for your own command-line tools location), does it work?

[sbehnke]

For libc++, the open source macOS toolchain will need to have the SDKROOT environment variable set. On my system, without Xcode installed, but with the CommandLineTools installed, my SDKROOT can be set to this:

export SDKROOT="/Library/Developer/CommandLineTools/SDKs/MacOSX15.3.sdk"

If you set this environment variable and try again (adjusting for your own command-line tools location), does it work?

It does work, but I'm going to add:

export SDKROOT=$(xcrun --sdk macosx --show-sdk-path)

to my .zshrc. Thank you!

[cmcgee1024]

Thanks @sbehnke for testing this out.

I think that the fix here is to try to set SDKROOT environment if it's not already set by the user, and set it to what xcrun --show-sdk-path reports on macOS only. SwiftPM has its own mechanisms to manage the SDK's. It's the other drivers such as clang and clang++ that don't have that information.

[sbehnke]

Sounds good to me, thanks for the investigation.