Change in module behavior for clang in GitHub

Question

I've inherited a GitHub job I don't understand well and that has stopped compiling.

It creates a .a library for iOS.

The source consists of a set of modules, each declared thus:

export module x;  
import y;  
using namespace nam_y;  
namespace nam_x  
{  
   export void x() {  
     do something  
   }  
}

compile.sh handles each module as below. This used to work, but now -fmodules-ts produces an error. Removing it produces many errors. I assume clang has been updated sometime this year? What syntax should I be using now?

This is the new error with fmodules-ts:

clang++: error: unknown argument: '-fmodules-ts'
$CXX -std=c++20 -fmodules-ts \
    -target "$TARGET" \
    -isysroot "$SDK_PATH" \
    -fprebuilt-module-path="$BUILD_DIR" \
    -x c++-module --precompile src/x.ixx -o build/x.pcm

Have you tried dropping the flag that Clang doesn't recognize? C++20 supports modules.. fmodules-ts provides for an experimental version of module support... I doubt it is supported for C++20. — Questor
– Questor, Commented Nov 19 at 0:55

Questor · Accepted Answer · 2025-11-19 00:56:53Z

0

Delete the unrecognized flag ( e.g., -fmodules-ts )... C++20 provides support for modules natively. You don't need that flag if compiling with std=C++20, and its liekley that selecting C++20 means that -fmodules-ts is no longer a supported flag.

answered Nov 19 at 0:56

Questor

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3 Comments

Imer Satz Nov 19 at 1:27

The compile script I posted above worked perfectly until recently, using both c++20 and -fmodules flags. Dropping the flag produces these errors for each module: src/x.ixx:1:8: error: expected template 1 | export module x; | ^ src/x.ixx:1:8: error: unknown type name 'module' src/x.ixx:2:8: error: expected template 2 | export namespace nam_x | ^

Questor Nov 19 at 2:14

What version of clang are you using? you can find it by clang --version

Imer Satz Nov 19 at 3:06

Let's not underestimate my ignorance: Am I correct that if I am using a GitHub runner to build this ios library, that it is Xcode's clang that is being used? I'm running a Windows PC and pushing everything to GitHub. The macOS is reported as 15.7.1 which appears to use clang 17. Do I have control over any of this?

Imer Satz · Accepted Answer · 2025-11-19 20:16:12Z

0

If I specify


    runs-on: macos-14

in the workflow, I'm good if I continue to use the -fmodules-ts flag. Dunno when latest became 15 nor why that broke my code.

answered 2 days ago

Imer Satz

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