--std=c++20 -fno-modules still sets __has_feature(modules) to 1

[pkasting]

The following code should compile without error with --std=c++20 -fno-modules:

#if __has_feature(modules)
#error Module support found!
#endif

Actual result: error message.

[llvmbot]

@llvm/issue-subscribers-clang-frontend

[ChuanqiXu9]

The -fmodules and -fno-modules are prepared for clang modules extension instead of the standard c++ modules. This is documented in the preparing document: https://reviews.llvm.org/D131388

[ChuanqiXu9]

And it looks true currently we lack the ability to disable the standard c++ modules. May I ask the intention?

[llvmbot]

@llvm/issue-subscribers-c-20

[llvmbot]

@llvm/issue-subscribers-clang-modules

[sdefresne]

This is in the context of mixing Objective-C++ and Swift.

The Swift compiler generates code that looks like this:

#if __has_feature(modules)
@import Foundation;
#endif

We are currently building a code base that is C++17 and uses some Swift and Objective-C++ code. Our code base does not yet support building with clang modules support enabled. When trying to build with -std=c++20, this @import line is reached and causes compilation failure (since we do not pass -fmodules and passing it is not enough).

[ChuanqiXu9]

Oh, I understood it roughly. Currently both -std=c++20 and -fmodules will enable modules feature too. And it looks like we need to rename them or find a way to disable the cplusplusmodules specially.

[zmodem]

It looks like https://reviews.llvm.org/D106864 addressed the command-line flag problem?

[ChuanqiXu9]

Yeah, it looks true. Although it requires -Xclang -fno-cxx-modules. @pkasting could you give a try?

[rnk]

In the long run, we would like to avoid using Xclang flags in the Chromium build. Is there a plan to make this a driver flag?

[ChuanqiXu9]

Yeah, there is an issue tracking this: #55891. Although we don't have any concrete proposal yet...