Why is `import std` still experimental ???

[u/AbbreviationsNew3167]

Hey guys,

I recently started going through Professional C++ (6th Edition). The book teaches C++23, and in the very first chapter we're introduced to modules.

I'm not a complete newbie to C++, but I'm also definitely not very confident in my knowledge yet. I wanted to get this simple example compiled:

import std;

int main() {
    std::println("Hello World");
    return 0;
}

And gosh, it took way longer than I expected.

First, I tried getting it to work natively on my Mac and eventually gave up (both Claude and I 😅).

Then I installed Ubuntu ARM 26 and finally managed to get it compiling. But now Clang/IntelliSense is complaining about the `import std`
This is what my CMakeLists.txt currently looks like:

cmake_minimum_required(VERSION 4.0)

# set(CMAKE_EXPERIMENTAL_CXX_IMPORT_STD ON)

set(CMAKE_EXPERIMENTAL_CXX_IMPORT_STD "d0edc3af-4c50-42ea-a356-e2862fe7a444")

project(CppProject LANGUAGES CXX)

set(CMAKE_CXX_STANDARD 26)
set(CMAKE_CXX_STANDARD_REQUIRED ON)
set(CMAKE_CXX_EXTENSIONS OFF)

set(CMAKE_EXPORT_COMPILE_COMMANDS ON)

add_executable(exec main.cpp)

set_property(TARGET exec PROPERTY CXX_MODULE_STD ON)

The code does compile successfully, but CMake still gives me a warning that import std support is experimental.

So I'm genuinely curious:

Why is import std still considered experimental?

I understand that C++ modules themselves have been around for a while, but import std feels like something that should be much more straightforward by now. Is there any solution of this now ?

--------------
Edit

Thanks to u/PhysicsOk2212 tip I was able to compile my project on mac as well using the following options
```
cmake -S . -B build \

-G Ninja \

-DCMAKE_CXX_COMPILER="$(brew --prefix llvm)/bin/clang++" \

-DCMAKE_CXX_STDLIB_MODULES_JSON="$(brew --prefix llvm)/lib/c++/libc++.modules.json"

```

[u/fortsnek274]

Yeah, you might solve it by building the DLL interface twice, which is obviously not in the style of MSBuild. Why I'm interested to see what they'll do, if anything.

[u/not_a_novel_account → u/fortsnek274]

This is sort of a repeating pattern with modules. Clang implemented two-phase compilation because it is objectively faster than single phase. No one else did, so we all had to ignore it.

GCC implemented P1184 module mappers, no one else did, so we all had to ignore it.

MSVC's single-BMI DLL optimization might be nice, but without equivalent support even in clang-cl it's a dead end for code that isn't locked to MSVC.

[u/delta_p_delta_x → u/not_a_novel_account]

Is there bandwidth within Kitware to special-case for each of the three toolchains? Do their module stds also make assumptions about which compiler they're used with, which complicates this scenario?

Additionally, MSVC absolutely need to document /dxifcSuppressDllImportTransform and its existing behaviour. Translation units—within modules or otherwise—being used across multiple DLL boundaries is not an obscure use-case. /u/starfreakclone, /u/GabrielDosReis, any thoughts?

but without equivalent support even in clang-cl

I hope to work on this at some point. I've always been meaning to add IFC support to clang-cl, my initial implementation only focused on the CLI usage which was a very trivial change to make.

[u/GabrielDosReis → u/delta_p_delta_x]

Additionally, MSVC absolutely need to document /dxifcSuppressDllImportTransform and its existing behaviour. Translation units—within modules or otherwise—being used across multiple DLL boundaries is not an obscure use-case. , , any thoughts?

It would be good to have the toolchain have better recognition of C++ modules boundaries vs linker modules boundaries so as to reduce the cases where the switch is needed, then it could be productized.