ROOT Cling cannot run with libfmt

[YanzhaoW]

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Description

Hi,

I have encountered a compilation issue when using libfmt inside ROOT Cling interpreter.

Reproducer

1. Open the root:

root -l

2. include libfmt:

#include <fmt/format.h>

3. print hello world:

fmt::println("hello world");

The error message would be:

ROOT_prompt_3:1:14: error: call to consteval function 'fmt::fstring<>::fstring<12UL>' is not a constant expression
fmt::println("hello world");
             ^
note: cast that performs the conversions of a reinterpret_cast is not allowed in a constant expression
/opt/FairSoft/include/fmt/base.h:2736:54: note: in call to 'basic_string_view(&"hello world"[0])'
    if (FMT_USE_CONSTEVAL) parse_format_string<char>(s, checker(s, arg_pack()));
                                                     ^
ROOT_prompt_3:1:14: note: in call to 'fstring<12UL>("hello world")'
fmt::println("hello world");
             ^~~~~~~~~~~~~

ROOT version

6.36.00

Installation method

build from source

Operating system

Fedora 42

Additional context

libfmt version: 11.2.0
C++ standard: 23

[dpiparo]

Hi, thanks for the report.
The file seems to at this path /opt/FairSoft/include/fmt/base.h: could you please confirm that this is reproducible just with libfmt without any other fair component or patch on top of it?

[ferdymercury]

Maybe compiling FMT with FMT_USE_CONSTEVAL set to false or undefined helps?

[YanzhaoW]

@dpiparo

Thanks very much for your reply. I have installed libfmt with conan and still get the same error message:

ROOT_prompt_3:1:14: error: call to consteval function 'fmt::fstring<>::fstring<12UL>' is not a constant expression
fmt::println("hello world")
             ^
note: cast that performs the conversions of a reinterpret_cast is not allowed in a constant expression
/u/yanwang/.conan2/p/b/fmtd8601e6334d91/p/include/fmt/base.h:2736:54: note: in call to 'basic_string_view(&"hello world"[0])'
    if (FMT_USE_CONSTEVAL) parse_format_string<char>(s, checker(s, arg_pack()));
                                                     ^
ROOT_prompt_3:1:14: note: in call to 'fstring<12UL>("hello world")'
fmt::println("hello world")

To let the ROOT find the libfmt package, I did with this:

root [0] gInterpreter->AddIncludePath("-I/u/yanwang/.conan2/p/b/fmtd8601e6334d91/p/include")
root [1] #include <fmt/format.h>
root [2] gSystem->Load("/u/yanwang/.conan2/p/b/fmtd8601e6334d91/p/lib/libfmt.so.11.2.0")

[YanzhaoW]

Hi, @ferdymercury

Thanks for your comment. I don't think FMT_USE_CONSTEVAL is configurable from the user side. It seems it's automatically on with C++23 (or even 20).

[ferdymercury]

As workaround: Can you build fmt specifying maybe CmakeCxxStandard 17?

[ferdymercury]

Or rather try this solution fmtlib/fmt#2438

[YanzhaoW]

Ok, yeah, you're right, fmt::runtime("hello world") turns off this compile time check and it works properly with root. But It would be much better if the compile-time checking also works.

[YanzhaoW]

I dug a little bit deeper and found something really strange.

If I run the Cling with the Github action in the exact same docker image (yanzhaowang/r3bdev:fedora-latest) that I run locally, everything works really well. I got the compile time check from libfmt.

The log can be found here: https://github.com/YanzhaoW/test_repo/actions/runs/16224998731/job/45814746429

But if I pull the docker image and run a container locally, doing exactly the same thing, running the exactly the same script, I got the error from ROOT Cling. I have never known such an issue could occur even within the containment of Docker.

If you would like to reproduce the error, you could try with

docker pull yanzhaowang/r3bdev:fedora-latest
docker run -it --name test_fmt yanzhaowang/r3bdev:fedora-latest "/bin/bash"

then in the docker, write the following in the script text.C:

#include <fmt/format.h>
void test() { 
    fmt::println("hello world");
}

and run the root:

${SIMPATH}/bin/root -l test.C

[YanzhaoW]

Ok, I also compiled ROOT and libfmt with C++23 in a clean system. It does work perfectly. So I close this issue as the problem is somewhere else.

[dpiparo]

Thanks both for the follow up.

[github-actions]

Hi @YanzhaoW, @dpiparo,

It appears this issue is closed, but wasn't yet added to a project. Please add upcoming versions that will include the fix, or 'not applicable' otherwise.

Sincerely,
🤖