cmake ignores CMAKE_EXPERIMENTAL_CXX_IMPORT_STD flag regardless of when it is called

Question

I have a very simple script:

cmake_minimum_required(VERSION 4.0 FATAL_ERROR)
set(CMAKE_EXPERIMENTAL_CXX_IMPORT_STD "d0edc3af-4c50-42ea-a356-e2862fe7a444")
set(CMAKE_CXX_MODULE_STD 1)
project(import_std LANGUAGES CXX)
add_executable(main)
target_sources(main PRIVATE main.cpp)
target_compile_features(main PRIVATE cxx_std_23 INTERFACE cxx_std_23)

But cmake seems to never actually enable the flag:

Experimental `import std` support not enabled when detecting toolchain; it must be set before `CXX` is enabled (usually a `project()` call)

Nothing really seems to let me get around this.

For context I'm using clang for compilation and lld for linking.

set(CMAKE_EXPERIMENTAL_CXX_IMPORT_STD "d0edc3af-4c50-42ea-a356-e2862fe7a444") - and what exactly does that UUID refer to? We can't just guess that.. — Jesper Juhl
– Jesper Juhl, Commented May 18 at 17:06
@JesperJuhl the UUID is from the cmake github repository, experimental features are turned on with a UUID that changes every version, you can find it here github.com/Kitware/CMake/blob/master/Help/dev/experimental.rst — cose
– cose, Commented May 18 at 20:12
The exact value of UUID for experimenting features depends on the version of cmake executable. Do you actually use CMake built from the sources? E.g if you use CMake of version 4.0.0, then UUID differs: github.com/Kitware/CMake/blob/v4.0.0/Help/dev/… — Tsyvarev
– Tsyvarev, Commented May 18 at 20:59

cose · Accepted Answer · 2025-05-20 21:26:47Z

0

As Tsyvarev pointed out, i had simply used the wrong UUID, the UUID at main is not the same as the UUID of the latest release, i simply had to look in the v4.0.2 tag.

answered May 20 at 21:26

cose

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