How to know if the installed project is compiled with CXX_MODULE support?

[ClausKlein]

/Users/clausklein/Workspace/cpp/beman-project/exemplar/examples
bash-5.3$ CXX=g++-15 cmake -G Ninja -S . -B build --fresh 
-- The CXX compiler identification is GNU 15.2.0
-- Checking whether CXX compiler has -isysroot
-- Checking whether CXX compiler has -isysroot - yes
-- Checking whether CXX compiler supports OSX deployment target flag
-- Checking whether CXX compiler supports OSX deployment target flag - yes
-- Detecting CXX compiler ABI info
-- Detecting CXX compiler ABI info - done
-- Check for working CXX compiler: /usr/local/bin/g++-15 - skipped
-- Detecting CXX compile features
CMake Warning (dev) at /usr/local/share/cmake/Modules/Compiler/CMakeCommonCompilerMacros.cmake:226 (cmake_language):
  CMake's support for `import std;` in C++23 and newer is experimental.  It
  is meant only for experimentation and feedback to CMake developers.
Call Stack (most recent call first):
  /usr/local/share/cmake/Modules/CMakeDetermineCompilerSupport.cmake:110 (cmake_cxx_find_modules_json)
  /usr/local/share/cmake/Modules/CMakeTestCXXCompiler.cmake:83 (CMAKE_DETERMINE_COMPILER_SUPPORT)
  CMakeLists.txt:7 (project)
This warning is for project developers.  Use -Wno-dev to suppress it.

-- Detecting CXX compile features - done
-- Looking for __cpp_lib_ranges
-- Looking for __cpp_lib_ranges - not found
CMake Warning at CMakeLists.txt:26 (message):
  Missing range support! Skip: identity_as_default_projection


Examples to be built: identity_direct_usage
-- Configuring done (1.2s)
-- Generating done (0.0s)
-- Build files have been written to: /Users/clausklein/Workspace/cpp/beman-project/exemplar/examples/build
bash-5.3$ cd -
/Users/clausklein/Workspace/cpp/beman-project/exemplar/examples/build
bash-5.3$ ninja clean
[1/1] Cleaning all built files...
Cleaning... 0 files.
bash-5.3$ ninja -v
[1/10] /usr/local/bin/g++-15  -isystem /Users/clausklein/.local/include -std=gnu++23 -fmodule-only -E -x c++ /Users/clausklein/.local/share/beman.exemplar/modules/include/beman/exemplar/exemplar.cppm -MT CMakeFiles/beman__exemplar@synth_1a505f9e7846.dir/6c7a5fd0604b.bmi.ddi -MD -MF CMakeFiles/beman__exemplar@synth_1a505f9e7846.dir/6c7a5fd0604b.bmi.ddi.d -fmodules-ts -fdeps-file=CMakeFiles/beman__exemplar@synth_1a505f9e7846.dir/6c7a5fd0604b.bmi.ddi -fdeps-target=CMakeFiles/beman__exemplar@synth_1a505f9e7846.dir/6c7a5fd0604b.bmi -fdeps-format=p1689r5 -o CMakeFiles/beman__exemplar@synth_1a505f9e7846.dir/6c7a5fd0604b.bmi.ddi.i
[2/10] /usr/local/bin/g++-15   -std=gnu++23 -E -x c++ /usr/local/Cellar/gcc/15.2.0_1/include/c++/15/bits/std.compat.cc -MT CMakeFiles/__cmake_cxx_std_23.dir/usr/local/Cellar/gcc/15.2.0_1/include/c++/15/bits/std.compat.cc.o.ddi -MD -MF CMakeFiles/__cmake_cxx_std_23.dir/usr/local/Cellar/gcc/15.2.0_1/include/c++/15/bits/std.compat.cc.o.ddi.d -fmodules-ts -fdeps-file=CMakeFiles/__cmake_cxx_std_23.dir/usr/local/Cellar/gcc/15.2.0_1/include/c++/15/bits/std.compat.cc.o.ddi -fdeps-target=CMakeFiles/__cmake_cxx_std_23.dir/usr/local/Cellar/gcc/15.2.0_1/include/c++/15/bits/std.compat.cc.o -fdeps-format=p1689r5 -o CMakeFiles/__cmake_cxx_std_23.dir/usr/local/Cellar/gcc/15.2.0_1/include/c++/15/bits/std.compat.cc.o.ddi.i
[3/10] /usr/local/bin/g++-15   -std=gnu++23 -E -x c++ /usr/local/Cellar/gcc/15.2.0_1/include/c++/15/bits/std.cc -MT CMakeFiles/__cmake_cxx_std_23.dir/usr/local/Cellar/gcc/15.2.0_1/include/c++/15/bits/std.cc.o.ddi -MD -MF CMakeFiles/__cmake_cxx_std_23.dir/usr/local/Cellar/gcc/15.2.0_1/include/c++/15/bits/std.cc.o.ddi.d -fmodules-ts -fdeps-file=CMakeFiles/__cmake_cxx_std_23.dir/usr/local/Cellar/gcc/15.2.0_1/include/c++/15/bits/std.cc.o.ddi -fdeps-target=CMakeFiles/__cmake_cxx_std_23.dir/usr/local/Cellar/gcc/15.2.0_1/include/c++/15/bits/std.cc.o -fdeps-format=p1689r5 -o CMakeFiles/__cmake_cxx_std_23.dir/usr/local/Cellar/gcc/15.2.0_1/include/c++/15/bits/std.cc.o.ddi.i
[4/10] /usr/local/bin/cmake -E cmake_ninja_dyndep --tdi=CMakeFiles/__cmake_cxx_std_23.dir/CXXDependInfo.json --lang=CXX --modmapfmt=gcc --dd=CMakeFiles/__cmake_cxx_std_23.dir/CXX.dd @CMakeFiles/__cmake_cxx_std_23.dir/CXX.dd.rsp
[5/10] /usr/local/bin/cmake -E cmake_ninja_dyndep --tdi=CMakeFiles/beman__exemplar@synth_1a505f9e7846.dir/CXXDependInfo.json --lang=CXX --modmapfmt=gcc --dd='CMakeFiles/beman__exemplar@synth_1a505f9e7846.dir/CXX.dd' @'CMakeFiles/beman__exemplar@synth_1a505f9e7846.dir/CXX.dd'.rsp
[6/11] /usr/local/bin/g++-15  -isystem /Users/clausklein/.local/include -std=gnu++23 -E -x c++ /Users/clausklein/Workspace/cpp/beman-project/exemplar/examples/identity_direct_usage.cpp -MT CMakeFiles/beman.exemplar.examples.identity_direct_usage.dir/identity_direct_usage.cpp.o.ddi -MD -MF CMakeFiles/beman.exemplar.examples.identity_direct_usage.dir/identity_direct_usage.cpp.o.ddi.d -fmodules-ts -fdeps-file=CMakeFiles/beman.exemplar.examples.identity_direct_usage.dir/identity_direct_usage.cpp.o.ddi -fdeps-target=CMakeFiles/beman.exemplar.examples.identity_direct_usage.dir/identity_direct_usage.cpp.o -fdeps-format=p1689r5 -o CMakeFiles/beman.exemplar.examples.identity_direct_usage.dir/identity_direct_usage.cpp.o.ddi.i
[7/11] /usr/local/bin/cmake -E cmake_ninja_dyndep --tdi=CMakeFiles/beman.exemplar.examples.identity_direct_usage.dir/CXXDependInfo.json --lang=CXX --modmapfmt=gcc --dd=CMakeFiles/beman.exemplar.examples.identity_direct_usage.dir/CXX.dd @CMakeFiles/beman.exemplar.examples.identity_direct_usage.dir/CXX.dd.rsp
[8/11] ccache /usr/local/bin/g++-15   -std=gnu++23 -MD -MT CMakeFiles/__cmake_cxx_std_23.dir/usr/local/Cellar/gcc/15.2.0_1/include/c++/15/bits/std.cc.o -MF CMakeFiles/__cmake_cxx_std_23.dir/usr/local/Cellar/gcc/15.2.0_1/include/c++/15/bits/std.cc.o.d -fmodules-ts -fmodule-mapper=CMakeFiles/__cmake_cxx_std_23.dir/usr/local/Cellar/gcc/15.2.0_1/include/c++/15/bits/std.cc.o.modmap -MD -fdeps-format=p1689r5 -x c++ -o CMakeFiles/__cmake_cxx_std_23.dir/usr/local/Cellar/gcc/15.2.0_1/include/c++/15/bits/std.cc.o -c /usr/local/Cellar/gcc/15.2.0_1/include/c++/15/bits/std.cc
[9/11] /usr/local/bin/g++-15  -isystem /Users/clausklein/.local/include -std=gnu++23 -fmodule-only -MD -MT 'CMakeFiles/beman__exemplar@synth_1a505f9e7846.dir/6c7a5fd0604b.bmi' -MF CMakeFiles/beman__exemplar@synth_1a505f9e7846.dir/6c7a5fd0604b.bmi.d -fmodules-ts -fmodule-mapper=CMakeFiles/beman__exemplar@synth_1a505f9e7846.dir/6c7a5fd0604b.bmi.modmap -MD -fdeps-format=p1689r5 -x c++ -o 'CMakeFiles/beman__exemplar@synth_1a505f9e7846.dir/6c7a5fd0604b.bmi' -c /Users/clausklein/.local/share/beman.exemplar/modules/include/beman/exemplar/exemplar.cppm
[10/11] ccache /usr/local/bin/g++-15  -isystem /Users/clausklein/.local/include -std=gnu++23 -MD -MT CMakeFiles/beman.exemplar.examples.identity_direct_usage.dir/identity_direct_usage.cpp.o -MF CMakeFiles/beman.exemplar.examples.identity_direct_usage.dir/identity_direct_usage.cpp.o.d -fmodules-ts -fmodule-mapper=CMakeFiles/beman.exemplar.examples.identity_direct_usage.dir/identity_direct_usage.cpp.o.modmap -MD -fdeps-format=p1689r5 -x c++ -o CMakeFiles/beman.exemplar.examples.identity_direct_usage.dir/identity_direct_usage.cpp.o -c /Users/clausklein/Workspace/cpp/beman-project/exemplar/examples/identity_direct_usage.cpp
FAILED: [code=1] CMakeFiles/beman.exemplar.examples.identity_direct_usage.dir/identity_direct_usage.cpp.o 
ccache /usr/local/bin/g++-15  -isystem /Users/clausklein/.local/include -std=gnu++23 -MD -MT CMakeFiles/beman.exemplar.examples.identity_direct_usage.dir/identity_direct_usage.cpp.o -MF CMakeFiles/beman.exemplar.examples.identity_direct_usage.dir/identity_direct_usage.cpp.o.d -fmodules-ts -fmodule-mapper=CMakeFiles/beman.exemplar.examples.identity_direct_usage.dir/identity_direct_usage.cpp.o.modmap -MD -fdeps-format=p1689r5 -x c++ -o CMakeFiles/beman.exemplar.examples.identity_direct_usage.dir/identity_direct_usage.cpp.o -c /Users/clausklein/Workspace/cpp/beman-project/exemplar/examples/identity_direct_usage.cpp
/Users/clausklein/Workspace/cpp/beman-project/exemplar/examples/identity_direct_usage.cpp:7:1: fatal error: unknown compiled module interface: no such module
    7 | import std;
      | ^~~~~~
compilation terminated.
ninja: build stopped: subcommand failed.
bash-5.3$ 

[ClausKlein]

diff --git a/examples/CMakeLists.txt b/examples/CMakeLists.txt
index ed8ed52..763d28c 100644
--- a/examples/CMakeLists.txt
+++ b/examples/CMakeLists.txt
@@ -1,5 +1,19 @@
 # SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 
+cmake_minimum_required(VERSION 3.30...4.3)
+
+include(../infra/cmake/enable-experimental-import-std.cmake)
+
+project(
+    beman.exemplar
+    LANGUAGES CXX
+    VERSION 2.4.1
+)
+
+if(PROJECT_IS_TOP_LEVEL)
+    find_package(beman.exemplar 2.4.1 EXACT REQUIRED)
+endif()
+
 set(ALL_EXAMPLES identity_direct_usage)
 
 # Example `identity_as_default_projection` need ranges support:

[ClausKlein]

@ednolan I would expect that BEMAN_EXEMPLAR_USE_MODULES is set from the imported package?

[ednolan]

I would expect that BEMAN_EXEMPLAR_USE_MODULES is set from the imported package?

No— build-time cache variables are not leaked into consumers by the exported package config. Closing as not-a-defect.

[ednolan]

Actually, reopening. The code that you provided isn't a supported use case, but there does need to be a CMake-level mechanism to answer the original question, "How to know if the installed project is compiled with CXX_MODULE support?"

[ednolan]

I'm looking into adding the feature in the following way:

# in Config.cmake.in, after the targets include
set_target_properties(
    beman::exemplar PROPERTIES
    BEMAN_EXEMPLAR_HAS_MODULES @BEMAN_EXEMPLAR_USE_MODULES@
)

Then consumers can query the property.

I'll put up a pull request later.

[ClausKlein]

I would expect that BEMAN_EXEMPLAR_USE_MODULES is set from the imported package?

No— build-time cache variables are not leaked into consumers by the exported package config. Closing as not-a-defect.

Note: A target property is already set:
set_target_properties(beman.exemplar PROPERTIES CXX_MODULE_STD ON)

[ClausKlein]

@ednolan there are more possible config mismatch problems:

CXX=g++-15 cmake -G Ninja -S . -B build --fresh -D CMAKE_CXX_STANDARD=26 -D BEMAN_EXEMPLAR_USE_MODULES=1
# ...
[15/21] /usr/local/bin/g++-15  -isystem /Users/clausklein/.local/include -std=gnu++23 -fmodule-only -MD -MT 'CMakeFiles/beman__exemplar@synth_1a505f9e7846.dir/6c7a5fd0604b.bmi' -MF CMakeFiles/beman__exemplar@synth_1a505f9e7846.dir/6c7a5fd0604b.bmi.d -fmodules-ts -fmodule-mapper=CMakeFiles/beman__exemplar@synth_1a505f9e7846.dir/6c7a5fd0604b.bmi.modmap -MD -fdeps-format=p1689r5 -x c++ -o 'CMakeFiles/beman__exemplar@synth_1a505f9e7846.dir/6c7a5fd0604b.bmi' -c /Users/clausklein/.local/share/beman.exemplar/modules/include/beman/exemplar/exemplar.cppm
FAILED: [code=1] CMakeFiles/beman__exemplar@synth_1a505f9e7846.dir/6c7a5fd0604b.bmi 
/usr/local/bin/g++-15  -isystem /Users/clausklein/.local/include -std=gnu++23 -fmodule-only -MD -MT 'CMakeFiles/beman__exemplar@synth_1a505f9e7846.dir/6c7a5fd0604b.bmi' -MF CMakeFiles/beman__exemplar@synth_1a505f9e7846.dir/6c7a5fd0604b.bmi.d -fmodules-ts -fmodule-mapper=CMakeFiles/beman__exemplar@synth_1a505f9e7846.dir/6c7a5fd0604b.bmi.modmap -MD -fdeps-format=p1689r5 -x c++ -o 'CMakeFiles/beman__exemplar@synth_1a505f9e7846.dir/6c7a5fd0604b.bmi' -c /Users/clausklein/.local/share/beman.exemplar/modules/include/beman/exemplar/exemplar.cppm
In module imported at /Users/clausklein/.local/share/beman.exemplar/modules/include/beman/exemplar/exemplar.cppm:3:1:
std: error: language dialect differs 'C++26', expected 'C++23'
std: error: failed to read compiled module: Bad file data
std: note: compiled module file is 'CMakeFiles/__cmake_cxx_std_23.dir/std.gcm'
std: fatal error: returning to the gate for a mechanical issue
compilation terminated.
[16/21] ccache /usr/local/bin/g++-15   -std=gnu++26 -MD -MT CMakeFiles/__cmake_cxx_std_26.dir/usr/local/Cellar/gcc/15.2.0_1/include/c++/15/bits/std.compat.cc.o -MF CMakeFiles/__cmake_cxx_std_26.dir/usr/local/Cellar/gcc/15.2.0_1/include/c++/15/bits/std.compat.cc.o.d -fmodules-ts -fmodule-mapper=CMakeFiles/__cmake_cxx_std_26.dir/usr/local/Cellar/gcc/15.2.0_1/include/c++/15/bits/std.compat.cc.o.modmap -MD -fdeps-format=p1689r5 -x c++ -o CMakeFiles/__cmake_cxx_std_26.dir/usr/local/Cellar/gcc/15.2.0_1/include/c++/15/bits/std.compat.cc.o -c /usr/local/Cellar/gcc/15.2.0_1/include/c++/15/bits/std.compat.cc
ninja: build stopped: subcommand failed.
bash-5.3$ 

[ednolan]

Thanks for pointing that out, I'll look into that as well.

[ClausKlein]

There is an additional problem it the project is compiled and installed with C++26:

bash-5.3$ make examples 
cmake -S examples -B examples/build -G Ninja \
	 -D BEMAN_USE_MODULES=YES \
	 -D BEMAN_USE_STD_MODULE=YES \
	 -D BEMAN_EXEMPLAR_USE_MODULES=YES \
	 -D CMAKE_CXX_STDLIB_MODULES_JSON=/usr/local/Cellar/llvm/22.1.8/lib/c++/libc++.modules.json \
	 -D CMAKE_BUILD_TYPE=Release \
	 -D CMAKE_CXX_STANDARD=26 -D CMAKE_CXX_EXTENSIONS=NO -D CMAKE_CXX_STANDARD_REQUIRED=YES \
	 --log-level=VERBOSE --fresh
-- The CXX compiler identification is Clang 22.1.8
-- Detecting CXX compiler ABI info
-- Detecting CXX compiler ABI info - done
-- Check for working CXX compiler: /usr/local/Cellar/llvm/22.1.8/bin/clang++ - skipped
-- Detecting CXX compile features
CMake Warning (experimental) at /Users/clausklein/.local/share/cmake-4.4/Modules/Compiler/CMakeCommonCompilerMacros.cmake:226 (cmake_language):
  CMake's support for `import std;` in C++23 and newer is experimental.  It
  is meant only for experimentation and feedback to CMake developers.
Call Stack (most recent call first):
  /Users/clausklein/.local/share/cmake-4.4/Modules/CMakeDetermineCompilerSupport.cmake:157 (cmake_cxx_find_modules_json)
  /Users/clausklein/.local/share/cmake-4.4/Modules/CMakeTestCXXCompiler.cmake:81 (CMAKE_DETERMINE_COMPILER_SUPPORT)
  CMakeLists.txt:7 (project)
This warning is for project developers.  Use -Wno-author or -Wno-experimental
to suppress it.

-- Detecting CXX compile features - done
-- Looking for __cpp_lib_ranges
-- Looking for __cpp_lib_ranges - found
Examples to be built: identity_direct_usage;identity_as_default_projection
-- Configuring done (2.6s)
-- Generating done (0.1s)
CMake Warning (unused-cli):
  Manually-specified variables were not used by the project:

    BEMAN_USE_MODULES
    BEMAN_USE_STD_MODULE


-- Build files have been written to: /Users/clausklein/Workspace/cpp/beman-project/exemplar/examples/build
ninja -C examples/build
ninja: Entering directory `examples/build'
[13/25] Scanning /Users/clausklein/.local/share/beman.exemplar/modules/include/beman/exemplar/exemplar.cppm for CXX dependencies
Diagnostics while scanning dependencies for '/Users/clausklein/.local/share/beman.exemplar/modules/include/beman/exemplar/exemplar.cppm':
/Users/clausklein/.local/share/beman.exemplar/modules/include/beman/exemplar/exemplar.cppm:9:10: warning: '#include <filename>' attaches the declarations to the named module 'beman.exemplar', which is not usually intended; consider moving that directive before the module declaration
[19/28] Building CXX object CMakeFiles/__cmake_cxx_std_26.dir/usr/local/Cellar/llvm/22.1.8/share/libc++/v1/std.cppm.o
/usr/local/Cellar/llvm/22.1.8/share/libc++/v1/std.cppm:167:15: warning: 'std' is a reserved name for a module [-Wreserved-module-identifier]
  167 | export module std;
      |               ^
1 warning generated.
[20/28] Building CXX object CMakeFiles/__cmake_cxx_std_26.dir/usr/local/Cellar/llvm/22.1.8/share/libc++/v1/std.compat.cppm.o
/usr/local/Cellar/llvm/22.1.8/share/libc++/v1/std.compat.cppm:83:15: warning: 'std' is a reserved name for a module [-Wreserved-module-identifier]
   83 | export module std.compat;
      |               ^
1 warning generated.
[22/28] Building CXX object 'CMakeFiles/__cmake_cxx_std_26@synth_0.dir/5ffe088cfc4e.bmi'
/usr/local/Cellar/llvm/22.1.8/share/libc++/v1/std.cppm:167:15: warning: 'std' is a reserved name for a module [-Wreserved-module-identifier]
  167 | export module std;
      |               ^
1 warning generated.
[23/28] Building CXX object 'CMakeFiles/__cmake_cxx_std_23@synth_0.dir/5ffe088cfc4e.bmi'
/usr/local/Cellar/llvm/22.1.8/share/libc++/v1/std.cppm:167:15: warning: 'std' is a reserved name for a module [-Wreserved-module-identifier]
  167 | export module std;
      |               ^
1 warning generated.
[24/28] Building CXX object 'CMakeFiles/beman__exemplar@synth_0.dir/5f328837b835.bmi'
FAILED: [code=1] CMakeFiles/beman__exemplar@synth_0.dir/5f328837b835.bmi 
/usr/local/Cellar/llvm/22.1.8/bin/clang++  -isystem /Users/clausklein/.local/include -stdlib=libc++ -O3 -DNDEBUG -std=c++23 -MD -MT 'CMakeFiles/beman__exemplar@synth_0.dir/5f328837b835.bmi' -MF CMakeFiles/beman__exemplar@synth_0.dir/5f328837b835.bmi.d @CMakeFiles/beman__exemplar@synth_0.dir/5f328837b835.bmi.modmap -o 'CMakeFiles/beman__exemplar@synth_0.dir/5f328837b835.bmi' --precompile /Users/clausklein/.local/share/beman.exemplar/modules/include/beman/exemplar/exemplar.cppm
error: C++26 was enabled in precompiled file 'CMakeFiles/__cmake_cxx_std_23@synth_0.dir/5ffe088cfc4e.bmi' but is currently disabled
error: precompiled file 'CMakeFiles/__cmake_cxx_std_23@synth_0.dir/5ffe088cfc4e.bmi' cannot be loaded due to a configuration mismatch with the current compilation [-Wmodule-file-config-mismatch]
/Users/clausklein/.local/share/beman.exemplar/modules/include/beman/exemplar/exemplar.cppm:7:2: error: unknown type name 'pragma'
    7 | #pragma clang diagnostic push
      |  ^
/Users/clausklein/.local/share/beman.exemplar/modules/include/beman/exemplar/exemplar.cppm:8:8: error: expected ';' after top level declarator
    8 | #pragma clang diagnostic ignored "-Winclude-angled-in-module-purview"
      |        ^
      |        ;
In file included from /Users/clausklein/.local/share/beman.exemplar/modules/include/beman/exemplar/exemplar.cppm:9:
In file included from /Users/clausklein/.local/include/beman/exemplar/exemplar.hpp:14:
/Users/clausklein/.local/include/beman/exemplar/identity.hpp:44:16: error: use of undeclared identifier 'std'
   44 |         return std::forward<T>(t);
      |                ^
/Users/clausklein/.local/include/beman/exemplar/identity.hpp:44:29: error: 'T' does not refer to a value
   44 |         return std::forward<T>(t);
      |                             ^
/Users/clausklein/.local/include/beman/exemplar/identity.hpp:42:21: note: declared here
   42 |     template <class T>
      |                     ^
/Users/clausklein/.local/include/beman/exemplar/identity.hpp:52:2: error: unknown type name 'endif'
   52 | #endif // BEMAN_EXEMPLAR_USE_MODULES() &&
      |  ^
/Users/clausklein/.local/include/beman/exemplar/identity.hpp:55:7: error: expected ';' after top level declarator
   55 | #endif // BEMAN_EXEMPLAR_IDENTITY_HPP
      |       ^
8 errors generated.
ninja: build stopped: subcommand failed.
make: *** [examples] Error 1
bash-5.3$ 

FYI: same problem has boost: boostorg/any#42