Generator expression not evaluated

Usage
1

Is this supposed to work:

cmake_minimum_required(VERSION 3.25)
 
project(TEST)
 
add_executable(test_ext dummy.cpp)
add_library(lib_a STATIC dummy.cpp)
add_library(lib_b STATIC dummy.cpp)
 
target_link_libraries(lib_a
    PUBLIC
        $<1:lib_b>
)
 
target_link_libraries(test_ext
    PUBLIC
        $<TARGET_PROPERTY:lib_a,LINK_LIBRARIES>
)

Running commands:

touch dummy.cpp
cmake -S . -B out -G Ninja
cmake --build out

Fails with:

ninja: error: build.ninja:63: bad $-escape (literal $ must be written as $$)
   LINK_LIBRARIES = $<1:lib_b>
                    ^ near here
2

It looks like test_ext’s link line isn’t recursively expanding genexes; lib_a is fine. Full stanza:

build test_ext: CXX_EXECUTABLE_LINKER__test_ext_ CMakeFiles/test_ext.dir/dummy.cpp.o
  LINK_LIBRARIES = $<1:lib_b>
  OBJECT_DIR = CMakeFiles/test_ext.dir
  POST_BUILD = :
  PRE_LINK = :
  TARGET_COMPILE_PDB = CMakeFiles/test_ext.dir/
  TARGET_FILE = test_ext
  TARGET_PDB = test_ext.pdb

Cc: @brad.king

3

No.

It isn’t supposed to. See the GENEX_EVAL expression.

This is not typical usage. The LINK_LIBRARIES of a target are meant to be private. Use just

target_link_libraries(test_ext PUBLIC lib_a)

This looks like a x-y problem. Please describe what you are really trying to do, perhaps in a new topic dedicated to your use case rather than the genex error.

4

Thanks for the information. Even after reading the linked documentation it’s not easy to understand how generator expressions are evaluated.

I discovered this problem by adding:

target_link_libraries(lib_a
    PUBLIC
        $<1:lib_b>
)

It took me a while to realise that the build failed because someone had used in a completely different place (scope was actually PRIVATE and not PUBLIC as in my first message):

target_link_libraries(test_ext
    PRIVATE
        $<TARGET_PROPERTY:lib_a,LINK_LIBRARIES>
)

I agree that this is not typical usage and I don’t understand why it was used in this case. But there it was and it made the build fail. Luckily I can modify that part of the code too and remove this redundant generator expression. It would be nice if CMake could detect this kind of errors and print a useful error message instead of creating invalid build.ninja file.

5

Part of the confusion is that in this example all generator expressions are evaluated:

cmake_minimum_required(VERSION 3.25)

project(TEST)

add_executable(test_ext dummy.cpp)
add_library(lib_a STATIC dummy.cpp)
add_library(lib_b STATIC dummy.cpp)

target_include_directories(lib_a
    PUBLIC
        $<1:${CMAKE_CURRENT_LIST_DIR}/foo_dir>
)

target_include_directories(test_ext
    PRIVATE
        $<TARGET_PROPERTY:lib_a,INTERFACE_INCLUDE_DIRECTORIES>
)

After running commands:

touch dummy.cpp
mkdir foo_dir
cmake -S . -B out -G Ninja

There’s this in build.ninja:

build CMakeFiles/test_ext.dir/dummy.cpp.o: CXX_COMPILER__test_ext_ /host/workdir/tmp/cmake/gen_exp_inc/dummy.cpp || cmake_object_order_depends_target_test_ext
  DEP_FILE = CMakeFiles/test_ext.dir/dummy.cpp.o.d
  INCLUDES = -I/host/workdir/tmp/cmake/gen_exp_inc/foo_dir
  OBJECT_DIR = CMakeFiles/test_ext.dir
  OBJECT_FILE_DIR = CMakeFiles/test_ext.dir
  TARGET_COMPILE_PDB = CMakeFiles/test_ext.dir/
  TARGET_PDB = test_ext.pdb

There’s foo_dir in INCLUDES as one expects. It’s not easy to understand why generator expressions are evaluated here but not in the first example.