[CMake] Problem with foreach() iteration over a list that uses generator expressions

[Tadeusz A. Kadłubowski]

Hi,

I apologize for initially sending this message to
cmake-developers at cmake.org, rather than here. I was confused and
mistook the proper mailing list.

I have an `INTERFACE IMPORTED` library target that depends on a bunch
of optional dependencies. Please see the snippet below:

cmake_minimum_required(VERSION 3.14)

add_library(example INTERFACE IMPORTED)
set_property(TARGET example PROPERTY INTERFACE_LINK_LIBRARIES
$<$<CONFIG:Debug>:${undefined_dependency_one}
${undefined_dependency_two}>
)

It so happens that both of the optional dependencies
`${undefined_dependency_one}` and `${undefined_dependency_two}` are
set to empty/undefined on my system, which is perfectly fine.

I want to do some postprocessing to the dependencies  of my target:

get_target_property(all_dependencies example INTERFACE_LINK_LIBRARIES)
foreach(lib IN LISTS all_dependencies)
    message(STATUS "dependency on " ${lib})
endforeach()

This `foreach()` loop gets confused about the generator expression.
The output of the `message(STATUS ...)` calls in the snippet above is:

-- dependency on $<$<CONFIG:Debug>:
-- dependency on >

Please note that the generator expression got mangled into two pieces
by the `foreach()` loop. I was expecting to see a single line of
`$<$<CONFIG:Debug>:>`

I experimented with different versions of this test case and found
variants that do not get mangled in the `foreach()` loop. In a
modification when there is only one empty/undefined dependency the
`foreach()` loop works as expected.

set_property(TARGET example PROPERTY INTERFACE_LINK_LIBRARIES
$<$<CONFIG:Debug>:${undefined_dependency_one}>
)

A modification in which each empty/undefined dependency is surrounded
with its own generator expression works correctly too:

set_property(TARGET example PROPERTY INTERFACE_LINK_LIBRARIES
$<$<CONFIG:Debug>:${undefined_dependency_one}>
$<$<CONFIG:Debug>:${undefined_dependency_two}>
)

Is that a bug in how CMake handles generator expressions? Am I
misunderstanding some CMake language concepts here?

(all code tested with CMake 3.14.3 on Ubuntu 18.04)

I would be grateful on any feedback about what's going on in that
`foreach` loop.
--

Tadeusz Andrzej Kadłubowski | Senior Software Engineer
Berlin, Germany
tadeusz.kadlubowski at pix4d.com
www.pix4d.com



-- 
Tadeusz Andrzej Kadłubowski | Senior Software Engineer
Berlin, Germany
tadeusz.kadlubowski at pix4d.com
www.pix4d.com