As this is effectively a breaking change for downstream projects that link against the fcl target, could it make sense to add a add_library(fcl ALIAS fcl::fcl) instruction to fcl-config.cmake.in? Eventually by adding a deprecation message if somebody still uses it, something like:

add_library(fcl ALIAS fcl::fcl)
if(CMAKE_VERSION VERSION_GREATER_EQUAL 3.17)
  set_property(TARGET fcl PROPERTY DEPRECATION "Deprecated target. Please use fcl::fcl instead.")
endif()

I didn't want to include an alias without any warning, but thank you for pointing me to the new DEPRECATION property.

It doesn't work on an ALIAS target

CMake Error at CMake/fcl-config.cmake:47 (set_property):
  set_property can not be used on an ALIAS target.

but this would work (or using VERSION_LESS for CMake <3.7):

add_library(fcl INTERFACE IMPORTED)
set_target_properties(fcl PROPERTIES INTERFACE_LINK_LIBRARIES "fcl::fcl")
if(CMAKE_VERSION VERSION_GREATER_EQUAL 3.17)
  set_property(TARGET fcl PROPERTY DEPRECATION "Deprecated target. Please use fcl::fcl instead.")
endif()

Great, thanks! I think fcl requires CMake 3.10, so using VERSION_GREATER_EQUAL is ok.

Great, I'll update the PR accordingly then.

While fcl requires CMake 3.10, a consumer project of the config file can have a lower version. In this case, this would lead to the following error:

CMake Error at CMake/fcl-config.cmake:46 (if):
  if given arguments:

    "CMAKE_VERSION" "VERSION_GREATER_EQUAL" "3.17"

Changing this to use VERSION_LESS however works fine on a project with CMake 3.0 linking against fcl (first version with CMakeFindDependencyMacro module).

While fcl requires CMake 3.10, a consumer project of the config file can have a lower version. In this case, this would lead to the following error:

Thanks, indeed I was not thinking of that case.

Probably for people that consume fcl via CMake's FetchContent module, it could also make sense to add a fcl::fcl alias target for the build, i.e. add in

add_library(${PROJECT_NAME}::${PROJECT_NAME} ALIAS ${PROJECT_NAME})

Good point, I've just added this to the PR.