How do I Install bundled interface dependencies with modern CMake?

Question

What is the proper way to install bundled interface dependencies in Modern CMake?

I have a library MyLib that has an interface dependency on libDep (MyLib.hpp contains #include <libDep.h>). Anything that depends on MyLib also transitively depends on libDep.

libDep is a single header taken from a gist so I have included it part of the source-tree of MyLib

$ tree
.
├── CMake
│   ├── MyLibConfig.cmake.in
│   └── modules
│       └── FindlibDep.cmake
├── CMakeLists.txt
├── include
│   └── MyLib
│       └── MyLib.hpp
├── src
│   └── MyLib.cpp
└── third_party
    └── libDep
        └── libDep.h

I would like to install libDep with myLib, in with a path like include/MyLib/third_party/libDep.

---

Here is the CMakeLists for MyLib

cmake_minimum_required(VERSION 3.3.0)
Project(MyLib
    DESCRIPTION "Library with bundled interface dependency"
    LANGUAGES CXX)

include(GNUInstallDirs)
include(CMakePackageConfigHelpers)

set(CMAKE_MODULE_PATH ${CMAKE_SOURCE_DIR}/CMake/modules)

# Find LibDep dependency
find_package(libDep REQUIRED)

# MyLib library
add_library(MyLib STATIC
    ${CMAKE_SOURCE_DIR}/src/MyLib.cpp)

target_include_directories(MyLib
    PUBLIC $<BUILD_INTERFACE:${CMAKE_SOURCE_DIR}/include/MyLib>
    PUBLIC $<INSTALL_INTERFACE:include/MyLib>)

target_link_libraries(MyLib
    INTERFACE libDep)

MyLib locates libDep with FindlibDep.cmake located in CMAKE_MODULE_PATH

find_path(LibDep_INCLUDE_DIR
    NAMES LibDep.hpp
    PATHS third_party/libDep)
    PATH_SUFFIXES Mylib/third_party/libDep)

mark_as_advanced(LibDep_FOUND LibDep_INCLUDE_DIR)

include(FindPackageHandleStandardArgs)
find_package_handle_standard_args(LibDep
    REQUIRED_VARS
        LibDep_INCLUDE_DIR
)

if(LibDep_FOUND)
    set(LibDep_INCLUDE_DIRS ${LibDep_INCLUDE_DIR})
endif()

if(LibDep_FOUND AND NOT TARGET MyLib::LibDep)
    add_library(MyLib::LibDep INTERFACE IMPORTED)
    set_target_properties(MyLib::LibDep PROPERTIES
        INTERFACE_INCLUDE_DIRECTORIES ${LibDep_INCLUDE_DIR})
endif()

---

I install MyLib like

install(TARGETS MyLib
    EXPORT MyLibTargets
    LIBRARY DESTINATION lib
    ARCHIVE DESTINATION lib
    RUNTIME DESTINATION bin
    INCLUDES DESTINATION include)

include(CMakePackageConfigHelpers)
write_basic_package_version_file(MyLibConfigVersion.cmake)

install(EXPORT MyLibTargets
    FILE MyLibTargets.cmake
    NAMESPACE MyLib::
    DESTINATION lib/cmake/MyLib)

configure_file(CMake/MyLibConfig.cmake.in MyLibConfig.cmake @ONLY)
install(FILES
    "${CMAKE_CURRENT_BINARY_DIR}/MyLibConfig.cmake"
    "${CMAKE_CURRENT_BINARY_DIR}/MyLibConfigVersion.cmake"
    DESTINATION lib/cmake/MyLib)

install(DIRECTORY ${MyLib_PUBLIC_INCLUDE_DIR}
    DESTINATION ${CMAKE_INSTALL_INCLUDEDIR})

# Package with CPack
include(InstallRequiredSystemLibraries)
include(CPack)

MyLibConfig.cmake.in declares a dependency on libDep

include(CMakeFindDependencyMacro)

# Dependencies
add_library(libDep REQUIRED)

# Add the targets file
include("${CMAKE_CURRENT_LIST_DIR}/MyLibTargets.cmake")

libDep is installed into MyLib's tree with

install(DIRECTORY ${libDep_INCLUDE_DIRS}
DESTINATION ${CMAKE_INSTALL_INCLUDEDIR}/MyLib/third_party)

and I copy the find module with

install(FILES
    ${CMAKE_MODULE_PATH}/FindlibDep.cmake
    DESTINATION lib/cmake/MyLib)

---

Unfortunately users of MyLib don't see FindlibDep.cmake by default

$ cmake ..
CMake Error at /usr/share/cmake-3.10/Modules/CMakeFindDependencyMacro.cmake:48 (find_package):
  By not providing "FindlibDep.cmake" in CMAKE_MODULE_PATH this project has
  asked CMake to find a package configuration file provided by "libDep", but
  CMake did not find one.

  Could not find a package configuration file provided by "libDep" with any
  of the following names:

    libDepConfig.cmake
    libDep-config.cmake

  Add the installation prefix of "libDep" to CMAKE_PREFIX_PATH or set
  "libDep_DIR" to a directory containing one of the above files.  If "libDep"
  provides a separate development package or SDK, be sure it has been
  installed.
Call Stack (most recent call first):
  /usr/local/lib/cmake/MyLib/MyLibConfig.cmake:5 (find_dependency)
  CMakeLists.txt:9 (find_package)

-- Configuring incomplete, errors occurred!

The user can manually locate FindlibDep.cmake and add it to their CMAKE_MODULE_PATH but that shouldn't be required.

---

A reproduction of this issue is available on GitHub.

Answer 1

You need to use the PROJECT_ variable for the source dir instead of the CMAKE_ variable:

set(CMAKE_MODULE_PATH ${CMAKE_MODULE_PATH} ${PROJECT_SOURCE_DIR}/CMake/modules)

PROJECT_SOURCE_DIR gets the source dir from the most recently executed project() directive (in this case, your library).

You should also be adding your current CMAKE_MODULE_PATH to the new path, as shown above.