cmake the following imported targets are referenced, but are missing

Question

I have a repository with two libraries (liba and libb) whereas liba depends on libb. They are part of a single repository and are built using a single cmake "context". The file structure is shown below:

├── CMakeLists.txt
├── liba
│   ├── CMakeLists.txt
│   ├── internal
│   │   └── private.hh
│   ├── module.cc
│   ├── module.hh
└── libb
    ├── CMakeLists.txt
    ├── other.cc
    └── other.hh

Everything compiles and installs without any issues. Although, if I try to create a new project that depends on liba. Like so:

cmake_minimum_required(VERSION 3.5)

find_package(Threads REQUIRED)
find_package(OpenCV REQUIRED)
find_package(liba REQUIRED)

add_executable(exec exec.cc)
target_link_libraries(exec PRIVATE is::liba)

I get the following error:

CMake Error at CMakeLists.txt:5 (find_package):
  Found package configuration file:

    /home/hodor/is-sdk/lib/cmake/liba/libaConfig.cmake

  but it set liba_FOUND to FALSE so package "liba" is considered to be NOT
  FOUND.  Reason given by package:

  The following imported targets are referenced, but are missing: is::libb

What I am missing here? Repository link for context

CMakeLists that generates liba:

cmake_minimum_required(VERSION 3.5)
include(GNUInstallDirs)

find_package(OpenCV REQUIRED core)
find_package(Threads REQUIRED)

list(APPEND liba_public_headers 
  "module.hh"
)

list(APPEND liba_private_headers 
  "internal/private.hh"
)

list(APPEND liba_sources 
  "module.cc"
  ${liba_public_headers}
  ${liba_private_headers}
)

add_library(liba ${liba_sources})

target_include_directories(
  liba
 PUBLIC
  $<BUILD_INTERFACE:${CMAKE_CURRENT_LIST_DIR}> # for headers when building
  $<BUILD_INTERFACE:${CMAKE_CURRENT_BINARY_DIR}> # for generated files in build mode
  $<INSTALL_INTERFACE:include/is/liba> # for clients in install mode
 PRIVATE
  ${OpenCV_INCLUDE_DIRS}
)

target_link_libraries(liba 
 PRIVATE
  is::libb
  ${OpenCV_LIBRARIES} 
  Threads::Threads
)

set_property(TARGET liba PROPERTY CXX_STANDARD 11)

install(
  TARGETS liba EXPORT libaTargets
  LIBRARY  DESTINATION "${CMAKE_INSTALL_LIBDIR}"
  ARCHIVE  DESTINATION "${CMAKE_INSTALL_LIBDIR}"
  RUNTIME  DESTINATION "${CMAKE_INSTALL_BINDIR}"
  INCLUDES  DESTINATION "${CMAKE_INSTALL_INCLUDEDIR}"
)

install(FILES ${liba_public_headers} DESTINATION ${CMAKE_INSTALL_INCLUDEDIR}/is/liba)

install(
  EXPORT libaTargets
  FILE libaConfig.cmake
  NAMESPACE is::
  DESTINATION ${CMAKE_INSTALL_LIBDIR}/cmake/liba
)

CMakeLists that generates libb:

cmake_minimum_required(VERSION 3.5)
include(GNUInstallDirs)

list(APPEND libb_public_headers 
  "other.hh"
)

list(APPEND libb_sources 
  "other.cc"
  ${libb_public_headers}
)

add_library(libb ${libb_sources})

target_include_directories(
  libb
 PUBLIC
  $<BUILD_INTERFACE:${CMAKE_CURRENT_LIST_DIR}> # for headers when building
  $<BUILD_INTERFACE:${CMAKE_CURRENT_BINARY_DIR}> # for generated files in build mode
  $<INSTALL_INTERFACE:include/is/libb> # for clients in install mode
)

install(
  TARGETS libb EXPORT libbTargets
  LIBRARY  DESTINATION "${CMAKE_INSTALL_LIBDIR}"
  ARCHIVE  DESTINATION "${CMAKE_INSTALL_LIBDIR}"
  RUNTIME  DESTINATION "${CMAKE_INSTALL_BINDIR}"
  INCLUDES  DESTINATION "${CMAKE_INSTALL_INCLUDEDIR}"
)

install(FILES ${libb_public_headers} DESTINATION ${CMAKE_INSTALL_INCLUDEDIR}/is/libb)

install(
  EXPORT libbTargets
  FILE libbConfig.cmake
  NAMESPACE is::
  DESTINATION ${CMAKE_INSTALL_LIBDIR}/cmake/libb
)

add_library(is::libb ALIAS libb)

Top level CMakeLists that includes liba and libb:

cmake_minimum_required(VERSION 3.5)

add_subdirectory(liba)
add_subdirectory(libb)

Answer 1

After watching C++Now 2017: Daniel Pfeifer “Effective CMake" I now realized what was missing in my configuration. As mentioned by @Tsyvarev the problem was about libaConfig.cmake.

When exporting a library with dependencies you should export mylibraryTargets.cmake, like so:

install(
  EXPORT libaTargets
  FILE libaTargets.cmake
  NAMESPACE is::
  DESTINATION ${CMAKE_INSTALL_LIBDIR}/cmake/liba
)

And then manually (a bit disappointed here I have to say) write a mylibraryConfig.cmake with all the dependencies like so:

include(CMakeFindDependencyMacro)

find_dependency(libb)
find_dependency(OpenCV)
find_dependency(Threads)

include("${CMAKE_CURRENT_LIST_DIR}/libaTargets.cmake")