CMake can't find header from dependency of imported project - cmake

Question

I have a CMake project C that depends on project B, which in turn depends on project A. I've created a superbuild that builds all three. Project C uses headers from project B, and my CMakeLists for Project C looks like this:
cmake_minimum_required(VERSION 3.10)
project(code_c)
find_package(b REQUIRED)
add_library(c c.c)
target_link_libraries(c PUBLIC b)
target_include_directories(c INTERFACE
$<INSTALL_INTERFACE:include>)
install(TARGETS c EXPORT c)
install(FILES c.h DESTINATION include)
export(PACKAGE c)
export(TARGETS c FILE "${PROJECT_BINARY_DIR}/bTargets.cmake")
install(EXPORT c FILE aTargets.cmake DESTINATION lib/cmake/c)
include(CMakePackageConfigHelpers)
configure_package_config_file(${CMAKE_CURRENT_SOURCE_DIR}/Config.cmake.in
"${CMAKE_CURRENT_BINARY_DIR}/cConfig.cmake"
INSTALL_DESTINATION lib/cmake/c)
install(FILES
"${CMAKE_CURRENT_BINARY_DIR}/cConfig.cmake"
DESTINATION lib/cmake/c)
install(EXPORT c
FILE cTargets.cmake
DESTINATION lib/cmake/c)
My CMakeLists for Project B looks like this:
cmake_minimum_required(VERSION 3.10)
project(code_b)
find_package(a REQUIRED)
add_library(b b.c)
target_link_libraries(b PUBLIC a)
target_include_directories(b INTERFACE
$<INSTALL_INTERFACE:include>)
install(TARGETS b EXPORT b)
install(FILES b.h DESTINATION include)
export(PACKAGE b)
export(TARGETS b FILE "${PROJECT_BINARY_DIR}/bTargets.cmake")
install(EXPORT b FILE aTargets.cmake DESTINATION lib/cmake/b)
include(CMakePackageConfigHelpers)
configure_package_config_file(${CMAKE_CURRENT_SOURCE_DIR}/Config.cmake.in
"${CMAKE_CURRENT_BINARY_DIR}/bConfig.cmake"
INSTALL_DESTINATION lib/cmake/b)
install(FILES
"${CMAKE_CURRENT_BINARY_DIR}/bConfig.cmake"
DESTINATION lib/cmake/b)
install(EXPORT b
FILE bTargets.cmake
DESTINATION lib/cmake/b)
My Config.cmake.in for Project B looks like
#PACKAGE_INIT#
include("${CMAKE_CURRENT_LIST_DIR}/bTargets.cmake")
check_required_components(b)
When I attempt to build Project C, I get
/Users/jhaiduce/Development/cmake_include_test/build/b/include/b.h:4:10: fatal error:
'a.h' file not found
#include "a.h"
^~~~~
1 error generated.
However, if I add find_package(a REQUIRED) to the CMakeLists.txt for Project A, then the include directories from target 'a' are added to the build and Project C compiles successfully. Should this be necessary? I would have expected that CMake would export Project A's targets along with Project B, making it unneccesary to call find_package(a) from Project C. Does the fact that Project C fails to build without this indicate a problem with the way I exported Project B?

Answer 1

I think in BConfig.cmake.in you should use:
include(CMakeFindDependencyMacro)
if(NOT A_FOUND AND NOT TARGET a)
find_dependency(A REQUIRED)
endif()
ref:
https://cmake.org/cmake/help/latest/module/CMakeFindDependencyMacro.html