Combined find_package/add_subdirectory not linking libraries

[spmcv]

Describe what you want to achieve.

I'm trying to follow the CMake example of Supporting Both.

Describe what you tried.

The external example works as expected. Unfortunately, the former encounters a namespace error. With extern/CMakeLists.txt

if(USE_EXTERNAL_JSON)
    message(DEBUG " using external library for nlohmann JSON")
    find_package(nlohmann_json 3.2.0 REQUIRED)
else()
    message(DEBUG " using git submodule for nlohmann JSON")
    set(JSON_BuildTests OFF CACHE INTERNAL "")
    add_subdirectory(nlohmann_json)
endif()

and CMakeLists.txt:

option(USE_EXTERNAL_JSON "Use an external nlohmann JSON library" ON)
# ...
add_subdirectory(extern)
# ...
target_link_libraries(${MY_TARGET} PRIVATE "nlohmann_json::nlohmann_json")

I get an error:

  Target "MyTarget" links to target "nlohmann_json::nlohmann_json" but
  the target was not found.  Perhaps a find_package() call is missing for an
  IMPORTED target, or an ALIAS target is missing?

However, when removing the namespace:

# ...
target_link_libraries(${MY_TARGET} PRIVATE "nlohmann_json")

Configure throws no error, but now build cannot find the header.

fatal error: 'nlohmann/json.hpp' file not found
#include <nlohmann/json.hpp>
         ^~~~~~~~~~~~~~~~~~~

Describe which system (OS, compiler) you are using.

macOS 10.14.6
CMake 3.14.5

Homebrew

$ brew --version
Homebrew 2.1.11
Homebrew/homebrew-core (git revision ab0ed; last commit 2019-10-02)
Homebrew/homebrew-cask (git revision ccc697; last commit 2019-10-02)

Describe which version of the library you are using.

nlohmann_json

$ brew info nlohmann-json
nlohmann-json: stable 3.7.0 (bottled), HEAD
JSON for modern C++
https://github.com/nlohmann/json
/usr/local/Cellar/nlohmann-json/3.7.0 (8 files, 1MB) *
  Poured from bottle on 2019-09-26 at 17:09:43
From: https://github.com/Homebrew/homebrew-core/blob/master/Formula/nlohmann-json.rb
==> Dependencies
Build: cmake ✘
==> Options
--HEAD
	Install HEAD version

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[martin-schulze-vireso]

Just in case someone stumbles upon this as I did:
I saw the exact same symptoms in an android build where a gradle project pulled in my own library via externalNativeBuild that in turn should pull in this library via add_subdirectory.

It turned out that I confused myself with the hierarchy of my larger project and only built the part of my CMake project tree which did not contain the add_subdirectory for this library. (Part of) my rough project layout:

parent/
  CMakeLists.txt
  third_party/nlohman_json/
   ...
   CMakeLists.txt
  my_library/
    CMakeLists.txt

The add_subdirectory(third_party/nlohmann_json) was done in parent/CMakeLists.txt but gradle ran cmake only on my_library/CMakeLists.txt, so it never saw that. After moving add_subdirectory(../third_part/nlohmann_json ${CMAKE_CURRENT_BINARY_DIR)/nlohmann_json) into my_library/CMakeLists.txt it worked as expected.