Fix the Meson CMake target for includedir or datadir outside the prefix

nlohmann_jsonTargets.cmake always prepended the prefix computed from the
file's location to includedir. With an absolute includedir outside the
prefix (as Nix passes for packages with a separate dev output), this gave
"<prefix>//abs/include"; with an absolute datadir outside the prefix, the
number of ".." was derived from the absolute path and the prefix resolved
to "/". In both cases find_package(nlohmann_json) failed with "Imported
target includes non-existent path". Use the absolute include directory in
these cases, as CMake's install(EXPORT) does, and check it in CI.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

.github/workflows/ubuntu.yml

@@ -63,6 +63,12 @@ jobs:
           grep -q 'JSON_USE_GLOBAL_UDLS=0;JSON_DIAGNOSTICS=1' install-options/share/cmake/nlohmann_json/nlohmann_jsonTargets.cmake
           cmake -S tests/cmake_import/project -B build-import-options -DCMAKE_PREFIX_PATH=${{ github.workspace }}/install-options
           cmake --build build-import-options
+      - name: Install with Meson and the include directory outside the prefix
+        run: |
+          meson setup build-meson-split --prefix=${{ github.workspace }}/install-split --includedir=${{ github.workspace }}/install-split-dev/include
+          meson install -C build-meson-split
+          cmake -S tests/cmake_import/project -B build-import-split -DCMAKE_PREFIX_PATH=${{ github.workspace }}/install-split
+          cmake --build build-import-split
 
   ci_infer:
     runs-on: ubuntu-latest

cmake/nlohmann_jsonTargets.cmake.in

@@ -4,7 +4,8 @@
 # do that, but as the library is header-only, the target only needs an include
 # directory, the C++ standard, and the compile definitions of the options that
 # differ from their defaults. Paths are computed relative to this file so that
-# the installation can be relocated (e.g., into a sysroot).
+# the installation can be relocated (e.g., into a sysroot), unless includedir or
+# datadir is outside the prefix.
 
 if(TARGET @PROJECT_NAME@::@NLOHMANN_JSON_TARGET_NAME@)
     return()
@@ -14,7 +15,7 @@ get_filename_component(_IMPORT_PREFIX "${CMAKE_CURRENT_LIST_DIR}/@NLOHMANN_JSON_
 
 add_library(@PROJECT_NAME@::@NLOHMANN_JSON_TARGET_NAME@ INTERFACE IMPORTED)
 set_target_properties(@PROJECT_NAME@::@NLOHMANN_JSON_TARGET_NAME@ PROPERTIES
-    INTERFACE_INCLUDE_DIRECTORIES "${_IMPORT_PREFIX}/@NLOHMANN_JSON_INCLUDE_INSTALL_DIR@"
+    INTERFACE_INCLUDE_DIRECTORIES "@NLOHMANN_JSON_INCLUDE_DIR@"
 )
 if(CMAKE_VERSION VERSION_LESS 3.8)
     set_target_properties(@PROJECT_NAME@::@NLOHMANN_JSON_TARGET_NAME@ PROPERTIES

meson.build

@@ -69,12 +69,22 @@ if not meson.is_subproject()
         install_dir: get_option('datadir') / 'pkgconfig',
     )
 
-    # CMake package config files, so that find_package(nlohmann_json) works
+    # CMake package config files, so that find_package(nlohmann_json) works. The
+    # include directory is given relative to the config files, so that the
+    # installation can be relocated. This is not possible if includedir or datadir
+    # is an absolute path outside the prefix (e.g., with the separate outputs of
+    # Nix); then the absolute include directory is used, as CMake does.
+    fs = import('fs')
     cmake_install_dir = get_option('datadir') / 'cmake' / meson.project_name()
     cmake_to_prefix = []
-    foreach component : cmake_install_dir.split('/')
-        cmake_to_prefix += '..'
-    endforeach
+    if fs.is_absolute(get_option('includedir')) or fs.is_absolute(cmake_install_dir)
+        cmake_include_dir = (get_option('prefix') / get_option('includedir')).replace('\\', '/')
+    else
+        foreach component : cmake_install_dir.split('/')
+            cmake_to_prefix += '..'
+        endforeach
+        cmake_include_dir = '${_IMPORT_PREFIX}/' + get_option('includedir')
+    endif
 
     cmake_conf = configuration_data()
     cmake_conf.set('PROJECT_NAME', meson.project_name())
@@ -82,7 +92,7 @@ if not meson.is_subproject()
     cmake_conf.set('PROJECT_VERSION_MAJOR', meson.project_version().split('.')[0])
     cmake_conf.set('NLOHMANN_JSON_TARGET_NAME', meson.project_name())
     cmake_conf.set('NLOHMANN_JSON_TARGETS_EXPORT_NAME', meson.project_name() + 'Targets')
-    cmake_conf.set('NLOHMANN_JSON_INCLUDE_INSTALL_DIR', get_option('includedir'))
+    cmake_conf.set('NLOHMANN_JSON_INCLUDE_DIR', cmake_include_dir)
     cmake_conf.set('NLOHMANN_JSON_CONFIG_TO_PREFIX', '/'.join(cmake_to_prefix))
     cmake_conf.set('NLOHMANN_JSON_COMPILE_DEFINITIONS', ';'.join(json_defines))