test: standalone public headers tests added

CMakeLists.txt

@@ -20,7 +20,7 @@
 # OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
 # SOFTWARE.
 
-cmake_minimum_required(VERSION 3.15)
+cmake_minimum_required(VERSION 3.19)
 project(mp-units-dev LANGUAGES CXX)
 
 list(APPEND CMAKE_MODULE_PATH "${PROJECT_SOURCE_DIR}/cmake")
@@ -71,3 +71,43 @@ add_subdirectory(docs)
 # add unit tests
 enable_testing()
 add_subdirectory(test)
+
+function(get_target_linked_targets target targets_out)
+    get_target_property(linked_libs ${target} INTERFACE_LINK_LIBRARIES)
+    foreach(linked_lib ${linked_libs})
+        if(NOT linked_lib IN_LIST targets)
+            if(TARGET ${linked_lib})
+                get_target_linked_targets(${linked_lib} child_targets)
+                list(APPEND targets ${linked_lib} ${child_targets})
+            endif()
+        endif()
+    endforeach()
+
+    list(REMOVE_DUPLICATES targets)
+    list(REMOVE_DUPLICATES link_libs)
+    set(${targets_out} ${targets} PARENT_SCOPE)
+endfunction()
+
+function(get_target_sources target paths_out)
+    get_target_linked_targets(${target} targets)
+    list(APPEND targets ${target})
+
+    foreach(t ${targets})
+        get_target_property(sources ${t} SOURCES)
+        if(sources)
+            get_target_property(source_dir ${t} SOURCE_DIR)
+            foreach(f ${sources})
+                file(REAL_PATH "${f}" path BASE_DIRECTORY "${source_dir}")
+                list(APPEND paths "${path}")
+            endforeach()
+        endif()
+    endforeach()
+
+    set(${paths_out} ${paths} PARENT_SCOPE)
+endfunction()
+
+# tests for standalone headers
+include(TestHeaders)
+get_target_sources(mp-units::mp-units sources)
+add_header_test(test_headers HEADERS ${sources})
+target_link_libraries(test_headers PRIVATE mp-units::mp-units)