GitLab

If a source file already has the HEADER_FILE_ONLY source property set to true, that source file should be excluded from the unity build. As of CMake 3.16.0-rc3, this is not the case. A simple demonstrator project that verifies the issue:

CMakeLists.txt:

cmake_minimum_required(VERSION 3.16)
project(unitybuild)

set(CMAKE_UNITY_BUILD YES)
add_library(checker ok1.cpp ok2.cpp notOk.cpp)
set_source_files_properties(notOk.cpp PROPERTIES HEADER_FILE_ONLY TRUE)

ok1.cpp

#define ok1_added 1
int ok1() { return 23; }  // Just to ensure there's something to compile, prevents warnings

ok2.cpp

#define ok2_added 1
int ok2() { return 57; }  // Just to ensure there's something to compile, prevents warnings

notOk.cpp

#if defined(ok1_added) || defined(ok2_added)
#error Was not excluded from unity build
#endif
int notOk() { return 3; }  // Just to ensure there's something to compile, prevents warnings