From 2a05f931e7fc8b00f3c8aa2faa7d7e2b5742a229 Mon Sep 17 00:00:00 2001
From: Mateusz Pusz <mateusz.pusz@gmail.com>
Date: Wed, 24 Apr 2024 20:17:39 +0200
Subject: [PATCH] build: `MP_UNITS_BUILD_AS_SYSTEM_HEADERS` restored (but
 renamed)

---
 CMakeLists.txt                                 | 2 ++
 docs/getting_started/installation_and_usage.md | 8 ++++++++
 src/CMakeLists.txt                             | 7 +++++++
 src/cmake/AddMPUnitsModule.cmake               | 4 ++++
 4 files changed, 21 insertions(+)

diff --git a/CMakeLists.txt b/CMakeLists.txt
index e43b7220..2cb45efc 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -42,6 +42,8 @@ enable_ccache(BASE_DIR ${PROJECT_SOURCE_DIR})
 option(${projectPrefix}DEV_IWYU "Enables include-what-you-use" OFF)
 
 if(${projectPrefix}DEV_IWYU)
+    set(${projectPrefix}BUILD_AS_SYSTEM_HEADERS ON)
+
     include(include-what-you-use)
     enable_iwyu(
         MAPPING_FILE "${PROJECT_SOURCE_DIR}/.mp-units.imp"
diff --git a/docs/getting_started/installation_and_usage.md b/docs/getting_started/installation_and_usage.md
index aba8cc37..8ecd0e5f 100644
--- a/docs/getting_started/installation_and_usage.md
+++ b/docs/getting_started/installation_and_usage.md
@@ -223,6 +223,14 @@ tools.build:compiler_executables={"c": "gcc-12", "cpp": "g++-12"}
 
 ### CMake options
 
+[`MP_UNITS_BUILD_AS_SYSTEM_HEADERS`](#MP_UNITS_BUILD_AS_SYSTEM_HEADERS){ #MP_UNITS_BUILD_AS_SYSTEM_HEADERS }
+
+:   [:octicons-tag-24: 2.2.0][as system headers support] · :octicons-milestone-24: `ON`/`OFF` (Default: `OFF`)
+
+    Exports library as system headers.
+
+    [as system headers support]: https://github.com/mpusz/mp-units/releases/tag/v2.2.0
+
 [`MP_UNITS_BUILD_CXX_MODULES`](#MP_UNITS_BUILD_CXX_MODULES){ #MP_UNITS_BUILD_CXX_MODULES }
 
 :   [:octicons-tag-24: 2.2.0][build_cxx_modules support] · :octicons-milestone-24: `ON`/`OFF` (Default: `OFF`)
diff --git a/src/CMakeLists.txt b/src/CMakeLists.txt
index 6ca25bda..ff1408f5 100644
--- a/src/CMakeLists.txt
+++ b/src/CMakeLists.txt
@@ -36,9 +36,16 @@ include(CheckLibcxxInUse)
 check_libcxx_in_use(${projectPrefix}LIBCXX)
 
 # project build options
+option(${projectPrefix}BUILD_AS_SYSTEM_HEADERS "Exports library as system headers" OFF)
+message(STATUS "${projectPrefix}BUILD_AS_SYSTEM_HEADERS: ${${projectPrefix}BUILD_AS_SYSTEM_HEADERS}")
+
 option(${projectPrefix}BUILD_CXX_MODULES "Add C++ modules to the list of default targets" OFF)
 message(STATUS "${projectPrefix}BUILD_CXX_MODULES: ${${projectPrefix}BUILD_CXX_MODULES}")
 
+if(${projectPrefix}BUILD_AS_SYSTEM_HEADERS)
+    set(${projectPrefix}_AS_SYSTEM SYSTEM)
+endif()
+
 # project API settings
 function(cache_var_values name)
     set_property(CACHE ${projectPrefix}${name} PROPERTY STRINGS ${ARGN})
diff --git a/src/cmake/AddMPUnitsModule.cmake b/src/cmake/AddMPUnitsModule.cmake
index b4da85c8..be007c90 100644
--- a/src/cmake/AddMPUnitsModule.cmake
+++ b/src/cmake/AddMPUnitsModule.cmake
@@ -70,6 +70,10 @@ function(add_mp_units_module name target_name)
         target_sources(
             ${target_name} PUBLIC FILE_SET HEADERS BASE_DIRS ${CMAKE_CURRENT_SOURCE_DIR}/include FILES ${ARG_HEADERS}
         )
+        target_include_directories(
+            ${target_name} ${${projectPrefix}_AS_SYSTEM} ${${projectPrefix}TARGET_SCOPE}
+            $<BUILD_INTERFACE:${CMAKE_CURRENT_SOURCE_DIR}/include> $<INSTALL_INTERFACE:include>
+        )
     endif()
 
     if(${projectPrefix}BUILD_CXX_MODULES)