fix: support basic dual includes (#2804)

CMakeLists.txt

@@ -45,8 +45,13 @@ if(NOT pybind11_FIND_QUIETLY)
   message(STATUS "pybind11 v${pybind11_VERSION} ${pybind11_VERSION_TYPE}")
 endif()
 
+# Avoid infinite recursion if tests include this as a subdirectory
+if(DEFINED PYBIND11_MASTER_PROJECT)
+  set(PYBIND11_TEST OFF)
+endif()
+
 # Check if pybind11 is being used directly or via add_subdirectory
-if(CMAKE_SOURCE_DIR STREQUAL PROJECT_SOURCE_DIR)
+if(CMAKE_SOURCE_DIR STREQUAL PROJECT_SOURCE_DIR AND NOT DEFINED PYBIND11_MASTER_PROJECT)
   ### Warn if not an out-of-source builds
   if(CMAKE_CURRENT_SOURCE_DIR STREQUAL CMAKE_CURRENT_BINARY_DIR)
     set(lines
@@ -161,12 +166,24 @@ endif()
 # You can also place ifs *in* the Config.in, but not here.
 
 # This section builds targets, but does *not* touch Python
-
-# Build the headers-only target (no Python included):
-# (long name used here to keep this from clashing in subdirectory mode)
-add_library(pybind11_headers INTERFACE)
-add_library(pybind11::pybind11_headers ALIAS pybind11_headers) # to match exported target
-add_library(pybind11::headers ALIAS pybind11_headers) # easier to use/remember
+# Non-IMPORT targets cannot be defined twice
+if(NOT TARGET pybind11_headers)
+  # Build the headers-only target (no Python included):
+  # (long name used here to keep this from clashing in subdirectory mode)
+  add_library(pybind11_headers INTERFACE)
+  add_library(pybind11::pybind11_headers ALIAS pybind11_headers) # to match exported target
+  add_library(pybind11::headers ALIAS pybind11_headers) # easier to use/remember
+
+  target_include_directories(
+    pybind11_headers ${pybind11_system} INTERFACE $<BUILD_INTERFACE:${pybind11_INCLUDE_DIR}>
+                                                  $<INSTALL_INTERFACE:${CMAKE_INSTALL_INCLUDEDIR}>)
+
+  target_compile_features(pybind11_headers INTERFACE cxx_inheriting_constructors cxx_user_literals
+                                                     cxx_right_angle_brackets)
+else()
+  # It is invalid to install a target twice, too.
+  set(PYBIND11_INSTALL OFF)
+endif()
 
 include("${CMAKE_CURRENT_SOURCE_DIR}/tools/pybind11Common.cmake")
 
@@ -177,14 +194,6 @@ elseif(USE_PYTHON_INCLUDE_DIR AND DEFINED PYTHON_INCLUDE_DIR)
   file(RELATIVE_PATH CMAKE_INSTALL_INCLUDEDIR ${CMAKE_INSTALL_PREFIX} ${PYTHON_INCLUDE_DIRS})
 endif()
 
-# Fill in headers target
-target_include_directories(
-  pybind11_headers ${pybind11_system} INTERFACE $<BUILD_INTERFACE:${pybind11_INCLUDE_DIR}>
-                                                $<INSTALL_INTERFACE:${CMAKE_INSTALL_INCLUDEDIR}>)
-
-target_compile_features(pybind11_headers INTERFACE cxx_inheriting_constructors cxx_user_literals
-                                                   cxx_right_angle_brackets)
-
 if(PYBIND11_INSTALL)
   install(DIRECTORY ${pybind11_INCLUDE_DIR}/pybind11 DESTINATION ${CMAKE_INSTALL_INCLUDEDIR})
   set(PYBIND11_CMAKECONFIG_INSTALL_DIR

tests/test_cmake_build/CMakeLists.txt

@@ -25,7 +25,7 @@ function(pybind11_add_build_test name)
   endif()
 
   if(NOT ARG_INSTALL)
-    list(APPEND build_options "-DPYBIND11_PROJECT_DIR=${pybind11_SOURCE_DIR}")
+    list(APPEND build_options "-Dpybind11_SOURCE_DIR=${pybind11_SOURCE_DIR}")
   else()
     list(APPEND build_options "-DCMAKE_PREFIX_PATH=${pybind11_BINARY_DIR}/mock_install")
   endif()
@@ -79,3 +79,6 @@ if(PYBIND11_INSTALL)
 endif()
 
 add_dependencies(check test_cmake_build)
+
+add_subdirectory(subdirectory_target EXCLUDE_FROM_ALL)
+add_subdirectory(subdirectory_embed EXCLUDE_FROM_ALL)

tests/test_cmake_build/subdirectory_embed/CMakeLists.txt

@@ -16,7 +16,7 @@ set(PYBIND11_INSTALL
     CACHE BOOL "")
 set(PYBIND11_EXPORT_NAME test_export)
 
-add_subdirectory(${PYBIND11_PROJECT_DIR} pybind11)
+add_subdirectory("${pybind11_SOURCE_DIR}" pybind11)
 
 # Test basic target functionality
 add_executable(test_subdirectory_embed ../embed.cpp)
@@ -24,7 +24,7 @@ target_link_libraries(test_subdirectory_embed PRIVATE pybind11::embed)
 set_target_properties(test_subdirectory_embed PROPERTIES OUTPUT_NAME test_cmake_build)
 
 add_custom_target(check_subdirectory_embed $<TARGET_FILE:test_subdirectory_embed>
-                                           ${PROJECT_SOURCE_DIR}/../test.py)
+                                           "${PROJECT_SOURCE_DIR}/../test.py")
 
 # Test custom export group -- PYBIND11_EXPORT_NAME
 add_library(test_embed_lib ../embed.cpp)

tests/test_cmake_build/subdirectory_function/CMakeLists.txt

@@ -11,7 +11,7 @@ endif()
 
 project(test_subdirectory_function CXX)
 
-add_subdirectory("${PYBIND11_PROJECT_DIR}" pybind11)
+add_subdirectory("${pybind11_SOURCE_DIR}" pybind11)
 pybind11_add_module(test_subdirectory_function ../main.cpp)
 set_target_properties(test_subdirectory_function PROPERTIES OUTPUT_NAME test_cmake_build)
 

tests/test_cmake_build/subdirectory_target/CMakeLists.txt

@@ -11,7 +11,7 @@ endif()
 
 project(test_subdirectory_target CXX)
 
-add_subdirectory(${PYBIND11_PROJECT_DIR} pybind11)
+add_subdirectory("${pybind11_SOURCE_DIR}" pybind11)
 
 add_library(test_subdirectory_target MODULE ../main.cpp)
 set_target_properties(test_subdirectory_target PROPERTIES OUTPUT_NAME test_cmake_build)