FindDoxygen: Allow generated files with USE_STAMP_FILE

Fixes: #20597

Modules/FindDoxygen.cmake

@@ -999,9 +999,11 @@ doxygen_add_docs() for target ${targetName}")
     foreach(_item IN LISTS DOXYGEN_INPUT)
         get_filename_component(_abs_item "${_item}" ABSOLUTE
                                BASE_DIR "${_args_WORKING_DIRECTORY}")
-        if(EXISTS "${_abs_item}" AND
-           NOT IS_DIRECTORY "${_abs_item}" AND
-           NOT IS_SYMLINK "${_abs_item}")
+        get_source_file_property(_isGenerated "${_abs_item}" GENERATED)
+        if(_isGenerated OR
+           (EXISTS "${_abs_item}" AND
+            NOT IS_DIRECTORY "${_abs_item}" AND
+            NOT IS_SYMLINK "${_abs_item}"))
             list(APPEND _sources "${_abs_item}")
         elseif(_args_USE_STAMP_FILE)
             message(FATAL_ERROR "Source does not exist or is not a file:\n"

Tests/FindDoxygen/StampFile/CMakeLists.txt

@@ -3,6 +3,7 @@ project(TestFindDoxygen VERSION 1.0 LANGUAGES NONE)
 
 find_package(Doxygen REQUIRED)
 
+set(DOXYGEN_OUTPUT_DIRECTORY noFiles)
 doxygen_add_docs(docsWithoutFilesWithStamp USE_STAMP_FILE)
 if(NOT EXISTS "${PROJECT_BINARY_DIR}/Doxyfile.docsWithoutFilesWithStamp")
   message(FATAL_ERROR "Missing generated file: Doxyfile.docsWithoutFilesWithStamp")
@@ -11,6 +12,7 @@ if(NOT TARGET docsWithoutFilesWithStamp)
   message(FATAL_ERROR "Target docsWithoutFilesWithStamp not created")
 endif()
 
+set(DOXYGEN_OUTPUT_DIRECTORY withFiles)
 doxygen_add_docs(docsWithFilesWithStamp main.cpp main2.cpp USE_STAMP_FILE)
 if(NOT EXISTS "${PROJECT_BINARY_DIR}/Doxyfile.docsWithFilesWithStamp")
   message(FATAL_ERROR "Missing generated file: Doxyfile.docsWithFilesWithStamp")
@@ -19,6 +21,23 @@ if(NOT TARGET docsWithFilesWithStamp)
   message(FATAL_ERROR "Target docsWithFilesWithStamp not created")
 endif()
 
+# Confirm that doxygen_add_docs() doesn't cause a fatal error if given a
+# source file that is generated at build time
+file(REMOVE ${CMAKE_CURRENT_BINARY_DIR}/genDox.cpp)
+add_custom_command(OUTPUT genDox.cpp
+  COMMAND ${CMAKE_COMMAND} -E copy ${CMAKE_CURRENT_SOURCE_DIR}/main2.cpp genDox.cpp
+  DEPENDS main2.cpp
+)
+set(DOXYGEN_OUTPUT_DIRECTORY withGenFiles)
+doxygen_add_docs(docsWithGenFilesWithStamp
+  main.cpp
+  ${CMAKE_CURRENT_BINARY_DIR}/genDox.cpp
+  USE_STAMP_FILE
+)
 
 add_custom_target(allDocTargets)
-add_dependencies(allDocTargets docsWithoutFilesWithStamp docsWithFilesWithStamp)
+add_dependencies(allDocTargets
+  docsWithoutFilesWithStamp
+  docsWithFilesWithStamp
+  docsWithGenFilesWithStamp
+)