Tests: Extend BuildDepends test to cover OBJECT_DEPENDS

The actual file timestamp dependency is known to not work on
Visual Studio or Xcode generators.  Tolerate such failure for
these generators (Tests/CustomCommand already covers using
OBJECT_DEPENDS to pull a custom command into a target, and
that still works with these generators).

Tests/BuildDepends/CMakeLists.txt

@@ -69,6 +69,9 @@ file(WRITE ${BuildDepends_BINARY_DIR}/Project/link_depends_no_shared_exe.h
   "#define link_depends_no_shared_exe_value 0\n")
 set(link_depends_no_shared_check_txt ${BuildDepends_BINARY_DIR}/Project/link_depends_no_shared_check.txt)
 
+file(WRITE ${BuildDepends_BINARY_DIR}/Project/object_depends.txt "0\n")
+set(object_depends_check_txt ${BuildDepends_BINARY_DIR}/Project/object_depends_check.txt)
+
 file(WRITE ${BuildDepends_BINARY_DIR}/Project/external.in "external original\n")
 file(WRITE ${BuildDepends_BINARY_DIR}/Project/multi1-in.txt "multi1-in original\n")
 file(WRITE ${BuildDepends_BINARY_DIR}/Project/multi2-stamp.txt "multi2-stamp original\n")
@@ -246,6 +249,8 @@ file(WRITE ${BuildDepends_BINARY_DIR}/Project/zot_macro_tgt.hxx
 file(WRITE ${BuildDepends_BINARY_DIR}/Project/link_depends_no_shared_lib.h
   "#define link_depends_no_shared_lib_value 0\n")
 
+file(WRITE ${BuildDepends_BINARY_DIR}/Project/object_depends.txt "1\n")
+
 if(TEST_LINK_DEPENDS)
   file(WRITE ${TEST_LINK_DEPENDS} "2")
 endif()
@@ -359,6 +364,20 @@ else()
     "Targets link_depends_no_shared_lib and link_depends_no_shared_exe not both built.")
 endif()
 
+if(EXISTS "${object_depends_check_txt}")
+  file(STRINGS "${object_depends_check_txt}" object_depends_check LIMIT_COUNT 1)
+  if("${object_depends_check}" STREQUAL "1")
+    message(STATUS "object_depends exe is newer than object_depends.txt as expected.")
+  elseif(CMAKE_GENERATOR MATCHES "Visual Studio|Xcode")
+    message(STATUS "Known limitation: OBJECT_DEPENDS does not work on ${CMAKE_GENERATOR}")
+  else()
+    message(SEND_ERROR "Project did not rebuild properly: object_depends exe is not newer than object_depends.txt.")
+  endif()
+else()
+  message(SEND_ERROR "Project did not rebuild properly.  "
+    "object_depends exe and object_depends.txt are not both present.")
+endif()
+
 if(EXISTS ${BuildDepends_BINARY_DIR}/Project/external.out)
   file(STRINGS ${BuildDepends_BINARY_DIR}/Project/external.out external_out)
   if("${external_out}" STREQUAL "external changed")

Tests/BuildDepends/Project/CMakeLists.txt

@@ -182,3 +182,15 @@ if(TEST_MULTI3)
   set_property(SOURCE multi3-real.txt multi3-dummy.txt PROPERTY SYMBOLIC 1)
   add_custom_target(multi3 ALL DEPENDS multi3-real.txt)
 endif()
+
+add_executable(object_depends object_depends.cxx)
+set_property(SOURCE object_depends.cxx PROPERTY OBJECT_DEPENDS
+  ${CMAKE_CURRENT_BINARY_DIR}/object_depends.txt)
+add_custom_target(object_depends_check ALL
+  COMMAND ${CMAKE_COMMAND}
+   -Dexe=$<TARGET_FILE:object_depends>
+   -Dout=${CMAKE_CURRENT_BINARY_DIR}/object_depends_check.txt
+   -Dtxt=${CMAKE_CURRENT_BINARY_DIR}/object_depends.txt
+   -P ${CMAKE_CURRENT_SOURCE_DIR}/object_depends_check.cmake
+  )
+add_dependencies(object_depends_check object_depends)

Tests/BuildDepends/Project/object_depends.cxx

@@ -0,0 +1 @@
+int main() { return 0; }

Tests/BuildDepends/Project/object_depends_check.cmake

@@ -0,0 +1,7 @@
+if(NOT EXISTS "${txt}" OR NOT EXISTS "${exe}")
+  file(REMOVE "${out}")
+elseif("${exe}" IS_NEWER_THAN "${txt}")
+  file(WRITE "${out}" "1\n")
+else()
+  file(WRITE "${out}" "0\n")
+endif()