FetchContent: Relative SOURCE_DIR override cannot be a hard error

Fixes: #21624

Modules/FetchContent.cmake

@@ -1053,7 +1053,14 @@ function(FetchContent_Populate contentName)
     # so no population is required. The build directory may still be specified
     # by the declared details though.
 
-    if(NOT EXISTS "${FETCHCONTENT_SOURCE_DIR_${contentNameUpper}}")
+    if(NOT IS_ABSOLUTE "${FETCHCONTENT_SOURCE_DIR_${contentNameUpper}}")
+      # Don't check this directory because we don't know what location it is
+      # expected to be relative to. We can't make this a hard error for backward
+      # compatibility reasons.
+      message(WARNING "Relative source directory specified. This is not safe, "
+        "as it depends on the calling directory scope.\n"
+        "  FETCHCONTENT_SOURCE_DIR_${contentNameUpper} --> ${FETCHCONTENT_SOURCE_DIR_${contentNameUpper}}")
+    elseif(NOT EXISTS "${FETCHCONTENT_SOURCE_DIR_${contentNameUpper}}")
       message(FATAL_ERROR "Manually specified source directory is missing:\n"
         "  FETCHCONTENT_SOURCE_DIR_${contentNameUpper} --> ${FETCHCONTENT_SOURCE_DIR_${contentNameUpper}}")
     endif()

Tests/RunCMake/FetchContent/ManualSourceDirectoryRelative-stderr.txt

@@ -0,0 +1,3 @@
+ *Relative source directory specified.  This is not safe, as it depends on
+ *the calling directory scope.
++ *FETCHCONTENT_SOURCE_DIR_WITHPROJECT --> WithProject

Tests/RunCMake/FetchContent/ManualSourceDirectoryRelative.cmake

@@ -0,0 +1 @@
+include(ManualSourceDirectory.cmake)

Tests/RunCMake/FetchContent/RunCMakeTest.cmake

@@ -21,6 +21,11 @@ run_cmake_with_options(ManualSourceDirectory
 run_cmake_with_options(ManualSourceDirectoryMissing
   -D "FETCHCONTENT_SOURCE_DIR_WITHPROJECT=${CMAKE_CURRENT_LIST_DIR}/ADirThatDoesNotExist"
 )
+# Need to use :STRING to prevent CMake from automatically converting it to an
+# absolute path
+run_cmake_with_options(ManualSourceDirectoryRelative
+  -D "FETCHCONTENT_SOURCE_DIR_WITHPROJECT:STRING=WithProject"
+)
 
 function(run_FetchContent_DirOverrides)
   set(RunCMake_TEST_BINARY_DIR ${RunCMake_BINARY_DIR}/DirOverrides-build)