diff --git a/clang-tools-extra/clangd/quality/CompletionModel.cmake b/clang-tools-extra/clangd/quality/CompletionModel.cmake
--- a/clang-tools-extra/clangd/quality/CompletionModel.cmake
+++ b/clang-tools-extra/clangd/quality/CompletionModel.cmake
@@ -4,8 +4,11 @@
 # ${CMAKE_CURRENT_BINARY_DIR}. The generated header
 # will define a C++ class called ${cpp_class} - which may be a
 # namespace-qualified class name.
+
+# TODO: use CMAKE_CURRENT_FUNCTION_LIST_DIR after CMake 3.17
+set(CompletionModelCodegen_py_dir ${CMAKE_CURRENT_LIST_DIR})
 function(gen_decision_forest model filename cpp_class)
-  set(model_compiler ${CMAKE_SOURCE_DIR}/../clang-tools-extra/clangd/quality/CompletionModelCodegen.py)
+  set(model_compiler ${CompletionModelCodegen_py_dir}/CompletionModelCodegen.py)
   
   set(output_dir ${CMAKE_CURRENT_BINARY_DIR})
   set(header_file ${output_dir}/${filename}.h)