diff --git a/pkgs/development/compilers/llvm/12/clang/default.nix b/pkgs/development/compilers/llvm/12/clang/default.nix
index 4721241730e..d6ee3752441 100644
--- a/pkgs/development/compilers/llvm/12/clang/default.nix
+++ b/pkgs/development/compilers/llvm/12/clang/default.nix
@@ -17,6 +17,9 @@ let
       mv clang-* clang
       sourceRoot=$PWD/clang
       unpackFile ${clang-tools-extra_src}
+      mv clang-tools-extra-* $sourceRoot/tools/extra
+      substituteInPlace $sourceRoot/tools/extra/clangd/quality/CompletionModel.cmake \
+        --replace ' ''${CMAKE_SOURCE_DIR}/../clang-tools-extra' ' ''${CMAKE_SOURCE_DIR}/tools/extra'
     '';
 
     nativeBuildInputs = [ cmake python3 ]