diff --git a/src/libexpr/eval.cc b/src/libexpr/eval.cc index 5351ca0c34..517a1151ec 100644 --- a/src/libexpr/eval.cc +++ b/src/libexpr/eval.cc @@ -922,11 +922,19 @@ void ExprOpConcatLists::eval(EvalState & state, Env & env, Value & v) void EvalState::concatLists(Value & v, unsigned int nrLists, Value * * lists) { nrListConcats++; - + + Value * nonEmpty = 0; unsigned int len = 0; for (unsigned int n = 0; n < nrLists; ++n) { forceList(*lists[n]); - len += lists[n]->list.length; + unsigned int l = lists[n]->list.length; + len += l; + if (l) nonEmpty = lists[n]; + } + + if (nonEmpty && len == nonEmpty->list.length) { + v = *nonEmpty; + return; } mkList(v, len);