eval: switch up ExprOpConcatLists error position again

This is a minor adjustment to 2b22dae1ba /
I3e9d7c1c7a6599a8e68302448bbb961d051002b7

Basically having the error point to the operand instead of the operator
kind of makes sense, but it would require error spans to *truly* make
sense, and in the meantime maintaining the logic is more hassle than it
is worth.

Change-Id: Idac98dd77a0f4a6cb386cb74e0d4eb5fd2f503e5

lix/libexpr/eval-expr.cc

@@ -356,12 +356,14 @@ Value ExprOpConcatLists::eval(EvalState & state, Env & env)
 {
     state.ctx.stats.nrListConcats++;
 
-    /* We don't call into `concatLists` as that loses the position information of the expressions. */
+    /* We could simply call into `state.concatLists`, but that would add a redundant trace to our errors,
+     * and to fix that we would need to make the error on it and `forceList` optional, and *sigh*
+     */
 
     Value v1 = e1->eval(state, env);
-    state.checkList(v1, env, *e1);
+    state.checkList(v1, env, *this); // Pass in `this` instead of `e1` to make the error point to the `++`
     Value v2 = e2->eval(state, env);
-    state.checkList(v2, env, *e2);
+    state.checkList(v2, env, *this); // Pass in `this` instead of `e2` to make the error point to the `++`
 
     size_t l1 = v1.listSize(), l2 = v2.listSize(), len = l1 + l2;
 

tests/functional2/lang/list/eval-fail-2.err.exp

@@ -1,5 +1,5 @@
 error: expected a list but found an integer: 8
-       at /pwd/in.nix:1:1:
+       at /pwd/in.nix:1:2:
             1| 8++1
-             | ^
+             |  ^
             2|