c++: Fix a pasto in the PR120471 fix [PR120940]

No idea how this slipped in, I'm terribly sorry.
Strangely nothing in the testsuite has caught this, so I've added
a new test for that.

2025-07-03  Jakub Jelinek  <jakub@redhat.com>

	PR c++/120940
	* typeck.cc (cp_build_array_ref): Fix a pasto.

	* g++.dg/parse/pr120940.C: New test.
	* g++.dg/warn/Wduplicated-branches9.C: New test.

(cherry picked from commit dc90649)

Author: jakubjelinek

gcc/cp/typeck.cc

@@ -3814,7 +3814,7 @@ cp_build_array_ref (location_t loc, tree array, tree idx,
       tree op0, op1, op2;
       op0 = TREE_OPERAND (array, 0);
       op1 = TREE_OPERAND (array, 1);
-      op2 = TREE_OPERAND (array, 1);
+      op2 = TREE_OPERAND (array, 2);
       if (TREE_SIDE_EFFECTS (idx) || !tree_invariant_p (idx))
 	{
 	  /* If idx could possibly have some SAVE_EXPRs, turning

gcc/testsuite/g++.dg/parse/pr120940.C

@@ -0,0 +1,18 @@
+// PR c++/120940
+// { dg-do run }
+
+int a[8] = { 1, 2, 3, 4, 5, 6, 7, 8 };
+int b[8] = { 9, 10, 11, 12, 13, 14, 15, 16 };
+
+__attribute__((noipa)) int
+foo (int x, int y)
+{
+  return (x ? a : b)[y];
+}
+
+int
+main ()
+{
+  if (foo (1, 4) != 5 || foo (0, 6) != 15)
+    __builtin_abort ();
+}

gcc/testsuite/g++.dg/warn/Wduplicated-branches9.C

@@ -0,0 +1,11 @@
+// PR c++/120940
+// { dg-do compile }
+// { dg-options "-Wduplicated-branches" }
+
+static char a[16][8], b[16][8];
+
+char *
+foo (int x, int y)
+{
+  return (x ? a : b)[y];
+}