cgen: move the condition of an iterator to a temp var (#8989)

vlib/v/gen/c/cgen.v

@@ -1480,16 +1480,20 @@ fn (mut g Gen) for_in_stmt(node ast.ForInStmt) {
 			return
 		}
 		ret_typ := next_fn.return_type
+		t_expr := g.new_tmp_var()
+		g.write('${g.typ(node.cond_type)} $t_expr = ')
+		g.expr(node.cond)
+		g.writeln(';')
 		g.writeln('while (1) {')
 		t := g.new_tmp_var()
-		receiver_styp := g.typ(next_fn.params[0].typ)
+		receiver_typ := next_fn.params[0].typ
+		receiver_styp := g.typ(receiver_typ)
 		fn_name := receiver_styp.replace_each(['*', '', '.', '__']) + '_next'
 		g.write('\t${g.typ(ret_typ)} $t = ${fn_name}(')
-		if !node.cond_type.is_ptr() {
+		if !node.cond_type.is_ptr() && receiver_typ.is_ptr() {
 			g.write('&')
 		}
-		g.expr(node.cond)
-		g.writeln(');')
+		g.writeln('$t_expr);')
 		g.writeln('\tif (!${t}.ok) { break; }')
 		val := if node.val_var in ['', '_'] { g.new_tmp_var() } else { node.val_var }
 		val_styp := g.typ(node.val_type)

vlib/v/tests/for_in_iterator_test.v

@@ -34,3 +34,11 @@ fn test_for_in_empty_iterator() {
 	}
 	assert vals == []
 }
+
+fn test_for_in_iterator_with_tmp_expr() {
+	mut vals := []int{}
+	for val in doubler(2, 64) {
+		vals << val
+	}
+	assert vals == [2, 4, 8, 16, 32, 64]
+}