// Phase 29ad P0: Pattern6 reverse scan contract violation
// Expect: JoinIR freeze (reverse step must be i = i - 1)

static box Main {
    last_index_bad_step(s, ch) {
        local i = s.length() - 1
        loop(i >= 0) {
            if s.substring(i, i + 1) == ch {
                return i
            }
            i = i - 2
        }
        return -1
    }

    main() {
        return Main.last_index_bad_step("abc", "b")
    }
}