fix

    // Break self-referential expansion cycles. GNU make aborts with

    // "Recursive variable references itself"; we instead treat the recursive

    // reference as empty so a single legitimate self-append still terminates.

    // thread_local so concurrent recipe/export expansions under -j each get

    // their own recursion guard rather than corrupting a shared set.

    static thread_local std::set<std::string> expanding_;

    if (!expanding_.insert(name).second) return "";

    std::string out = do_expand(value, mf, auto_vars);

    expanding_.erase(name);

    // break self-referential cycles (e.g. "X = $(X) more") that would otherwise

    // recurse until the process exhausts memory. Matches GNU make, which refuses

    // to expand a variable that references itself.

    mutable std::set<std::string> expanding_;

    // (The recursion guard itself lives as a thread_local inside

    // expand_recursive_var so concurrent expansions under -j stay independent.)

    // Expand a recursively-flavored variable's value while guarding against

    // cycles; returns "" if 'name' is already on the expansion stack.

    std::string expand_recursive_var(const std::string& name, const std::string& value,