c++filt and boost demangle failure

#include <iostream>
#include <boost/core/demangle.hpp>

template <typename T>
auto mangled (T t) {
    return typeid(t).name();
}

template <typename T>
auto demangled (T t) {
    return boost::core::demangle(mangled(t));
}

#define SYM struct { using symbolic = void; }
#define TSYM(Name) struct Name { using symbolic = void; }

template <typename... Us> TSYM(ListType);

template <typename... Ts>
auto list (Ts...) {
    return ListType<Ts...>{};
}

template <typename T, typename U>
auto operator+ (T t, U u) {
    SYM plus;
    return list(plus, t, u);
}

int main () {
    SYM a;
    SYM b;
    auto sum1 = a + b;
    auto sum2 = a + b + a;

    std::cout << mangled(a) << std::endl
              << demangled(a) << std::endl
              << mangled(b) << std::endl
              << demangled(b) << std::endl
              << mangled(sum1) << std::endl
              << demangled(sum1) << std::endl
              << mangled(sum2) << std::endl
              << demangled(sum2) << std::endl;
    return 0;
}

/* OUTPUT
Z4mainEUt_
main::{unnamed type#1}
Z4mainEUt0_
main::{unnamed type#2}
8ListTypeIJZplIZ4mainEUt_Z4mainEUt0_EDaT_T0_EUt_S1_S2_EE
ListType<auto operator+<main::{unnamed type#1}, main::{unnamed type#2}>(main::{unnamed type#1}, main::{unnamed type#2})::{unnamed type#1}, {unnamed type#1}, main::{unnamed type#1}>
8ListTypeIJZplIS_IJZS0_IZ4mainEUt_Z4mainEUt0_EDaT_T0_EUt_S1_S2_EES1_EDaS3_S4_EUt_S6_S1_EE
8ListTypeIJZplIS_IJZS0_IZ4mainEUt_Z4mainEUt0_EDaT_T0_EUt_S1_S2_EES1_EDaS3_S4_EUt_S6_S1_EE
*/

/* OUTPUT PIPED TO 'c++filt -t'
main::{unnamed type#1}
main::{unnamed type#1}
main::{unnamed type#2}
main::{unnamed type#2}
ListType<auto operator+<main::{unnamed type#1}, main::{unnamed type#2}>(main::{unnamed type#1}, main::{unnamed type#2})::{unnamed type#1}, {unnamed type#1}, main::{unnamed type#1}>
ListType<auto operator+<main::{unnamed type#1}, main::{unnamed type#2}>(main::{unnamed type#1}, main::{unnamed type#2})::{unnamed type#1}, {unnamed type#1}, main::{unnamed type#1}>
8ListTypeIJZplIS_IJZS0_IZ4mainEUt_Z4mainEUt0_EDaT_T0_EUt_S1_S2_EES1_EDaS3_S4_EUt_S6_S1_EE
8ListTypeIJZplIS_IJZS0_IZ4mainEUt_Z4mainEUt0_EDaT_T0_EUt_S1_S2_EES1_EDaS3_S4_EUt_S6_S1_EE
*/