Solved, by moving the decode() inside the lambda, and adding the `.template ` for GCC (MSVC was happy w/o it...)
A miraculous inspiration I guess. Unusual the ML is silent like this. Must have been a question asked the wrong way. --DD
static void decode(Decoder& d, std::variant<Ts...>& var) {
const size_t index = d.decodeUnionIndex();
if (index >= sizeof...(Ts)) {
throw avro::Exception("Invalid Union index");
}
boost::mp11::mp_with_index<sizeof...(Ts)>(
index, [&](auto Idx) {
constexpr size_t alternative_index = Idx;
avro::decode(d, var.template emplace<alternative_index>());
}
);
}