gcc implements the letter of the standard and allows this:

template <typename... T>
void foo(T&... ts) {
    [...&us=ts]{};
}

While that is valid per the latest working draft, and is the wording specified in P0780R2, the direction we really want to go in is to put the ... on the other side of the &. That is, the declaration of 'us' should look the same as the declaration of 'ts'. This is CWG 2378.

In other words, the correct syntax should be:

template <typename... T>
void foo(T&... ts) {
    [&...us=ts]{};
}

clang implements the CWG 2378 rule, this issue should hopefully be resolved by the time C++20 ships.