this issue is from Trevor Gray.

in [exec.then]/p5, the impls-for<decayed-typeof<then-cpo>>::check-types function is specified as follows:

template<class Sndr, class... Env>
  static consteval void check-types();

Effects: Equivalent to:

auto cs = get_completion_signatures<child-type<Sndr>, FWD-ENV-T(Env)...>();
auto fn = []<class... Ts>(set_value_t(*)(Ts...)) {
  if constexpr (!invocable<remove_cvref_t<data-type<Sndr>>, Ts...>)
    throw unspecified-exception();
};
cs.for-each(overload-set{fn, [](auto){}});

where unspecified-exception is a type derived from exception.

the line auto fn = []<class... Ts>(set_value_t(*)(Ts...)) { is correct when then-cpo is then but not when it is upon_error or upon_stopped.

for upon_error it should be:

auto fn = []<class... Ts>(set_error_t(*)(Ts...)) {

and for upon_stopped it should be:

auto fn = []<class... Ts>(set_stopped_t(*)(Ts...)) {

We can achieve that by replacing set_value_t in the problematic line with decayed-typeof<set-cpo>.

Proposed resolution

In [exec.then]/p5, change the line:

auto fn = []<class... Ts>(set_value_t(*)(Ts...)) {

to:

auto fn = []<class... Ts>(decayed-typeof<set-cpo>(*)(Ts...)) {

(where decayed-typeof and set-cpo are in italics).