Proposed resolution:

This wording is relative to N5014.

1. Modify [exec.then] as indicated:

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

   -5- Effects: Equivalent to:

   auto cs = get_completion_signatures<child-type<Sndr>, FWD-ENV-T(Env)...>();
   auto fn = []<class... Ts>(set_value_tdecayed-typeof<set-cpo>(*)(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 following has been reported by Trevor Gray:

In [exec.then] p5, the impls-for<decayed-typeof<then-cpo>>::check-types unction 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>.