improve the diagnostics of invalid fork expressions (#1544)

Author: ericniebler

include/exec/fork.hpp

@@ -117,7 +117,7 @@ namespace exec {
     struct _opstate_t {
       using operation_state_concept = stdexec::operation_state_t;
       using _env_t = stdexec::__call_result_t<stdexec::__env::__fwd_fn, stdexec::env_of_t<Rcvr>>;
-      using _child_completions_t = stdexec::completion_signatures_of_t<Sndr, _env_t>;
+      using _child_completions_t = stdexec::__completion_signatures_of_t<Sndr, _env_t>;
       using _domain_t = stdexec::__early_domain_of_t<Sndr, stdexec::__none_such>;
       using _when_all_sndr_t = fork_t::_when_all_sndr_t<_child_completions_t, Closures, _domain_t>;
       using _child_opstate_t =
@@ -233,10 +233,12 @@ namespace exec {
       using namespace stdexec;
       using _domain_t = __early_domain_of_t<Sndr, __none_such>;
       using _child_t = __copy_cvref_t<Self, Sndr>;
-      using _child_completions_t = completion_signatures_of_t<_child_t, __fwd_env_t<Env>...>;
+      using _child_completions_t = __completion_signatures_of_t<_child_t, __fwd_env_t<Env>...>;
       using __decay_copyable_results_t = stdexec::__decay_copyable_results_t<_child_completions_t>;
 
-      if constexpr (!__decay_copyable_results_t::value) {
+      if constexpr (!stdexec::__valid_completion_signatures<_child_completions_t>) {
+        return _child_completions_t{};
+      } else if constexpr (!__decay_copyable_results_t::value) {
         return _ERROR_<
           _WHAT_<>(PREDECESSOR_RESULTS_ARE_NOT_DECAY_COPYABLE),
           _IN_ALGORITHM_(exec::fork_t)>();