P3923R0: Re NB US 221-339 (C++26 CD)

Fixes NB US 221-339 (C++26 CD).

Author: eisenwave

source/exec.tex

@@ -4204,6 +4204,27 @@
 if \tcode{Tag} denotes a type other than \tcode{set_value_t} or
 if the expression \tcode{f(auto(shape), auto(shape), args...)} is well-formed.
 
+\begin{itemdecl}
+template<class Sndr, class... Env>
+  static consteval void \exposid{check-types}();
+\end{itemdecl}
+
+\begin{itemdescr}
+\pnum
+\effects
+Equivalent to:
+\begin{codeblock}
+auto cs = get_completion_signatures<@\exposid{child-type}@<Sndr>, @\exposid{FWD-ENV-T}@(Env)...>();
+auto fn = []<class... Ts>(set_value_t(*)(Ts...)) {
+  using data_type = @\exposid{data-type}@<Sndr>;
+  if constexpr (!invocable<remove_cvref_t<@\exposid{child-type}@<data_type>&,
+                           remove_cvref_t<@\exposid{data-type}@<data_type>>, Ts&...>)
+    throw @\placeholder{unspecified-exception}@();
+};
+cs.@\exposid{for-each}@(@\exposid{overload-set}@(fn, [](auto){}));
+\end{codeblock}
+\end{itemdescr}
+
 \pnum
 The exposition-only class template \exposid{impls-for}\iref{exec.snd.expos}
 is specialized for \tcode{bulk_unchunked_t} as follows:
@@ -4212,6 +4233,9 @@
   template<>
   struct @\exposid{impls-for}@<bulk_unchunked_t> : @\exposid{default-impls}@ {
     static constexpr auto @\exposid{complete}@ = @\seebelow@;
+
+    template<class Sndr, class... Env>
+      static consteval void \exposid{check-types}();
   };
 }
 \end{codeblock}
@@ -4253,7 +4277,9 @@
 \begin{codeblock}
 auto cs = get_completion_signatures<@\exposid{child-type}@<Sndr>, @\exposid{FWD-ENV-T}@(Env)...>();
 auto fn = []<class... Ts>(set_value_t(*)(Ts...)) {
-  if constexpr (!@\libconcept{invocable}@<remove_cvref_t<@\exposid{data-type}@<Sndr>>, Ts&...>)
+  using data_type = @\exposid{data-type}@<Sndr>;
+  if constexpr (!@\libconcept{invocable}@<remove_cvref_t<@\exposid{child-type}@<data_type>&,
+                           remove_cvref_t<@\exposid{data-type}@<data_type>>, Ts&...>)
     throw @\placeholder{unspecified-exception}@();
 };
 cs.@\exposid{for-each}@(@\exposid{overload-set}@(fn, [](auto){}));