Merge 2025-06 LWG Motion 22

P3682R0 Remove std::execution::split

Author: tkoeppe

source/exec.tex

@@ -633,7 +633,6 @@
   struct @\libglobal{bulk_t}@ { @\unspec@ };
   struct @\libglobal{bulk_chunked_t}@ { @\unspec@ };
   struct @\libglobal{bulk_unchunked_t}@ { @\unspec@ };
-  struct @\libglobal{split_t}@ { @\unspec@ };
   struct @\libglobal{when_all_t}@ { @\unspec@ };
   struct @\libglobal{when_all_with_variant_t}@ { @\unspec@ };
   struct @\libglobal{into_variant_t}@ { @\unspec@ };
@@ -657,7 +656,6 @@
   inline constexpr bulk_t @\libglobal{bulk}@{};
   inline constexpr bulk_chunked_t @\libglobal{bulk_chunked}@{};
   inline constexpr bulk_unchunked_t @\libglobal{bulk_unchunked}@{};
-  inline constexpr split_t @\libglobal{split}@{};
   inline constexpr when_all_t @\libglobal{when_all}@{};
   inline constexpr when_all_with_variant_t @\libglobal{when_all_with_variant}@{};
   inline constexpr into_variant_t @\libglobal{into_variant}@{};
@@ -4014,377 +4012,6 @@
 ignore cancellation requests.
 \end{note}
 
-\rSec3[exec.split]{\tcode{execution::split}}
-
-\pnum
-\tcode{split} adapts an arbitrary sender
-into a sender that can be connected multiple times.
-
-\pnum
-Let \exposid{split-env} be the type of an environment
-such that, given an instance \tcode{env},
-the expression \tcode{get_stop_token(env)} is well-formed and
-has type \tcode{inplace_stop_token.}
-
-\pnum
-The name \tcode{split} denotes a pipeable sender adaptor object.
-For a subexpression \tcode{sndr}, let \tcode{Sndr} be \tcode{decltype((sndr))}.
-If \tcode{\libconcept{sender_in}<Sndr, \exposid{split-env}>} is \tcode{false},
-\tcode{split(sndr)} is ill-formed.
-
-\pnum
-Otherwise, the expression \tcode{split(sndr)} is expression-equivalent to:
-\begin{codeblock}
-transform_sender(@\exposid{get-domain-early}@(sndr), @\exposid{make-sender}@(split, {}, sndr))
-\end{codeblock}
-except that \tcode{sndr} is evaluated only once.
-\begin{note}
-The default implementation of \tcode{transform_sender}
-will have the effect of connecting the sender to a receiver.
-It will return a sender with a different tag type.
-\end{note}
-
-\pnum
-Let \exposid{local-state} denote the following exposition-only class template:
-
-\begin{codeblock}
-namespace std::execution {
-  struct @\exposid{local-state-base}@ {                                     // \expos
-    virtual ~@\exposid{local-state-base}@() = default;
-    virtual void @\exposid{notify}@() noexcept = 0;                         // \expos
-  };
-
-  template<class Sndr, class Rcvr>
-  struct @\exposid{local-state}@ : @\exposid{local-state-base}@ {                       // \expos
-    using @\exposid{on-stop-callback}@ =                                    // \expos
-      stop_callback_for_t<stop_token_of_t<env_of_t<Rcvr>>, @\exposid{on-stop-request}@>;
-
-    @\exposid{local-state}@(Sndr&& sndr, Rcvr& rcvr) noexcept;
-    ~@\exposid{local-state}@();
-
-    void @\exposid{notify}@() noexcept override;
-
-  private:
-    optional<@\exposid{on-stop-callback}@> @\exposid{on_stop}@;                         // \expos
-    @\exposid{shared-state}@<Sndr>* @\exposid{sh_state}@;                               // \expos
-    Rcvr* @\exposid{rcvr}@;                                                 // \expos
-  };
-}
-\end{codeblock}
-
-\begin{itemdecl}
-@\exposid{local-state}@(Sndr&& sndr, Rcvr& rcvr) noexcept;
-\end{itemdecl}
-
-\begin{itemdescr}
-\pnum
-\effects
-Equivalent to:
-\begin{codeblock}
-auto& [_, data, _] = sndr;
-this->@\exposid{sh_state}@ = data.sh_state.get();
-this->@\exposid{sh_state}@->@\exposid{inc-ref}@();
-this->@\exposid{rcvr}@ = addressof(rcvr);
-\end{codeblock}
-\end{itemdescr}
-
-\begin{itemdecl}
-~@\exposid{local-state}@();
-\end{itemdecl}
-
-\begin{itemdescr}
-\pnum
-\effects
-Equivalent to:
-\begin{codeblock}
-sh_state->@\exposid{dec-ref}@();
-\end{codeblock}
-\end{itemdescr}
-
-\begin{itemdecl}
-void @\exposid{notify}@() noexcept override;
-\end{itemdecl}
-
-\begin{itemdescr}
-\pnum
-\effects
-Equivalent to:
-\begin{codeblock}
-@\exposid{on_stop}@.reset();
-visit(
-  [this](const auto& tupl) noexcept -> void {
-    apply(
-      [this](auto tag, const auto&... args) noexcept -> void {
-        tag(std::move(*@\exposid{rcvr}@), args...);
-      },
-      tupl);
-  },
-  @\exposid{sh_state}@->result);
-\end{codeblock}
-\end{itemdescr}
-
-\pnum
-Let \exposid{split-receiver} denote
-the following exposition-only class template:
-\begin{codeblock}
-namespace std::execution {
-  template<class Sndr>
-  struct @\exposid{split-receiver}@ {                                       // \expos
-    using receiver_concept = receiver_t;
-
-    template<class Tag, class... Args>
-    void @\exposid{complete}@(Tag, Args&&... args) noexcept {               // \expos
-      using tuple_t = @\exposid{decayed-tuple}@<Tag, Args...>;
-      try {
-        @\exposid{sh_state}@->result.template emplace<tuple_t>(Tag(), std::forward<Args>(args)...);
-      } catch (...) {
-        using tuple_t = tuple<set_error_t, exception_ptr>;
-        @\exposid{sh_state}@->result.template emplace<tuple_t>(set_error, current_exception());
-      }
-      @\exposid{sh_state}@->notify();
-    }
-
-    template<class... Args>
-    void set_value(Args&&... args) && noexcept {
-      @\exposid{complete}@(execution::set_value, std::forward<Args>(args)...);
-    }
-
-    template<class Error>
-    void set_error(Error&& err) && noexcept {
-      @\exposid{complete}@(execution::set_error, std::forward<Error>(err));
-    }
-
-    void set_stopped() && noexcept {
-      @\exposid{complete}@(execution::set_stopped);
-    }
-
-    struct @\exposid{env}@ {                                                // \expos
-      @\exposid{shared-state}@<Sndr>* @\exposid{sh-state}@;                             // \expos
-
-      inplace_stop_token query(get_stop_token_t) const noexcept {
-        return @\exposid{sh-state}@->stop_src.get_token();
-      }
-    };
-
-    @\exposid{env}@ get_env() const noexcept {
-      return @\exposid{env}@{@\exposid{sh_state}@};
-    }
-
-    @\exposid{shared-state}@<Sndr>* @\exposid{sh_state}@;                               // \expos
-  };
-}
-\end{codeblock}
-
-\pnum
-Let \exposid{shared-state} denote the following exposition-only class template:
-\begin{codeblock}
-namespace std::execution {
-  template<class Sndr>
-  struct @\exposid{shared-state}@ {
-    using @\exposid{variant-type}@ = @\seebelow@;                             // \expos
-    using @\exposid{state-list-type}@ = @\seebelow@;                          // \expos
-
-    explicit @\exposid{shared-state}@(Sndr&& sndr);
-
-    void @\exposid{start-op}@() noexcept;                                   // \expos
-    void @\exposid{notify}@() noexcept;                                     // \expos
-    void @\exposid{inc-ref}@() noexcept;                                    // \expos
-    void @\exposid{dec-ref}@() noexcept;                                    // \expos
-
-    inplace_stop_source @\exposid{stop_src}@{};                             // \expos
-    @\exposid{variant-type}@ @\exposid{result}@{};                                      // \expos
-    @\exposid{state-list-type}@ @\exposid{waiting_states}@;                             // \expos
-    atomic<bool> @\exposid{completed}@{false};                              // \expos
-    atomic<size_t> @\exposid{ref_count}@{1};                                // \expos
-    connect_result_t<Sndr, @\exposid{split-receiver}@<Sndr>> @\exposid{op_state}@;      // \expos
-  };
-}
-\end{codeblock}
-
-\pnum
-Let \tcode{Sigs} be a pack of the arguments
-to the \tcode{completion_signatures} specialization
-named by \tcode{completion_signatures_of_t<Sndr>}.
-For type \tcode{Tag} and pack \tcode{Args},
-let \exposid{as-tuple} be an alias template
-such that \tcode{\exposid{as-tuple}<Tag(Args...)>} denotes
-the type \tcode{\exposid{decayed-tuple}<Tag, Args...>}.
-Then \exposid{variant-type} denotes the type
-\begin{codeblock}
-variant<tuple<set_stopped_t>, tuple<set_error_t, exception_ptr>, @\exposid{as-tuple}@<Sigs>...>
-\end{codeblock}
-but with duplicate types removed.
-
-\pnum
-Let \exposid{state-list-type} be a type
-that stores a list of pointers to \exposid{local-state-base} objects and
-that permits atomic insertion.
-
-\begin{itemdecl}
-explicit @\exposid{shared-state}@(Sndr&& sndr);
-\end{itemdecl}
-
-\begin{itemdescr}
-\pnum
-\effects
-Initializes \exposid{op_state} with the result of
-\tcode{connect(std::forward<Sndr>(sndr), \exposid{split-re\-ceiver}\{this\})}.
-
-\pnum
-\ensures
-\exposid{waiting_states} is empty, and \exposid{completed} is \tcode{false}.
-\end{itemdescr}
-
-\begin{itemdecl}
-void @\exposid{start-op}@() noexcept;
-\end{itemdecl}
-
-\begin{itemdescr}
-\pnum
-\effects
-Evaluates \tcode{\exposid{inc-ref}()}.
-If \tcode{stop_src.stop_requested()} is \tcode{true},
-evaluates \tcode{\exposid{notify}()};
-otherwise, evaluates \tcode{start(\exposid{op_state})}.
-\end{itemdescr}
-
-\begin{itemdecl}
-void @\exposid{notify}@() noexcept;
-\end{itemdecl}
-
-\begin{itemdescr}
-\pnum
-\effects
-Atomically does the following:
-\begin{itemize}
-\item
-Sets \tcode{completed} to \tcode{true}, and
-\item
-Exchanges \tcode{waiting_states} with an empty list,
-storing the old value in a local \tcode{prior_states}.
-\end{itemize}
-Then, for each pointer \tcode{p} in \tcode{prior_states},
-evaluates \tcode{p->\exposid{notify}()}.
-Finally, evaluates \tcode{\exposid{dec-ref}()}.
-\end{itemdescr}
-
-\begin{itemdecl}
-void @\exposid{inc-ref}@() noexcept;
-\end{itemdecl}
-
-\begin{itemdescr}
-\pnum
-\effects
-Increments \exposid{ref_count}.
-\end{itemdescr}
-
-\begin{itemdecl}
-void @\exposid{dec-ref}@() noexcept;
-\end{itemdecl}
-
-\begin{itemdescr}
-\pnum
-\effects
-Decrements \exposid{ref_count}.
-If the new value of \exposid{ref_count} is \tcode{0},
-calls \tcode{delete this}.
-
-\pnum
-\sync
-If an evaluation of \tcode{\exposid{dec-ref}()} does not
-decrement the \tcode{ref_count} to \tcode{0} then
-synchronizes with the evaluation of \tcode{dec-ref()}
-that decrements \tcode{ref_count} to \tcode{0}.
-\end{itemdescr}
-
-\pnum
-Let \exposid{split-impl-tag} be an empty exposition-only class type.
-Given an expression \tcode{sndr},
-the expression \tcode{split.transform_sender(sndr)} is equivalent to:
-\begin{codeblock}
-auto&& [tag, _, child] = sndr;
-auto* sh_state = new @\exposid{shared-state}@{std::forward_like<decltype((sndr))>(child)};
-return @\exposid{make-sender}@(@\exposid{split-impl-tag}@(), @\exposid{shared-wrapper}@{sh_state, tag});
-\end{codeblock}
-where \exposid{shared-wrapper} is an exposition-only class
-that manages the reference count of the \exposid{shared-state} object
-pointed to by sh_state.
-\exposid{shared-wrapper} models \libconcept{copyable}
-with move operations nulling out the moved-from object,
-copy operations incrementing the reference count
-by calling \tcode{sh_state->\exposid{inc-ref}()}, and
-assignment operations performing a copy-and-swap operation.
-The destructor has no effect if sh_state is null;
-otherwise, it decrements the reference count
-by evaluating \tcode{sh_state->\exposid{dec-ref}()}.
-
-\pnum
-The exposition-only class template \exposid{impls-for}\iref{exec.snd.general}
-is specialized for \exposid{split-impl-tag} as follows:
-\begin{codeblock}
-namespace std::execution {
-  template<>
-  struct @\exposid{impls-for}@<@\exposid{split-impl-tag}@> : @\exposid{default-impls}@ {
-    static constexpr auto @\exposid{get-state}@ = @\seebelow@;
-    static constexpr auto @\exposid{start}@ = @\seebelow@;
-  };
-}
-\end{codeblock}
-
-\pnum
-The member
-\tcode{\exposid{impls-for}<\exposid{split-impl-tag}>::\exposid{get-state}}
-is initialized with a callable object equivalent to
-the following lambda expression:
-\begin{codeblock}
-[]<class Sndr>(Sndr&& sndr, auto& rcvr) noexcept {
-  return @\exposid{local-state}@{std::forward<Sndr>(sndr), rcvr};
-}
-\end{codeblock}
-
-\pnum
-The member
-\tcode{\exposid{impls-for}<\exposid{split-impl-tag}>::\exposid{start}}
-is initialized with a callable object
-that has a function call operator equivalent to the following:
-\begin{codeblock}
-template<class Sndr, class Rcvr>
-void operator()(@\exposid{local-state}@<Sndr, Rcvr>& state, Rcvr& rcvr) const noexcept;
-\end{codeblock}
-
-\effects
-If \tcode{state.\exposid{sh_state}->\exposid{completed}} is \tcode{true},
-evaluates \tcode{state.\exposid{notify}()} and returns.
-Otherwise, does the following in order:
-\begin{itemize}
-\item
-Evaluates
-\begin{codeblock}
-state.@\exposid{on_stop}@.emplace(
-  get_stop_token(get_env(rcvr)),
-  @\exposid{on-stop-request}@{state.@\exposid{sh_state}@->@\exposid{stop_src}@});
-\end{codeblock}
-\item
-Then atomically does the following:
-\begin{itemize}
-\item
-Reads the value \tcode{c} of
-\tcode{state.\exposid{sh_state}->\exposid{completed}}, and
-\item
-Inserts \tcode{addressof(state)} into
-\tcode{state.\exposid{sh_state}->\exposid{waiting_states}}
-if \tcode{c} is \tcode{false}.
-\end{itemize}
-\item
-If \tcode{c} is \tcode{true},
-calls \tcode{state.\exposid{notify}()} and returns.
-\item
-Otherwise,
-if \tcode{addressof(state)} is the first item added to
-\tcode{state.\exposid{sh_state}->\exposid{waiting_states}},
-evaluates \tcode{state.\exposid{sh_state}->\exposid{start-op}()}.
-\end{itemize}
-
 \rSec3[exec.when.all]{\tcode{execution::when_all}}
 
 \pnum