Merge pull request #159 from cplusplus/port-read-to-basic-sender

port the `read` sender factory to `basic-sender`

execution.bs

@@ -5773,55 +5773,20 @@ template<class Domain, class Tag, sender Sender, class... Args>
     completes synchronously in its start operation with a value completion
     result equal to a value read from the receiver's associated environment.
 
-2. `read` is a customization point object of the unspecified class type:
+2. `read` is a customization point object. For some query object `q` of type `Q`,
+    the expression `read(q)` is expression-equivalent to
+    <code><i>make-sender</i>(read, q)</code>.
 
-    <pre highlight="c++">
-    template&lt;class Tag>
-      struct <i>read-sender</i>; // exposition only
-
-    struct <i>read-t</i> { // exposition only
-      template&lt;class Tag>
-        constexpr <i>read-sender</i>&lt;Tag> operator()(Tag) const noexcept {
-          return {};
-        }
-    };
-    </pre>
-
-3. <code><i>read-sender</i></code> is the exposition-only class template:
+3. The exposition-only class template <code><i>impls-for</i></code> ([exec.snd.general])
+    is specialized for `read` as follows:
 
     <pre highlight="c++">
-    template&lt;class Tag>
-      struct <i>read-sender</i> { // exposition only
-        using sender_concept = sender_t;
-        template&lt;class R>
-          struct <i>operation-state</i> { // exposition only
-            R r_; // exposition only
-
-            friend void tag_invoke(start_t, <i>operation-state</i>& s) noexcept {
-              <i>TRY-SET-VALUE</i>(std::move(s.r_), Tag{}(get_env(s.r_)));
-            }
-          };
-
-        template&lt;receiver R>
-        friend <i>operation-state</i>&lt;decay_t&lt;R>> tag_invoke(connect_t, <i>read-sender</i>, R && r) {
-          return { std::forward&lt;R>(r) };
-        }
-
-        template&lt;class Env>
-            requires <i>callable</i>&lt;Tag, Env>
-          friend auto tag_invoke(get_completion_signatures_t, <i>read-sender</i>, Env)
-            -> completion_signatures&lt;
-              set_value_t(<i>call-result-t</i>&lt;Tag, Env>), set_error_t(exception_ptr)>; <i>// not defined</i>
-
-        template&lt;class Env>
-            requires <i>nothrow-callable</i>&lt;Tag, Env>
-          friend auto tag_invoke(get_completion_signatures_t, <i>read-sender</i>, Env)
-            -> completion_signatures&lt;set_value_t(<i>call-result-t</i>&lt;Tag, Env>)>; <i>// not defined</i>
-
-        friend empty_env tag_invoke(get_env_t, const <i>read-sender</i>&) noexcept {
-          return {};
-        }
+    template&lt;>
+    struct <i>impls-for</i>&lt;tag_t&lt;read>> : <i>default-impls</i> {
+      static constexpr auto start = [](auto query, auto& rcvr) noexcept -> void {
+        <i>TRY-SET-VALUE</i>(std::move(rcvr), query(get_env(rcvr)));
       };
+    };
     </pre>
 
 ### Sender adaptors <b>[exec.adapt]</b> ### {#spec-execution.senders.adapt}