make sync_wait hard error instead of SFINAE when passed a bad sender

cmake/clangd_compile_info.cmake

@@ -21,7 +21,7 @@ set(CMAKE_EXPORT_COMPILE_COMMANDS ON)
 # Symlink the compile command output to the source dir, where clangd will find it.
 set(compile_commands_file "${CMAKE_BINARY_DIR}/compile_commands.json")
 set(compile_commands_link "${CMAKE_SOURCE_DIR}/compile_commands.json")
-message(STATUS "Creating symlink from ${compile_commands_link} to ${compile_commands_file}...")
+message(STATUS "Creating symlink from \"${compile_commands_file}\" to \"${compile_commands_link}\"...")
 stdexec_execute_non_fatal_process(COMMAND
   "${CMAKE_COMMAND}" -E rm -f "${compile_commands_link}")
 stdexec_execute_non_fatal_process(COMMAND

include/stdexec/__detail/__basic_sender.hpp

@@ -140,7 +140,7 @@ namespace stdexec {
         };
 
       static constexpr auto get_completion_signatures = //
-        []<class _Sender>(_Sender&& __sndr, auto&&...) noexcept {
+        []<class _Sender>(_Sender&&, auto&&...) noexcept {
           static_assert(
             __mnever<tag_of_t<_Sender>>,
             "No customization of get_completion_signatures for this sender tag type.");

include/stdexec/__detail/__completion_signatures.hpp

@@ -46,7 +46,7 @@ namespace stdexec {
   concept __valid_completion_signatures = //
     __same_as<__ok_t<_Completions>, __msuccess> && __sigs::__is_completion_signatures<_Completions>;
 
-  template <class _Sender, class _Env>
+  template <class _Sender, class... _Env>
   using __unrecognized_sender_error = //
-    __mexception<_UNRECOGNIZED_SENDER_TYPE_<>, _WITH_SENDER_<_Sender>, _WITH_ENVIRONMENT_<_Env>>;
+    __mexception<_UNRECOGNIZED_SENDER_TYPE_<>, _WITH_SENDER_<_Sender>, _WITH_ENVIRONMENT_<_Env>...>;
 } // namespace stdexec

include/stdexec/__detail/__continues_on.hpp

@@ -63,7 +63,7 @@ namespace stdexec {
       }
 
       template <class _Sender, class _Env>
-      static auto transform_sender(_Sender&& __sndr, const _Env& __env) {
+      static auto transform_sender(_Sender&& __sndr, const _Env&) {
         return __sexpr_apply(static_cast<_Sender&&>(__sndr), __transform_sender_fn());
       }
     };

include/stdexec/__detail/__env.hpp

@@ -24,8 +24,8 @@
 #include "__tag_invoke.hpp"
 #include "__tuple.hpp"
 
-#include <type_traits>
 #include <functional> // IWYU pragma: keep for unwrap_reference_t
+#include <type_traits>
 #include <utility>
 
 STDEXEC_PRAGMA_PUSH()
@@ -211,7 +211,7 @@ namespace stdexec {
     struct get_domain_t {
       template <class _Ty>
         requires tag_invocable<get_domain_t, const _Ty&>
-      constexpr auto operator()(const _Ty& __ty) const noexcept
+      constexpr auto operator()(const _Ty&) const noexcept
         -> __decay_t<tag_invoke_result_t<get_domain_t, const _Ty&>> {
         static_assert(
           nothrow_tag_invocable<get_domain_t, const _Ty&>,
@@ -361,8 +361,6 @@ namespace stdexec {
       STDEXEC_ATTRIBUTE((nodiscard)) constexpr auto query(_Query) const noexcept -> const _Value& {
         return __value;
       }
-
-      auto operator=(const prop&) -> prop& = delete;
     };
 
     template <class _Query, class _Value>
@@ -398,8 +396,6 @@ namespace stdexec {
         return tag_invoke(
           __q, env::__get_1st<_Query, _Args...>(*this), static_cast<_Args&&>(__args)...);
       }
-
-      auto operator=(const env&) -> env& = delete;
     };
 
     // specialization for two envs to avoid warnings about elided braces
@@ -433,8 +429,6 @@ namespace stdexec {
         return tag_invoke(
           __q, env::__get_1st<_Query, _Args...>(*this), static_cast<_Args&&>(__args)...);
       }
-
-      auto operator=(const env&) -> env& = delete;
     };
 
     template <class... _Envs>
@@ -461,8 +455,6 @@ namespace stdexec {
       auto query(_Key) const noexcept -> const _Value& {
         return __value_;
       }
-
-      auto operator=(const __with&) -> __with& = delete;
     };
 
     template <class _Value, class _Tag, class... _Tags>
@@ -494,8 +486,6 @@ namespace stdexec {
           -> tag_invoke_result_t<_Tag, __cvref_env_t> {
           return tag_invoke(_Tag(), __env_);
         }
-
-        auto operator=(const __t&) -> __t& = delete;
       };
     };
 
@@ -539,8 +529,6 @@ namespace stdexec {
           query(_Key) const noexcept(nothrow_tag_invocable<_Key, __cvref_env_t>) -> decltype(auto) {
           return tag_invoke(_Key(), __env_);
         }
-
-        auto operator=(const __t&) -> __t& = delete;
       };
     };
 
@@ -573,8 +561,6 @@ namespace stdexec {
         -> __call_result_t<const _Fun&, _Tag> {
         return __fun_(_Tag());
       }
-
-      auto operator=(const __from&) -> __from& = delete;
     };
 
     template <class _Fun>

include/stdexec/__detail/__senders.hpp

@@ -30,6 +30,7 @@
 #include "__transform_completion_signatures.hpp"
 #include "__transform_sender.hpp"
 #include "__type_traits.hpp"
+#include <type_traits>
 
 namespace stdexec {
   /////////////////////////////////////////////////////////////////////////////
@@ -335,4 +336,54 @@ namespace stdexec {
   template <class _Sender>
   concept __well_formed_sender = __detail::__well_formed_sender<
     __minvoke<__with_default_q<__completion_signatures_of_t, dependent_completions>, _Sender>>;
+
+  // Used to report a meaningful error message when the sender_in<Sndr, Env> concept check fails.
+  template <class _Sender, class... _Env>
+  auto __diagnose_sender_concept_failure() {
+    if constexpr (!enable_sender<_Sender>) {
+      static_assert(
+        enable_sender<_Sender>,
+        "The given type is not a sender because stdexec::enable_sender<Sender> is false. Either "
+        "give the type a nested ::sender_concept typedef that is an alias for stdexec::sender_t, "
+        "or else specialize the stdexec::enable_sender boolean trait for this type to true.");
+    } else if constexpr (!__detail::__consistent_completion_domains<_Sender>) {
+      static_assert(
+        __detail::__consistent_completion_domains<_Sender>,
+        "The completion schedulers of the sender do not have consistent domains. This is likely a "
+        "bug in the sender implementation.");
+    } else if constexpr (!move_constructible<__decay_t<_Sender>>) {
+      static_assert(
+        move_constructible<__decay_t<_Sender>>, //
+        "The sender type is not move-constructible.");
+    } else if constexpr (!constructible_from<__decay_t<_Sender>, _Sender>) {
+      static_assert(
+        constructible_from<__decay_t<_Sender>, _Sender>,
+        "The sender cannot be decay-copied. Did you forget a std::move?");
+    } else {
+      using _Completions = __completion_signatures_of_t<_Sender, _Env...>;
+      if constexpr (__same_as<_Completions, __unrecognized_sender_error<_Sender, _Env...>>) {
+        static_assert(
+          __mnever<_Completions>,
+          "The sender type was not able to report its completion signatures when asked. This is "
+          "either because it lacks the necessary member functions, or because the member functions "
+          "were ill-formed.\n\nA sender can declare its completion signatures in one of two ways:\n"
+          "  1. By defining a nested type alias named `completion_signatures` that is a\n"
+          "     specialization of stdexec::completion_signatures<...>.\n"
+          "  2. By defining a member function named `get_completion_signatures` that returns a\n"
+          "     specialization of stdexec::completion_signatures<...>.");
+      } else if constexpr (__merror<_Completions>) {
+        static_assert(
+          !__merror<_Completions>,
+          "Trying to compute the sender's completion signatures resulted in an error. See the rest "
+          "of the compiler diagnostic for clues. Look for the string \"_ERROR_\".");
+        // MSVC needs more encouragement to print the type of the error.
+        STDEXEC_MSVC(_Completions __what = 0;)
+      } else {
+        static_assert(
+          __valid_completion_signatures<_Completions>,
+          "The stdexec::sender_in<Sender, Environment> concept check has failed. This is likely a "
+          "bug in the sender implementation.");
+      }
+    }
+  }
 } // namespace stdexec

include/stdexec/__detail/__sync_wait.hpp

@@ -178,55 +178,61 @@ namespace stdexec {
       _Sender,
       __env>>;
 
-#if STDEXEC_EDG()
-    // It requires some hoop-jumping to get the NVHPC compiler to report a meaningful
-    // diagnostic for SFINAE failures.
-    template <class _Sender>
-    auto __diagnose_error() {
-      if constexpr (!sender_in<_Sender, __env>) {
-        using _Completions = __completion_signatures_of_t<_Sender, __env>;
-        if constexpr (__merror<_Completions>) {
-          return _Completions();
-        } else {
-          constexpr __mstring __diag =
-            "The stdexec::sender_in<Sender, Environment> concept check has failed."_mstr;
-          return __sync_wait_error<__diag, _Sender>();
-        }
-      } else if constexpr (!__valid_sync_wait_argument<_Sender>) {
-        return __sync_wait_error<__too_many_successful_completions_diag, _Sender>();
-      } else if constexpr (!sender_to<_Sender, __sync_receiver_for_t<_Sender>>) {
-        constexpr __mstring __diag =
-          "Failed to connect the given sender to sync_wait's internal receiver. "
-          "The stdexec::connect(Sender, Receiver) expression is ill-formed."_mstr;
-        return __sync_wait_error<__diag, _Sender>();
-      } else {
-        constexpr __mstring __diag = "Unknown concept check failure."_mstr;
-        return __sync_wait_error<__diag, _Sender>();
-      }
-    }
-
-    template <class _Sender>
-    using __error_description_t = decltype(__sync_wait::__diagnose_error<_Sender>());
-#endif
-
     ////////////////////////////////////////////////////////////////////////////
     // [execution.senders.consumers.sync_wait]
     struct sync_wait_t {
-      template <sender_in<__env> _Sender>
-        requires __valid_sync_wait_argument<_Sender>
-              && __has_implementation_for<sync_wait_t, __early_domain_of_t<_Sender>, _Sender>
-      auto operator()(_Sender&& __sndr) const -> std::optional<__value_tuple_for_t<_Sender>> {
-        using _Domain = __late_domain_of_t<_Sender, __env>;
-        return stdexec::apply_sender(_Domain(), *this, static_cast<_Sender&&>(__sndr));
+      template <class _Sender>
+      auto operator()(_Sender&& __sndr) const {
+        if constexpr (!sender_in<_Sender, __env>) {
+          stdexec::__diagnose_sender_concept_failure<_Sender, __env>();
+        } else {
+          using __domain_t = __late_domain_of_t<_Sender, __env>;
+          constexpr auto __success_completion_count =
+            __v<value_types_of_t<_Sender, __env, __types, __msize::__f>>;
+          static_assert(
+            __success_completion_count != 0,
+            "The argument to stdexec::sync_wait() is a sender that cannot complete successfully. "
+            "stdexec::sync_wait() requires a sender that can complete successfully in exactly one "
+            "way. In other words, the sender's completion signatures must include exactly one "
+            "signature of the form `set_value_t(value-types...)`.");
+          static_assert(
+            __success_completion_count <= 1,
+            "The sender passed to stdexec::sync_wait() can complete successfully in "
+            "more than one way. Use stdexec::sync_wait_with_variant() instead.");
+          if constexpr (1 == __success_completion_count) {
+            using __sync_wait_receiver = __receiver_t<_Sender>;
+            constexpr bool __no_custom_sync_wait = __same_as<__domain_t, default_domain>;
+            if constexpr (__no_custom_sync_wait && sender_to<_Sender, __sync_wait_receiver>) {
+              // using __connect_result = connect_result_t<_Sender, __sync_wait_receiver>;
+              // if constexpr (!operation_state<__connect_result>) {
+              //   static_assert(
+              //     operation_state<__connect_result>,
+              //     "The `connect` member function of the sender passed to stdexec::sync_wait() does "
+              //     "not return an operation state. An operation state is required to have a "
+              //     "no-throw .start() member function.");
+              // } else
+              {
+                // success path, dispatch to the default domain's sync_wait
+                return default_domain().apply_sender(*this, static_cast<_Sender&&>(__sndr));
+              }
+            } else if constexpr (__no_custom_sync_wait) {
+              static_assert(
+                sender_to<_Sender, __sync_wait_receiver>,
+                "The sender passed to stdexec::sync_wait() does not have a .connect(<receiver>) "
+                "member function that accepts sync_wait's receiver.");
+            } else if constexpr (!__has_implementation_for<sync_wait_t, __domain_t, _Sender>) {
+              static_assert(
+                __has_implementation_for<sync_wait_t, __domain_t, _Sender>,
+                "The sender passed to stdexec::sync_wait() has a domain that does not provide a "
+                "usable implementation for sync_wait().");
+            } else {
+              // success path, dispatch to the custom domain's sync_wait
+              return stdexec::apply_sender(__domain_t(), *this, static_cast<_Sender&&>(__sndr));
+            }
+          }
+        }
       }
 
-#if STDEXEC_EDG()
-      // This is needed to get sensible diagnostics from nvc++
-      template <class _Sender, class _Error = __error_description_t<_Sender>>
-      auto operator()(_Sender&&, [[maybe_unused]] _Error __diagnostic = {}) const
-        -> std::optional<std::tuple<int>> = delete;
-#endif
-
       // clang-format off
       /// @brief Synchronously wait for the result of a sender, blocking the
       ///         current thread.

test/stdexec/algos/adaptors/test_then.cpp

@@ -184,7 +184,7 @@ namespace {
   struct then_test_domain {
     template <class Sender, class... Env>
       requires std::same_as<ex::tag_of_t<Sender>, ex::then_t>
-    static auto transform_sender(Sender&& sndr, Env&&...) {
+    static auto transform_sender(Sender&&, Env&&...) {
       return ex::just(std::string{"ciao"});
     }
   };

test/stdexec/algos/consumers/test_sync_wait.cpp

@@ -117,7 +117,7 @@ namespace {
     ex::sender auto snd = fallible_just{13} //
                         | ex::let_error(always(ex::just(std::string{"err"})));
     check_val_types<ex::__mset<pack<int>, pack<std::string>>>(snd);
-    static_assert(!std::invocable<ex::sync_wait_t, decltype(snd)>);
+    // static_assert(!std::invocable<ex::sync_wait_t, decltype(snd)>);
   }
 
   TEST_CASE(

test/stdexec/queries/test_forwarding_queries.cpp

@@ -17,6 +17,7 @@
 
 STDEXEC_PRAGMA_IGNORE_GNU("-Wdeprecated-declarations")
 STDEXEC_PRAGMA_IGNORE_EDG(deprecated_entity_with_custom_message)
+STDEXEC_PRAGMA_IGNORE_MSVC(4996)
 
 namespace ex = stdexec;