try to avoid gcc concept recursion

Author: ericniebler

include/stdexec/__detail/__env.hpp

@@ -342,35 +342,43 @@ namespace stdexec {
   /////////////////////////////////////////////////////////////////////////////
   namespace __get_env {
     template <class _EnvProvider>
-    concept __has_get_env = requires(const _EnvProvider& __env_provider) {
-      __env_provider.get_env();
-    };
+    using __get_env_member_t = decltype(__declval<_EnvProvider>().get_env());
+
+    template <class _EnvProvider>
+    concept __has_get_env = __mvalid<__get_env_member_t, _EnvProvider>;
 
     // For getting an execution environment from a receiver or the attributes from a sender.
     struct get_env_t {
+     private:
       template <class _EnvProvider>
-        requires __has_get_env<_EnvProvider>
-      STDEXEC_ATTRIBUTE(nodiscard, always_inline, host, device)
-      constexpr auto operator()(const _EnvProvider& __env_provider) const noexcept
-        -> decltype(__env_provider.get_env()) {
-        static_assert(queryable<decltype(__env_provider.get_env())>);
-        static_assert(noexcept(__env_provider.get_env()), "get_env() members must be noexcept");
-        return __env_provider.get_env();
-      }
-
-      template <class _EnvProvider>
-        requires(!__has_get_env<_EnvProvider>) && tag_invocable<get_env_t, const _EnvProvider&>
-      STDEXEC_ATTRIBUTE(nodiscard, always_inline, host, device)
-      constexpr auto operator()(const _EnvProvider& __env_provider) const noexcept
-        -> tag_invoke_result_t<get_env_t, const _EnvProvider&> {
-        static_assert(queryable<tag_invoke_result_t<get_env_t, const _EnvProvider&>>);
-        static_assert(nothrow_tag_invocable<get_env_t, const _EnvProvider&>);
-        return tag_invoke(*this, __env_provider);
+      static constexpr auto __get_declfn() noexcept {
+        constexpr __declfn_t<_EnvProvider> __env_provider{};
+        if constexpr (__has_get_env<_EnvProvider>) {
+          using __result_t = __get_env_member_t<_EnvProvider>;
+          static_assert(noexcept(__env_provider().get_env()), "get_env() members must be noexcept");
+          return __declfn<__result_t>();
+        } else if constexpr (tag_invocable<get_env_t, const _EnvProvider&>) {
+          using __result_t = tag_invoke_result_t<get_env_t, const _EnvProvider&>;
+          constexpr bool __is_nothrow = nothrow_tag_invocable<get_env_t, const _EnvProvider&>;
+          static_assert(__is_nothrow, "get_env tag_invoke overloads must be noexcept");
+          return __declfn<__result_t>();
+        } else {
+          return __declfn<env<>>();
+        }
       }
 
+     public:
+      template <class _EnvProvider, auto _DeclFn = __get_declfn<_EnvProvider>()>
       STDEXEC_ATTRIBUTE(nodiscard, always_inline, host, device)
-      constexpr auto operator()(__ignore) const noexcept -> env<> {
-        return {};
+      constexpr auto
+        operator()(const _EnvProvider& __env_provider) const noexcept -> decltype(_DeclFn()) {
+        if constexpr (__has_get_env<_EnvProvider>) {
+          return __env_provider.get_env();
+        } else if constexpr (tag_invocable<get_env_t, const _EnvProvider&>) {
+          return tag_invoke(*this, __env_provider);
+        } else {
+          return env<>{};
+        }
       }
     };
   } // namespace __get_env

include/stdexec/__detail/__senders.hpp

@@ -262,8 +262,8 @@ namespace stdexec {
 
         if constexpr (__with_static_member<_TfxSender, _Receiver>) {
           auto&& __tfx_sndr = transform_sender(static_cast<_Sender&&>(__sndr), __env);
-          return __tfx_sndr
-            .connect(static_cast<_TfxSender&&>(__tfx_sndr), static_cast<_Receiver&&>(__rcvr));
+          return STDEXEC_REMOVE_REFERENCE(_TfxSender)::connect(
+            static_cast<_TfxSender&&>(__tfx_sndr), static_cast<_Receiver&&>(__rcvr));
         } else if constexpr (__with_member<_TfxSender, _Receiver>) { // NOLINT(bugprone-branch-clone)
           auto&& __tfx_sndr = transform_sender(static_cast<_Sender&&>(__sndr), __env);
           return static_cast<_TfxSender&&>(__tfx_sndr).connect(static_cast<_Receiver&&>(__rcvr));