change __common_domain_t to always check for convertibility to default_domain

Author: ericniebler

include/stdexec/__detail/__domain.hpp

@@ -151,10 +151,23 @@ namespace STDEXEC
 
     // Common domain for a set of domains
     template <class... _Domains>
-    struct __common_domain
+    constexpr auto __common_domain_fn() noexcept
     {
-      using __t = __minvoke<__mtry_catch_q<std::common_type_t, __qq<__make_domain_t>>, _Domains...>;
-    };
+      if constexpr (__minvocable_q<std::common_type_t, _Domains...>)
+      {
+        return std::common_type_t<_Domains...>{};
+      }
+      // NOT TO SPEC: If each domain in Domains... is convertible to default_domain, then
+      // the common domain is default_domain.
+      else if constexpr (__minvocable_q<std::common_type_t, default_domain, _Domains...>)
+      {
+        return std::common_type_t<default_domain, _Domains...>{};
+      }
+      else
+      {
+        return __make_domain_t<_Domains...>{};
+      }
+    }
   }  // namespace __detail
 
   ////////////////////////////////////////////////////////////////////////////////////////////////
@@ -169,7 +182,7 @@ namespace STDEXEC
   using __completion_domain_of_t = __completion_domain_t<_Tag, env_of_t<_Sender>, _Env const &...>;
 
   template <class... _Domains>
-  using __common_domain_t = __t<__detail::__common_domain<_Domains...>>;
+  using __common_domain_t = decltype(__detail::__common_domain_fn<_Domains...>());
 
   template <class... _Domains>
   concept __has_common_domain =

test/CMakeLists.txt

@@ -70,6 +70,7 @@ set(stdexec_test_sources
     stdexec/algos/consumers/test_sync_wait.cpp
     stdexec/algos/consumers/test_spawn.cpp
     stdexec/detail/test_any.cpp
+    stdexec/detail/test_common_domain.cpp
     stdexec/detail/test_completion_signatures.cpp
     stdexec/detail/test_demangle.cpp
     stdexec/detail/test_utility.cpp

test/stdexec/detail/test_common_domain.cpp

@@ -0,0 +1,50 @@
+/*
+ * Copyright (c) 2022 Lucian Radu Teodorescu
+ *
+ * Licensed under the Apache License Version 2.0 with LLVM Exceptions
+ * (the "License"); you may not use this file except in compliance with
+ * the License. You may obtain a copy of the License at
+ *
+ *   https://llvm.org/LICENSE.txt
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#include <catch2/catch_all.hpp>
+#include <stdexec/execution.hpp>
+
+#include <test_common/receivers.hpp>
+
+namespace ex = STDEXEC;
+
+namespace
+{
+  struct gpu_domain
+  {};
+
+  struct thread_pool_domain : ex::default_domain
+  {};
+
+  struct parallel_runtime_domain : ex::default_domain
+  {};
+
+  template <class... Domains>
+  using common_domain_t = ex::__common_domain_t<Domains...>;
+
+  TEST_CASE("finding common domains", "[detail][domain]")
+  {
+    using common1 = common_domain_t<gpu_domain, gpu_domain>;
+    STATIC_REQUIRE(std::same_as<common1, gpu_domain>);
+
+    using common2 = common_domain_t<thread_pool_domain, parallel_runtime_domain>;
+    STATIC_REQUIRE(std::same_as<common2, ex::default_domain>);
+
+    using common3 = common_domain_t<gpu_domain, thread_pool_domain>;
+    STATIC_REQUIRE(std::same_as<common3, ex::indeterminate_domain<gpu_domain, thread_pool_domain>>);
+  }
+
+}  // namespace