Better detection of execution policies. (#1559)

---------

Co-authored-by: Eric Niebler <[email protected]>

Author: lucteo

CMakeLists.txt

@@ -379,6 +379,7 @@ if (STDEXEC_ENABLE_TBB)
     include(rapids-find)
     rapids_find_package(
         TBB REQUIRED
+        COMPONENTS tbb
         BUILD_EXPORT_SET stdexec-exports
         INSTALL_EXPORT_SET stdexec-exports
     )
@@ -397,10 +398,13 @@ if (STDEXEC_ENABLE_TBB)
       FILES ${tbbpool_headers}
     )
 
+    target_link_libraries(stdexec INTERFACE
+        TBB::tbb
+        )
+
     target_link_libraries(tbbpool
         INTERFACE
         STDEXEC::stdexec
-        TBB::tbb
         )
 
     install(TARGETS tbbpool

include/stdexec/__detail/__config.hpp

@@ -420,12 +420,18 @@ namespace stdexec {
 #  define STDEXEC_HAS_STD_MEMORY_RESOURCE() 0
 #endif
 
-#if STDEXEC_HAS_FEATURE(__cpp_lib_execution)
+#if defined(__cpp_lib_execution) && __cpp_lib_execution >= 201603L
 #  define STDEXEC_HAS_EXECUTION_POLICY() 1
 #else
 #  define STDEXEC_HAS_EXECUTION_POLICY() 0
 #endif
 
+#if defined(__cpp_lib_execution) && __cpp_lib_execution >= 	201902L
+#  define STDEXEC_HAS_UNSEQUENCED_EXECUTION_POLICY() 1
+#else
+#  define STDEXEC_HAS_UNSEQUENCED_EXECUTION_POLICY() 0
+#endif
+
 #ifdef STDEXEC_ASSERT
 #  error "Redefinition of STDEXEC_ASSERT is not permitted. Define STDEXEC_ASSERT_FN instead."
 #endif

include/stdexec/__detail/__execution_legacy.hpp

@@ -17,22 +17,24 @@
 
 #include "__config.hpp"
 
+#if STDEXEC_HAS_EXECUTION_POLICY()
+#include <execution>
+#endif
+
 namespace stdexec {
 
 #if STDEXEC_HAS_EXECUTION_POLICY()
 
   using sequenced_policy = std::execution::sequenced_policy;
   using parallel_policy = std::execution::parallel_policy;
   using parallel_unsequenced_policy = std::execution::parallel_unsequenced_policy;
-  using unsequenced_policy = std::execution::unsequenced_policy;
 
-  using seq = std::execution::seq;
-  using par = std::execution::par;
-  using par_unseq = std::execution::par_unseq;
-  using unseq = std::execution::unseq;
+  constexpr auto seq = std::execution::seq;
+  constexpr auto par = std::execution::par;
+  constexpr auto par_unseq = std::execution::par_unseq;
 
-  using std::execution::is_execution_policy_v;
-  using std::execution::is_execution_policy;
+  using std::is_execution_policy_v;
+  using std::is_execution_policy;
 
 #else
 
@@ -56,16 +58,9 @@ namespace stdexec {
     parallel_unsequenced_policy& operator=(const parallel_unsequenced_policy&) = delete;
   };
 
-  struct unsequenced_policy {
-    constexpr explicit unsequenced_policy(__hidden_construction) { };
-    unsequenced_policy(const unsequenced_policy&) = delete;
-    unsequenced_policy& operator=(const unsequenced_policy&) = delete;
-  };
-
   inline constexpr sequenced_policy seq{__hidden_construction{}};
   inline constexpr parallel_policy par{__hidden_construction{}};
   inline constexpr parallel_unsequenced_policy par_unseq{__hidden_construction{}};
-  inline constexpr unsequenced_policy unseq{__hidden_construction{}};
 
   template <typename>
   inline constexpr bool is_execution_policy_v = false;
@@ -79,11 +74,35 @@ namespace stdexec {
   template <>
   inline constexpr bool is_execution_policy_v<parallel_unsequenced_policy> = true;
 
-  template <>
-  inline constexpr bool is_execution_policy_v<unsequenced_policy> = true;
-
   template <class _T>
   struct is_execution_policy : std::bool_constant<is_execution_policy_v<_T>> { };
 
 #endif
+
+#if STDEXEC_HAS_UNSEQUENCED_EXECUTION_POLICY()
+
+  using unsequenced_policy = std::execution::unsequenced_policy;
+
+  constexpr auto unseq = std::execution::unseq;
+
+#else
+
+#if STDEXEC_HAS_EXECUTION_POLICY()
+  // already defined above
+  struct __hidden_construction { };
+#endif
+
+  struct unsequenced_policy {
+    constexpr explicit unsequenced_policy(__hidden_construction) { };
+    unsequenced_policy(const unsequenced_policy&) = delete;
+    unsequenced_policy& operator=(const unsequenced_policy&) = delete;
+  };
+
+  inline constexpr unsequenced_policy unseq{__hidden_construction{}};
+
+  template <>
+  inline constexpr bool is_execution_policy_v<unsequenced_policy> = true;
+
+#endif
+
 } // namespace stdexec