flatten the structure of env<> for the sake of msvc

fixes #1979

Author: ericniebler

include/stdexec/__detail/__env.hpp

@@ -21,6 +21,7 @@
 #include "__meta.hpp"
 #include "__query.hpp"
 #include "__tag_invoke.hpp"
+#include "__tuple.hpp"
 
 #include <exception>   // IWYU pragma: keep for std::terminate
 #include <functional>  // IWYU pragma: keep for unwrap_reference_t
@@ -29,6 +30,7 @@
 STDEXEC_PRAGMA_PUSH()
 STDEXEC_PRAGMA_IGNORE_EDG(probable_guiding_friend)
 STDEXEC_PRAGMA_IGNORE_EDG(type_qualifiers_ignored_on_reference)
+STDEXEC_PRAGMA_IGNORE_GNU("-Wmissing-braces")
 
 namespace STDEXEC
 {
@@ -113,7 +115,7 @@ namespace STDEXEC
         }
         else
         {
-          return env<_Env1, __fwd_env_t<_Env2>>{{static_cast<_Env1 &&>(__env1)},
+          return env<_Env1, __fwd_env_t<_Env2>>{static_cast<_Env1 &&>(__env1),
                                                 __fwd_fn()(static_cast<_Env2 &&>(__env2))};
         }
       }
@@ -201,70 +203,53 @@ namespace STDEXEC
   STDEXEC_HOST_DEVICE_DEDUCTION_GUIDE
     prop(_Query, _Value) -> prop<_Query, std::unwrap_reference_t<_Value>>;
 
-  //////////////////////////////////////////////////////////////////////
-  // env
-  template <class... _Envs>
-  struct env;
-
-  template <>
-  struct env<>
-  {
-    STDEXEC_ATTRIBUTE(nodiscard, host, device)
-    auto query() const = delete;
-  };
-
-  template <class _Env>
-  struct env<_Env> : _Env
-  {};
-
-  template <class _Env>
-  struct env<_Env &>
+  namespace __detail
   {
     template <class _Query, class... _Args>
-      requires __queryable_with<_Env, _Query, _Args...>
-    STDEXEC_ATTRIBUTE(nodiscard, always_inline, host, device)
-    constexpr auto query(_Query, _Args &&...__args) const
-      noexcept(__nothrow_queryable_with<_Env, _Query, _Args...>)
-        -> __query_result_t<_Env, _Query, _Args...>
+    struct __get_1st_env
     {
-      return __query<_Query>()(__env_, static_cast<_Args &&>(__args)...);
-    }
+      STDEXEC_ATTRIBUTE(host, device, always_inline)
+      constexpr void operator()() const noexcept {}
 
-    _Env &__env_;
-  };
+      template <class _Env0, class... _Envs>
+      STDEXEC_ATTRIBUTE(host, device, always_inline)
+      constexpr auto
+      operator()(_Env0 const &__env0, _Envs const &...__envs) const noexcept -> decltype(auto)
+      {
+        if constexpr (__callable<__query<_Query>, _Env0 const &, _Args...>)
+          return (__env0);  // load-bearing parentheses, do not remove
+        else
+          return (*this)(__envs...);
+      }
+    };
+  }  // namespace __detail
 
-  template <class _Env1, class _Env2>
-  struct env<_Env1, _Env2>
+  //////////////////////////////////////////////////////////////////////
+  // env
+  template <class... _Envs>
+  struct env
   {
+   private:
     template <class _Query, class... _Args>
-      requires __queryable_with<_Env1, _Query, _Args...>
-    STDEXEC_ATTRIBUTE(nodiscard, always_inline, host, device)
-    constexpr auto query(_Query, _Args &&...__args) const
-      noexcept(__nothrow_queryable_with<_Env1, _Query, _Args...>)
-        -> __query_result_t<_Env1, _Query, _Args...>
-    {
-      return __query<_Query>()(__env1_, static_cast<_Args &&>(__args)...);
-    }
+    using __1st_env_t =
+      __call_result_t<__detail::__get_1st_env<_Query, _Args...>, _Envs const &...>;
 
+   public:
     template <class _Query, class... _Args>
-      requires __queryable_with<_Env1, _Query, _Args...>
-            || __queryable_with<_Env2, _Query, _Args...>
+      requires __queryable_with<__1st_env_t<_Query, _Args...>, _Query, _Args...>
     STDEXEC_ATTRIBUTE(nodiscard, always_inline, host, device)
     constexpr auto query(_Query, _Args &&...__args) const
-      noexcept(__nothrow_queryable_with<_Env2, _Query, _Args...>)
-        -> __query_result_t<_Env2, _Query, _Args...>
+      noexcept(__nothrow_queryable_with<__1st_env_t<_Query, _Args...>, _Query, _Args...>)
+        -> __query_result_t<__1st_env_t<_Query, _Args...>, _Query, _Args...>
     {
-      return __query<_Query>()(__env2_, static_cast<_Args &&>(__args)...);
+      auto const &__env = __apply(__detail::__get_1st_env<_Query, _Args...>(), __envs_);
+      return __query<_Query>()(__env, static_cast<_Args &&>(__args)...);
     }
 
-    STDEXEC_ATTRIBUTE(no_unique_address) _Env1 __env1_;
-    STDEXEC_ATTRIBUTE(no_unique_address) _Env2 __env2_;
+    STDEXEC_ATTRIBUTE(no_unique_address)
+    __tuple<_Envs...> __envs_;
   };
 
-  template <class _Env1, class _Env2, class... _Envs>
-  struct env<_Env1, _Env2, _Envs...> : env<env<_Env1, _Env2>, _Envs...>
-  {};
-
   template <class... _Envs>
   STDEXEC_HOST_DEVICE_DEDUCTION_GUIDE env(_Envs...) -> env<std::unwrap_reference_t<_Envs>...>;
 

test/stdexec/queries/test_env.cpp

@@ -21,6 +21,8 @@
 
 namespace ex = STDEXEC;
 
+STDEXEC_PRAGMA_IGNORE_GNU("-Wmissing-braces")
+
 namespace
 {
   template <typename T>
@@ -106,4 +108,55 @@ namespace
     auto sch = ex::get_completion_scheduler<ex::set_value_t>(attrs, ex::env{});
     CHECK(std::same_as<decltype(sch), ex::inline_scheduler>);
   }
+
+#define DEFINE_QUERY(name)                              \
+  constexpr struct name ## _t : ex::__query<name ## _t> \
+  { } name{}
+
+  DEFINE_QUERY(query_0);
+  DEFINE_QUERY(query_1);
+  DEFINE_QUERY(query_2);
+  DEFINE_QUERY(query_3);
+  DEFINE_QUERY(query_4);
+  DEFINE_QUERY(query_5);
+  DEFINE_QUERY(query_6);
+  DEFINE_QUERY(query_7);
+  DEFINE_QUERY(query_8);
+  DEFINE_QUERY(query_9);
+  DEFINE_QUERY(query_10);
+  DEFINE_QUERY(query_11);
+  DEFINE_QUERY(query_12);
+
+  TEST_CASE("env supports lots of child envs without exceeding compiler limits", "[queries][env]")
+  {
+    auto env = ex::env{
+      ex::prop{ query_0,  0},
+      ex::prop{ query_1,  1},
+      ex::prop{ query_2,  2},
+      ex::prop{ query_3,  3},
+      ex::prop{ query_4,  4},
+      ex::prop{ query_5,  5},
+      ex::prop{ query_6,  6},
+      ex::prop{ query_7,  7},
+      ex::prop{ query_8,  8},
+      ex::prop{ query_9,  9},
+      ex::prop{query_10, 10},
+      ex::prop{query_11, 11},
+      ex::prop{query_12, 12}
+    };
+
+    CHECK(env.query(query_0) == 0);
+    CHECK(env.query(query_1) == 1);
+    CHECK(env.query(query_2) == 2);
+    CHECK(env.query(query_3) == 3);
+    CHECK(env.query(query_4) == 4);
+    CHECK(env.query(query_5) == 5);
+    CHECK(env.query(query_6) == 6);
+    CHECK(env.query(query_7) == 7);
+    CHECK(env.query(query_8) == 8);
+    CHECK(env.query(query_9) == 9);
+    CHECK(env.query(query_10) == 10);
+    CHECK(env.query(query_11) == 11);
+    CHECK(env.query(query_12) == 12);
+  }
 }  // namespace