Fix start_scheduler query of __receiver_proxy_base in __task_scheduler.hpp

include/stdexec/__detail/__task_scheduler.hpp

@@ -1,4 +1,4 @@
-/*
+/*
  * Copyright (c) 2026 NVIDIA Corporation
  *
  * Licensed under the Apache License Version 2.0 with LLVM Exceptions
@@ -847,14 +847,32 @@ namespace STDEXEC
       if (__value_type == __mtypeid<task_scheduler>)
       {
         auto& __val = *static_cast<std::optional<task_scheduler>*>(__dest);
-        if constexpr (__callable<get_start_scheduler_t, env_of_t<_Rcvr>>)
+
+        constexpr bool may_as_task_scheduler = []
         {
-          __val.emplace(get_start_scheduler(get_env(__rcvr_)));
-        }
-        else
+          if constexpr (__callable<get_start_scheduler_t, env_of_t<_Rcvr>>)
+          {
+            return __std::constructible_from<
+              task_scheduler,
+              __call_result_t<get_start_scheduler_t, env_of_t<_Rcvr>>>;
+          }
+          else
+          {
+            return false;
+          }
+        }();
+
+        if constexpr (may_as_task_scheduler)
         {
-          __val.emplace(inline_scheduler{});
+          try
+          {
+            __val.emplace(get_start_scheduler(get_env(__rcvr_)));
+            return;
+          }
+          catch (...)
+          {}
         }
+        __val.emplace(inline_scheduler{});
       }
     }
   }  // namespace __detail

test/stdexec/schedulers/test_parallel_scheduler.cpp

@@ -140,6 +140,13 @@ TEST_CASE("simple chain task on parallel scheduler", "[scheduler][parallel_sched
   (void) snd2;
 }
 
+TEST_CASE("parallel scheduler used in an already scheduled environment",
+          "[scheduler][parallel_scheduler]")
+{
+  auto snd = ex::schedule(ex::get_parallel_scheduler());
+  ex::sync_wait(ex::starts_on(ex::get_parallel_scheduler(), snd));
+}
+
 TEST_CASE("checks stop_token before starting the work", "[scheduler][parallel_scheduler]")
 {
   STDEXEC::parallel_scheduler sched = STDEXEC::get_parallel_scheduler();