handle nested let_value's when computing result domain

fixes #1218

Author: ericniebler

include/stdexec/execution.hpp

@@ -3571,8 +3571,11 @@ namespace stdexec {
           using _Domain = __result_domain_t<_Set, _Child, _Fun, _Env, _Sched>;
           if constexpr (__merror<_Domain>) {
             return _Domain();
+          } else if constexpr (same_as<_Domain, dependent_domain>) {
+            using _Domain2 = __late_domain_of_t<_Child, _Env>;
+            return __make_sexpr<__let_t<_Set, _Domain2>>((_Fun&&) __fun, (_Child&&) __child);
           } else {
-            static_assert(__none_of<_Domain, __none_such, dependent_domain>);
+            static_assert(!same_as<_Domain, __none_such>);
             return __make_sexpr<__let_t<_Set, _Domain>>((_Fun&&) __fun, (_Child&&) __child);
           }
         }

test/stdexec/algos/adaptors/test_let_value.cpp

@@ -316,4 +316,15 @@ namespace {
              | ex::let_value([](std::string& x) { return ex::just(x + ", world"); });
     wait_for_value(std::move(snd), std::string{"hallo"});
   }
+
+  TEST_CASE("let_value can nest", "[adaptors][let_value]") {
+    auto work = ex::just(2)                  //
+              | ex::let_value([](int x) {    //
+                  return ex::just()          //
+                       | ex::let_value([=] { //
+                           return ex::just(x);
+                         });
+                });
+    wait_for_value(std::move(work), 2);
+  }
 }