diff --git a/src/rpp/rpp/observables/observable.hpp b/src/rpp/rpp/observables/observable.hpp
index a0a0daea7..19bc89521 100644
--- a/src/rpp/rpp/observables/observable.hpp
+++ b/src/rpp/rpp/observables/observable.hpp
@@ -339,7 +339,8 @@ namespace rpp
             if constexpr (requires { typename std::decay_t<Op>::template operator_traits<Type>; })
             {
                 using result_type = typename std::decay_t<Op>::template operator_traits<Type>::result_type;
-                return observable<result_type, details::observables::make_chain_t<std::decay_t<Op>, Strategy>>{std::forward<Op>(op), std::move(m_strategy)};
+                if constexpr (requires { typename std::decay_t<Op>::template operator_traits<Type>::result_type; }) // narrow compilataion error a bit
+                    return observable<result_type, details::observables::make_chain_t<std::decay_t<Op>, Strategy>>{std::forward<Op>(op), std::move(m_strategy)};
             }
             else
             {
diff --git a/src/rpp/rpp/operators/finally.hpp b/src/rpp/rpp/operators/finally.hpp
index 88102784a..42c4c424b 100644
--- a/src/rpp/rpp/operators/finally.hpp
+++ b/src/rpp/rpp/operators/finally.hpp
@@ -53,7 +53,7 @@ namespace rpp::operators
      * @ingroup utility_operators
      * @see https://reactivex.io/documentation/operators/do.html
      */
-    template<std::invocable<> LastFn>
+    template<rpp::constraint::is_nothrow_invocable LastFn>
     auto finally(LastFn&& last_fn)
     {
         return details::finally_t<std::decay_t<LastFn>>{std::forward<LastFn>(last_fn)};
diff --git a/src/rpp/rpp/operators/fwd.hpp b/src/rpp/rpp/operators/fwd.hpp
index 131f04036..5f3b591f6 100644
--- a/src/rpp/rpp/operators/fwd.hpp
+++ b/src/rpp/rpp/operators/fwd.hpp
@@ -53,7 +53,7 @@ namespace rpp::operators
         requires (!utils::is_not_template_callable<Fn> || std::same_as<bool, std::invoke_result_t<Fn, rpp::utils::convertible_to_any>>)
     auto filter(Fn&& predicate);
 
-    template<std::invocable<> LastFn>
+    template<rpp::constraint::is_nothrow_invocable LastFn>
     auto finally(LastFn&& lastFn);
 
     template<typename Fn>
@@ -79,10 +79,6 @@ namespace rpp::operators
     template<template<typename> typename Subject = rpp::subjects::publish_subject>
     auto multicast();
 
-    template<typename Fn>
-        requires (!utils::is_not_template_callable<Fn> || rpp::constraint::observable<std::invoke_result_t<Fn, rpp::utils::convertible_to_any>>)
-    auto flat_map(Fn&& callable);
-
     template<rpp::constraint::observable TObservable, rpp::constraint::observable... TObservables>
         requires constraint::observables_of_same_type<std::decay_t<TObservable>, std::decay_t<TObservables>...>
     auto merge_with(TObservable&& observable, TObservables&&... observables);
diff --git a/src/rpp/rpp/operators/group_by.hpp b/src/rpp/rpp/operators/group_by.hpp
index 4e1d8d1fc..5aa4e0210 100644
--- a/src/rpp/rpp/operators/group_by.hpp
+++ b/src/rpp/rpp/operators/group_by.hpp
@@ -166,8 +166,8 @@ namespace rpp::operators::details
         template<rpp::constraint::decayed_type T>
         struct operator_traits
         {
-            static_assert(std::invocable<KeySelector, T>, "KeySelector is not invocacble with T");
-            static_assert(std::invocable<ValueSelector, T>, "ValueSelector is not invocable with T");
+            static_assert(!std::same_as<void, std::invoke_result_t<KeySelector, T>>, "KeySelector is not invocacble with T");
+            static_assert(!std::same_as<void, std::invoke_result_t<ValueSelector, T>>, "ValueSelector is not invocable with T");
             static_assert(std::strict_weak_order<KeyComparator, rpp::utils::decayed_invoke_result_t<KeySelector, T>, rpp::utils::decayed_invoke_result_t<KeySelector, T>>, "KeyComparator is not invocable with result of KeySelector");
 
             using result_type = grouped_observable<utils::decayed_invoke_result_t<KeySelector, T>, rpp::utils::decayed_invoke_result_t<ValueSelector, T>, group_by_observable_strategy<utils::decayed_invoke_result_t<ValueSelector, T>>>;
diff --git a/src/rpp/rpp/operators/map.hpp b/src/rpp/rpp/operators/map.hpp
index 74e3fad4d..43ec6d0d1 100644
--- a/src/rpp/rpp/operators/map.hpp
+++ b/src/rpp/rpp/operators/map.hpp
@@ -50,7 +50,7 @@ namespace rpp::operators::details
         template<rpp::constraint::decayed_type T>
         struct operator_traits
         {
-            static_assert(std::invocable<Fn, T>, "Fn is not invocable with T");
+            static_assert(!std::same_as<void, std::invoke_result_t<Fn, T>>, "Fn is not invocable with T");
 
             using result_type = std::invoke_result_t<Fn, T>;
 
diff --git a/src/rpp/rpp/utils/constraints.hpp b/src/rpp/rpp/utils/constraints.hpp
index 87e9d5792..8a47e751a 100644
--- a/src/rpp/rpp/utils/constraints.hpp
+++ b/src/rpp/rpp/utils/constraints.hpp
@@ -31,7 +31,7 @@ namespace rpp::constraint
     concept variadic_decayed_same_as = sizeof...(Types) == 1 && (decayed_same_as<Type, Types> && ...);
 
     template<typename T, typename Target>
-    concept static_pointer_convertible_to = requires { static_cast<Target*>(std::declval<T*>()); };
+    concept static_pointer_convertible_to = requires(T* ptr) { static_cast<Target*>(ptr); };
 
     template<typename R>
     concept iterable = requires(R& rng) {