sharded.hh: migrate to concepts

Migrates static_assert calls on invoke variants to idiomatic C++20 concept requirements. Requirements are duplicated on proxy functions for improved readability, loose coupling and faster failures.

Closes scylladb#2460

include/seastar/core/sharded.hh

@@ -291,6 +291,7 @@ public:
  /// \return Future that becomes ready once all calls have completed
  template <typename Func, typename... Args>
  requires std::invocable<Func, Service&, internal::sharded_unwrap_t<Args>...>
+ && std::is_same_v<futurize_t<std::invoke_result_t<Func, Service&, internal::sharded_unwrap_t<Args>...>>, future<>>
  future<> invoke_on_all(smp_submit_to_options options, Func func, Args... args) noexcept;
 
  /// Invoke a function on all instances of `Service`.
@@ -300,6 +301,7 @@ public:
  /// \ref smp::submit_to() called behind the scenes.
  template <typename Func, typename... Args>
  requires std::invocable<Func, Service&, internal::sharded_unwrap_t<Args>...>
+ && std::is_same_v<futurize_t<std::invoke_result_t<Func, Service&, internal::sharded_unwrap_t<Args>...>>, future<>>
  future<> invoke_on_all(Func func, Args... args) noexcept {
  try {
  return invoke_on_all(smp_submit_to_options{}, std::move(func), std::move(args)...);
@@ -320,6 +322,7 @@ public:
  /// processed the message.
  template <typename Func, typename... Args>
  requires std::invocable<Func, Service&, Args...>
+ && std::is_same_v<futurize_t<std::invoke_result_t<Func, Service&, Args...>>, future<>>
  future<> invoke_on_others(smp_submit_to_options options, Func func, Args... args) noexcept;
 
  /// Invoke a callable on all instances of \c Service except the instance
@@ -335,6 +338,7 @@ public:
  /// \ref smp::submit_to() called behind the scenes.
  template <typename Func, typename... Args>
  requires std::invocable<Func, Service&, Args...>
+ && std::is_same_v<futurize_t<std::invoke_result_t<Func, Service&, Args...>>, future<>>
  future<> invoke_on_others(Func func, Args... args) noexcept {
  try {
  return invoke_on_others(smp_submit_to_options{}, std::move(func), std::move(args)...);
@@ -796,11 +800,10 @@ sharded<Service>::invoke_on_all(smp_submit_to_options options, std::function<fut
 template <typename Service>
 template <typename Func, typename... Args>
 requires std::invocable<Func, Service&, internal::sharded_unwrap_t<Args>...>
+ && std::is_same_v<futurize_t<std::invoke_result_t<Func, Service&, internal::sharded_unwrap_t<Args>...>>, future<>>
 inline
 future<>
 sharded<Service>::invoke_on_all(smp_submit_to_options options, Func func, Args... args) noexcept {
- static_assert(std::is_same_v<futurize_t<std::invoke_result_t<Func, Service&, internal::sharded_unwrap_t<Args>...>>, future<>>,
- "invoke_on_all()'s func must return void or future<>");
  try {
  return invoke_on_all(options, invoke_on_multiple_func_type([func = std::move(func), args = std::tuple(std::move(args)...)] (Service& service) mutable {
  return std::apply([&service, &func] (Args&&... args) mutable {
@@ -815,11 +818,10 @@ sharded<Service>::invoke_on_all(smp_submit_to_options options, Func func, Args..
 template <typename Service>
 template <typename Func, typename... Args>
 requires std::invocable<Func, Service&, Args...>
+ && std::is_same_v<futurize_t<std::invoke_result_t<Func, Service&, Args...>>, future<>>
 inline
 future<>
 sharded<Service>::invoke_on_others(smp_submit_to_options options, Func func, Args... args) noexcept {
- static_assert(std::is_same_v<futurize_t<std::invoke_result_t<Func, Service&, Args...>>, future<>>,
- "invoke_on_others()'s func must return void or future<>");
  try {
  return invoke_on_all(options, [orig = this_shard_id(), func = std::move(func), args = std::tuple(std::move(args)...)] (Service& s) mutable -> future<> {
  return this_shard_id() == orig ? make_ready_future<>() : futurize_apply(func, std::tuple_cat(std::forward_as_tuple(s), args));;