asioexec::completion_token: Lvalue Completion Handler Invocation (#1566)

As added by 35a3e31 the completion
handler type used by the implementation of asioexec::completion_token
only supported rvalue-qualified invocation. This was due to the fact:

- Invocation of a completion handler is consumptive in the same way that
  calling set_value, set_error, or set_stopped on a receiver is (note
  those three operations require an rvalue-qualified operand), and
- Asio has invoked completion handlers with rvalue qualification since
  1.20.0 (standalone)/1.77 (Boost)

Unfortunately there are operations in the wild which do not rvalue
qualify their completion handlers when invoking them. For example the
operations provided by Boost.Beast (at least as of this writing).

Updated asioexec::completion_token to support the above-described
operations by removing the rvalue qualification from the function call
operator of its completion handler type.

Author: RobertLeahy

include/asioexec/completion_token.hpp

@@ -244,7 +244,7 @@ namespace asioexec {
       }
 
       template <typename... Args>
-      void operator()(Args&&... args) && noexcept {
+      void operator()(Args&&... args) noexcept {
         STDEXEC_ASSERT(self_);
         {
           const std::lock_guard l(self_->m_);

test/asioexec/test_completion_token.cpp

@@ -718,4 +718,15 @@ namespace {
     CHECK(lvalue_kind == value_category_receiver::kind::const_lvalue);
   }
 
+  TEST_CASE(
+    "Lvalue invocation of the completion handler is supported (for compatibility with Boost.Beast "
+    "perhaps among others)",
+    "[asioexec][completion_token]") {
+    const auto initiating_function = [](auto&& token) {
+      return asio_impl::async_initiate<decltype(token), void()>([](auto&& h) { h(); }, token);
+    };
+    auto op = ::stdexec::connect(initiating_function(completion_token), expect_value_receiver{});
+    ::stdexec::start(op);
+  }
+
 } // namespace