From 7d6f147592675ebe4b4ceafff78184a81773d747 Mon Sep 17 00:00:00 2001
From: Pavel Guzenfeld <pavelguzenfeld@users.noreply.github.com>
Date: Wed, 14 Oct 2026 00:22:08 +0300
Subject: [PATCH] fix: accept bare state + event as an internal transition
 (#317)

The row consumes the event with no action, exit or entry.
---
 include/boost/sml.hpp                         |  6 +-
 test/ft/CMakeLists.txt                        |  5 ++
 .../ft/issue_317_bare_internal_transition.cpp | 65 +++++++++++++++++++
 3 files changed, 73 insertions(+), 3 deletions(-)
 create mode 100644 test/ft/issue_317_bare_internal_transition.cpp

diff --git a/include/boost/sml.hpp b/include/boost/sml.hpp
index e4dfde27..bdff28f8 100644
--- a/include/boost/sml.hpp
+++ b/include/boost/sml.hpp
@@ -3167,13 +3167,13 @@ struct transition_sa<state<S2>, A> : transition<state<internal>, state<S2>, fron
   }
 };
 template <class S2, class E>
-struct transition<state<S2>, front::event<E>> {
+struct transition<state<S2>, front::event<E>> : transition<state<internal>, state<S2>, front::event<E>, always, none> {
+  constexpr transition(const state<S2> &, const front::event<E> &)
+      : transition<state<internal>, state<S2>, front::event<E>, always, none>{always{}, none{}} {}
   template <class T>
   constexpr auto operator=(const T &) const {
     return transition<T, state<S2>, front::event<E>, always, none>{always{}, none{}};
   }
-  const state<S2> &s2;
-  front::event<E> e;
 };
 template <class E, class G>
 struct transition_eg<front::event<E>, G> {
diff --git a/test/ft/CMakeLists.txt b/test/ft/CMakeLists.txt
index dc1d8393..7b4cf1e1 100644
--- a/test/ft/CMakeLists.txt
+++ b/test/ft/CMakeLists.txt
@@ -155,4 +155,9 @@ if(EXISTS ${CMAKE_CURRENT_SOURCE_DIR}/issue_552_sub_sm_dep_identity.cpp)
   add_test(test_issue_552_sub_sm_dep_identity test_issue_552_sub_sm_dep_identity)
 endif()
 
+if(EXISTS ${CMAKE_CURRENT_SOURCE_DIR}/issue_317_bare_internal_transition.cpp)
+  add_executable(test_issue_317_bare_internal_transition issue_317_bare_internal_transition.cpp)
+  add_test(test_issue_317_bare_internal_transition test_issue_317_bare_internal_transition)
+endif()
+
 add_subdirectory(errors)
diff --git a/test/ft/issue_317_bare_internal_transition.cpp b/test/ft/issue_317_bare_internal_transition.cpp
new file mode 100644
index 00000000..0be0be90
--- /dev/null
+++ b/test/ft/issue_317_bare_internal_transition.cpp
@@ -0,0 +1,65 @@
+// cppcheck-suppress missingIncludeSystem
+#include <boost/sml.hpp>
+
+namespace sml = boost::sml;
+
+namespace {
+struct ignored {};
+struct leave {};
+struct come_back {};
+
+struct counters {
+  int entries = 0;
+  int exits = 0;
+};
+struct count_entry {
+  void operator()(counters &c) const { ++c.entries; }
+};
+struct count_exit {
+  void operator()(counters &c) const { ++c.exits; }
+};
+
+const auto idle = sml::state<class idle_317>;
+const auto busy = sml::state<class busy_317>;
+}  // namespace
+
+test bare_row_on_initial_state_consumes_event_without_exit_or_entry = [] {
+  struct c {
+    auto operator()() const noexcept {
+      using namespace sml;
+      return make_transition_table(*idle + event<ignored>, idle + event<leave> = busy, busy + event<come_back> = idle,
+                                   idle + sml::on_entry<_> / count_entry{}, idle + sml::on_exit<_> / count_exit{});
+    }
+  };
+  counters calls{};
+  sml::sm<c> sm{calls};
+  expect(1 == calls.entries);
+
+  expect(sm.process_event(ignored{}));
+  expect(sm.is(idle));
+  expect(1 == calls.entries);
+  expect(0 == calls.exits);
+
+  expect(!sm.process_event(come_back{}));
+  expect(sm.is(idle));
+};
+
+test bare_row_on_non_initial_state_consumes_event_without_exit_or_entry = [] {
+  struct c {
+    auto operator()() const noexcept {
+      using namespace sml;
+      return make_transition_table(*idle + event<leave> = busy, busy + event<ignored>, busy + sml::on_entry<_> / count_entry{},
+                                   busy + sml::on_exit<_> / count_exit{});
+    }
+  };
+  counters calls{};
+  sml::sm<c> sm{calls};
+  expect(sm.process_event(leave{}));
+  expect(sm.is(busy));
+  expect(1 == calls.entries);
+
+  expect(sm.process_event(ignored{}));
+  expect(sm.is(busy));
+  expect(1 == calls.entries);
+  expect(0 == calls.exits);
+};