fix template depth explosion with large FSMs sharing dep types (issue #639)

[PavelGuzenfeld]

Problem

get_mapping<event<E>, event<E>, T, R> was calling unique_mappings_t on all k+1 state_mappings even though R's k entries were already unique. This caused O(k) nested recursion inside the O(N) outer unique_mappings_impl recursion, for a total instantiation depth of O(2N) when N transitions share the same event type.

With GCC's default -ftemplate-depth=900, an FSM with ~450 transitions on a single event type would hit the limit and fail to compile.

Root cause

// Before: O(k) inner recursion per step — O(2N) total depth
template <class E, class T, class R>
struct get_mapping<event<E>, event<E>, T, R>
    : aux::type_list<event_mappings<E, aux::apply_t<unique_mappings_t,
          aux::join_t<typename R::types, typename T::types>>>> {};

R::types already contains k unique state_mappings. Re-running unique_mappings_t on all k+1 items recurses k levels deep — unnecessarily, since only the single new entry from T needs to be merged.

Fix

Add a more-specific partial specialization after extend_mapping_t that:

1. Uses is_base_of (O(1)) to check if the new state_mapping's source state is already in R
2. If present: merges via extend_mapping_t (O(1) pack expansion)
3. If absent: appends the new state_mapping directly

// After: O(1) depth per merge step — O(N) total depth
template <class E, class NewSM, class... RSMs>
struct get_mapping_event_same_impl { ... };

template <class E, class NewSM, class... RSMs>
struct get_mapping<event<E>, event<E>, event_mappings<E, aux::inherit<NewSM>>,
                   event_mappings<E, aux::inherit<RSMs...>>>
    : get_mapping_event_same_impl<E, NewSM, RSMs...>::type {};

The original general specialization remains as a fallback for any edge cases.

Result

A 450-transition FSM with all transitions sharing the same event type now compiles at default template depth. The original would fail at ~450 transitions.

Test

test/ft/issue_639_repro.cpp — 50 same-event transitions compiled with -ftemplate-depth=100:

• Without fix: fails (depth ~100)
• With fix: compiles and runs correctly (depth ~55)

Fixes #639