fix: operator[]/operator/ SFINAE in trailing return type for MSVC C++20 (issue #515)

[PavelGuzenfeld]

Problem

On MSVC C++20, writing event<e>[!guard] or *state[!guard] fails to compile with a template argument deduction error. The same pattern works on GCC, Clang, and MSVC C++17.

Root cause: front::event::operator[] and front::state_impl::operator[] (and the corresponding operator/ overloads) express their SFINAE constraint as a default template parameter:

template <class T, typename enable_if<callable<bool, T>::value, int>::type = 0>
constexpr auto operator[](const T &t) const { ... }

MSVC C++20 has a known issue where this form of SFINAE can fail to deduce T when the argument is a nested dependent type (here: front::not_<zero_wrapper<Guard>> produced by !guard). Specifically, MSVC's C++20 subscript-operator lookup interacts poorly with default-parameter SFINAE, causing it to reject the overload before deduction completes.

Fix

Move the enable_if constraint to the trailing return type, which is the standard and MSVC-compatible form of expression-SFINAE:

template <class T>
constexpr auto operator[](const T &t) const
    -> enable_if_t<callable<bool, T>::value, transition_eg<event, zero_wrapper<T>>> { ... }

This form of SFINAE has identical semantics — substitution failure removes the overload from the candidate set — but is evaluated strictly after deduction completes, which works correctly on all compilers.

Affected overloads

• front::event::operator[] — event<e>[guard] / event<e>[!guard]
• front::event::operator/ — event<e>/action (same MSVC risk, fixed for consistency)
• front::state_impl::operator[] — *state[guard] / *state[!guard]
• front::state_impl::operator/ — *state/action (same MSVC risk, fixed for consistency)

Test

Added negated_guard_compiles_on_event_and_state in test/ft/guards.cpp that exercises:

• *state1 + event<event1>[!guard] (the primary reported failure)
• state2[!guard] (the state-subscript form)

Both patterns were already rejected by MSVC C++20 before this fix.

Compatibility

All existing GCC 14 and Clang tests pass unchanged (C++17 and C++20). The trailing-return-type form is valid C++14 and up.