fix undestructible stdexec::__task::__stop_callback_box

[Cra3z]

stdexec/include/stdexec/__detail/__task.hpp

Lines 151 to 176 in 3680678

	template <class _StopToken, class _StopSource>
	requires __not_same_as<__stop_source_token_t<_StopSource>, _StopToken>
	struct __stop_callback_box<_StopToken, _StopSource>
	{
	using __stop_variant_t = __variant<_StopSource, __stop_source_token_t<_StopSource>>;
	using __callback_fn_t = __forward_stop_request<_StopSource>;
	using __stop_callback_t = stop_callback_for_t<_StopToken, __callback_fn_t>;
	constexpr __stop_callback_box() {}
	void __register_callback(auto const & __has_env, __stop_variant_t& __stop)
	noexcept(__nothrow_constructible_from<__stop_callback_t, _StopToken, _StopSource&>)
	{
	std::construct_at(&__cb_, get_stop_token(get_env(__has_env)), __var::__get<0>(__stop));
	}
	void __reset_callback() noexcept
	{
	std::destroy_at(&__cb_);
	}
	union
	{
	__stop_callback_t __cb_;
	};
	};

Prior to the adoption of P3074 - trivial union in C++26, if __stop_callback_t has a non-trivial destructor, the destructor of __stop_callback_box is implicitly deleted, which in turn causes the destructor of stdexec::task::__awaiter (which is derived from __stop_callback_box) to be deleted.

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[ericniebler]

/ok to test 0ad2642

[ericniebler]

thanks!