`just` senders are move/copy constructible but not assignable

[msimberg]

See https://godbolt.org/z/GW7cfea31 for a reproducer. I would expect them to be assignable if they're copy/move constructible. The source seems to be __make_tuple (

stdexec/include/stdexec/__detail/__basic_sender.hpp

Lines 612 to 653 in 869fb2a

	namespace {
	constexpr auto __make_tuple = //
	[]<class _Tag, class... _Captures>(_Tag, _Captures&&... __captures) {
	return [=]<class _Cvref, class _Fun>(_Cvref __cvref, _Fun && __fun) mutable //
	noexcept(__nothrow_callable<_Fun, _Tag, __minvoke<_Captures, _Cvref>...>) //
	-> decltype(__f<__call_result_t<_Fun, _Tag, __minvoke<_Captures, _Cvref>...>>())
	requires __callable<_Fun, _Tag, __minvoke<_Captures, _Cvref>...>
	{
	return ((_Fun&&) __fun)(
	_Tag(), const_cast<__minvoke<_Captures, _Cvref>&&>(__captures.__value)...);
	};
	};
	} // anonymous namespace
	template <class _Tag>
	template <class _Data, class... _Child>
	constexpr auto __make_sexpr_t<_Tag>::operator()(_Data __data, _Child... __child) const {
	return __sexpr{__make_tuple(_Tag(), __detail::__mbc(__data), __detail::__mbc(__child)...)};
	}
	#else
	// Anonymous namespace here is to avoid symbol name collisions with the
	// lambda functions returned by __make_tuple.
	namespace {
	constexpr auto __make_tuple = //
	[]<class _Tag, class... _Captures>(_Tag, _Captures&&... __captures) {
	return [... __captures = (_Captures&&) __captures]<class _Cvref, class _Fun>(
	_Cvref, _Fun && __fun) mutable //
	noexcept(__nothrow_callable<_Fun, _Tag, __minvoke<_Cvref, _Captures>...>) //
	-> __call_result_t<_Fun, _Tag, __minvoke<_Cvref, _Captures>...>
	requires __callable<_Fun, _Tag, __minvoke<_Cvref, _Captures>...>
	{
	return ((_Fun&&) __fun)(
	_Tag(), const_cast<__minvoke<_Cvref, _Captures>&&>(__captures)...);
	};
	};
	} // anonymous namespace
	template <class _Tag>
	template <class _Data, class... _Child>
	constexpr auto __make_sexpr_t<_Tag>::operator()(_Data __data, _Child... __child) const {
	return __sexpr{__make_tuple(_Tag(), (_Data&&) __data, (_Child&&) __child...)};
	};

) which is a lambda with captures, which makes the just sender non-assignable. Is this expected or intentional? Does __make_tuple need to be implemented that way?