Remove unnecessary typename before dependent names

[localspook]

Now that the minimum clang version is 16 (#1657), the code can take advantage of implicit typename, right?

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[ericniebler]

you would think, but i've seen msvc struggle without the typename. i'll run the tests to confirm, but i think this pr is premature.

[ericniebler]

/ok to test c246aa2

[localspook]

Now that's quite bizarre. This is the pattern that's causing the failures:

template <typename T>
struct base {
    using type = int;
};

template <typename T>
struct derived : base<T> {
    using type = /*typename*/ derived::type; // Finds base<T>::type
};

template struct derived<int>;

Looks like (ironically) MSVC accepts it, Clang accepts it only since 18, and GCC doesn't accept it at all: https://godbolt.org/z/EK33394hP

[ericniebler]

template <typename T>
struct derived : base<T> {
    using type = /*typename*/ derived::type; // Finds base<T>::type
};

oh god that's terrible code. where are we doing that? i'd prefer:

template <typename T>
struct derived : base<T> {
    using type = /*typename*/ derived::base::type; // Finds base<T>::type
};

[localspook]

__state_t in:

stdexec/include/stdexec/__detail/__basic_sender.hpp

Lines 389 to 394 in 770f795

	template <class _Sexpr, class _Receiver>
	struct __op_state : __detail::__op_base<_Sexpr, _Receiver> {
	using __desc_t = typename __decay_t<_Sexpr>::__desc_t;
	using __tag_t = typename __desc_t::__tag;
	using __data_t = typename __desc_t::__data;
	using __state_t = typename __op_state::__state_t;

Finds the __state_t in the base class:

stdexec/include/stdexec/__detail/__basic_sender.hpp

Lines 219 to 222 in 770f795

	template <class _Sexpr, class _Receiver>
	struct __op_base : __immovable {
	using __tag_t = typename __decay_t<_Sexpr>::__tag_t;
	using __state_t = __state_type_t<__tag_t, _Sexpr, _Receiver>;

[ericniebler]

   using __state_t = typename __op_state::__state_t; 

can you try changing it to:

   using __state_t = typename __op_state::__op_base::__state_t; 

?

[localspook]

Pushed a commit with that change, please retest

[ericniebler]

/ok to test 6fa6ba9