[Bug libstdc++/128413] common_iterator's iter_move function unconditionally uses iter_rvalue_reference_t

[de34 at live dot cn via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=128413

--- Comment #1 from Jiang An <de34 at live dot cn> ---
The following example is rejected with libstdc++ because
iter_rvalue_reference_t  <non_iter_movable_iterator> is ill-formed.
https://godbolt.org/z/38aGdW56j

According to the standard, the return type should be declared as
`decltype(auto)` so ill-formed iter_rvalue_reference_t shouldn't cause hard
error when instantiation common_iterator.

```
#include <iterator>

template <class It>
class non_iter_movable_iterator {
  It it_;

public:
  using value_type      = std::iter_value_t<It>;
  using difference_type = std::iter_difference_t<It>;

  friend constexpr It base(const non_iter_movable_iterator& i) { return i.it_;
}

  explicit constexpr non_iter_movable_iterator(It it) : it_(it) {}

  constexpr decltype(auto) operator*() const { return *it_; }

  constexpr non_iter_movable_iterator& operator++() {
    ++it_;
    return *this;
  }
  constexpr void operator++(int) { ++it_; }

  friend constexpr void iter_move(const non_iter_movable_iterator&) {}
};

int main() {
  int* p{};
  non_iter_movable_iterator<int*> q{p};
  std::common_iterator<non_iter_movable_iterator<int*>,
std::unreachable_sentinel_t> it{q};
}
````