[libcxx-commits] [libcxx] [libc++] Resolve LWG4308: `std::optional<T&>::iterator` can't be a contiguous iterator for some T (PR #173948)

[A. Jiang via libcxx-commits]

================
@@ -19,6 +19,12 @@
 #include <type_traits>
 #include <utility>
 
+template <typename T>
+concept has_iterator = requires {
+  typename T::iterator;
+  typename T::const_iterator;
+};
+
----------------
frederick-vs-ja wrote:

I'd like to test that `optional<X&>` sometimes has neither `iterator` nor `const_iterator`, by using something like:
```C++
template <typename T>
concept has_no_iterator = !requires { typename T::iterator; } && !requires { typename T::const_iterator; };
```

https://github.com/llvm/llvm-project/pull/173948