[libcxx-commits] [libcxx] [libc++] Resolve LWG4308, correct `iterator` availability for `optional<T&>` (PR #173948)

[William Tran-Viet 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;
----------------
smallp-o-p wrote:

IMO, since the fix is pretty simple and we'd be adding all the testing stuff required for it, I included it in this PR. 

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