[libcxx-commits] [libcxx] [libc++] Resolve LWG4308: `std::optional<T&>::iterator` can't be a contiguous iterator for some 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;
+};
+
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smallp-o-p wrote:

I'm assuming the later comment regarding `optional<T&>` and `const_iterator` may affect this and whether it should be done in this PR? I'm assuming we'd just need to check for `optional<T&>` that it has `iterator` and not `const_iterator`.

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