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

[A. Jiang via libcxx-commits]

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@@ -19,6 +19,21 @@
 #include <type_traits>
 #include <utility>
 
+template <typename T>
+concept has_iterator = requires { typename T::iterator; };
+
+template <typename T>
+concept has_const_iterator = requires { typename T::const_iterator; };
+
+template <typename T>
+concept has_both_iterators = has_iterator<T> && has_const_iterator<T>;
+
+template <typename T>
+concept only_has_iterator = has_iterator<T> && !has_const_iterator<T>;
+
+template <typename T>
+concept has_no_iterators = !has_iterator<T> && !has_const_iterator<T>;
+
 template <typename T>
 constexpr bool test_range_concept() {
   return std::ranges::range<std::optional<T>>;
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frederick-vs-ja wrote:

It seems more senseful to make this a `concept` or `constexpr bool` variable template.

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