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

[A. Jiang via libcxx-commits]

================
@@ -56,7 +71,11 @@ constexpr bool test() {
     assert((std::is_same_v<typename decltype(it)::value_type, std::remove_cvref_t<T>>));
     assert((std::is_same_v<typename decltype(it)::reference, std::remove_reference_t<T>&>));
     assert((std::is_same_v<typename decltype(it2)::value_type, std::remove_cvref_t<T>>));
-    assert((std::is_same_v<typename decltype(it2)::reference, const std::remove_reference_t<T>&>));
+
+    // for optional<T&>, there is no const_iterator
+    if (!std::is_lvalue_reference_v<T>) {
+      assert((std::is_same_v<typename decltype(it2)::reference, const std::remove_reference_t<T>&>));
+    }
----------------
frederick-vs-ja wrote:

Maybe pre-existing. Why aren't `if constexpr` and `static_assert` used here?
```suggestion
    if constexpr (!std::is_lvalue_reference_v<T>) {
      static_assert(std::is_same_v<typename decltype(it2)::reference, const std::remove_reference_t<T>&>);
    }
```

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