The problem is with CommonReference<reference_t<I>, value_type_t<I>&>(). Imagine an iterator I such that reference_t<I> is std::unique_ptr<int>. The CommonReference first computes the common reference type to be std::unique_ptr<int>, then it tests that std::unique_ptr<int>& is ConvertibleTo std::unique_ptr<int>, which is of course false.

The fix is to instead be testing CommonReference<reference_t<I>&&, value_type_t<I>&>(). That causes the common reference to be computed as const std::unique_ptr<int>&, and std::unique_ptr<int>& is indeed convertible to that.

Proposed Resolution

(Includes the resolution for #339, accepted by LWG in Kona but not yet moved in full committee).

Change the Readable concept [iterators.readable] as follows:

template <class I>
concept bool Readable() {
  return requires {
      typename value_type_t<I>;
      typename reference_t<I>;
      typename rvalue_reference_t<I>;
    } &&
-   CommonReference<reference_t<I>, value_type_t<I>&>() &&
-   CommonReference<reference_t<I>, rvalue_reference_t<I>>() &&
-   CommonReference<rvalue_reference_t<I>, const value_type_t<I>&>();
+   CommonReference<reference_t<I>&&, value_type_t<I>&>() &&
+   CommonReference<reference_t<I>&&, rvalue_reference_t<I>&&>() &&
+   CommonReference<rvalue_reference_t<I>&&, const value_type_t<I>&>();
}