Writable should work with rvalues

[CaseyCarter]

During LWG Kona review of D0547R1. Writable is defined therein as:

template <class Out, class T>
concept bool Writable() {
  return requires(Out& o, T&& t) {
    *o = std::forward<T>(t);
  };
}

The concept only requires writability for lvalues, but it seems reasonable to require that writability is oblivious to value category.

Proposed Resolution

Accept the wording in P0547R1.

[ericniebler]

SUPERSEDED (Well, not really: this wording was incorporated into P0547R1)

Wording relative to D0547R1. Change the definition of the concept Writable in [iterators.writable] as follows:

 template <class Out, class T>
 concept bool Writable() {
-  return requires(Out& o, T&& t) {
-    *o = std::forward<T>(t);
+  return requires(Out& o1, Out&& o2, T&& t) {
+    *o1 = std::forward<T>(t);
+    *std::forward<Out>(o2) = std::forward<T>(t);
   };
 }

We need to combine this resolution with the one from #381.