LWG3549 view_interface is overspecified to derive from view_base

Author: jensmaurer

source/ranges.tex

@@ -1227,10 +1227,22 @@
 \indexlibraryglobal{enable_view}%
 \begin{itemdecl}
 template<class T>
-  inline constexpr bool enable_view = @\libconcept{derived_from}@<T, view_base>;
+  inline constexpr bool @\exposid{is-derived-from-view-interface}@ = @\seebelow@;     // \expos
+template<class T>
+  inline constexpr bool enable_view =
+    @\libconcept{derived_from}@<T, view_base> || @\exposid{is-derived-from-view-interface}@<T>;
 \end{itemdecl}
 
 \begin{itemdescr}
+\pnum
+For a type \tcode{T},
+\tcode{\exposid{is-derived-from-view-interface}<T>} is \tcode{true}
+if and only if
+\tcode{T} has exactly one public base class \tcode{view_interface<U>}
+for some type \tcode{U} and
+\tcode{T} has no base classes of type \tcode{view_interface<V>}
+for any other type \tcode{V}.
+
 \pnum
 \remarks
 Pursuant to \ref{namespace.std}, users may specialize \tcode{enable_view}
@@ -1363,7 +1375,7 @@
 namespace std::ranges {
   template<class D>
     requires is_class_v<D> && @\libconcept{same_as}@<D, remove_cv_t<D>>
-  class view_interface : public view_base {
+  class view_interface {
   private:
     constexpr D& @\exposid{derived}@() noexcept {                   // \expos
       return static_cast<D&>(*this);