LWG3746 optional's spaceship with U with a type derived from optional causes infinite constraint meta-recursion

Author: burblebee

source/utilities.tex

@@ -3131,9 +3131,9 @@
     class optional;
 
   template<class T>
-    constexpr bool @\exposid{is-optional}@ = false;                 // \expos
-  template<class T>
-    constexpr bool @\exposid{is-optional}@<optional<T>> = true;     // \expos
+    concept @\defexposconcept{is-derived-from-optional}@ = requires(const T& t) {       // \expos
+      []<class U>(const optional<U>&){ }(t);
+    };
 
   // \ref{optional.nullopt}, no-value state indicator
   struct nullopt_t{@\seebelow@};
@@ -3177,7 +3177,8 @@
   template<class T, class U> constexpr bool operator<=(const T&, const optional<U>&);
   template<class T, class U> constexpr bool operator>=(const optional<T>&, const U&);
   template<class T, class U> constexpr bool operator>=(const T&, const optional<U>&);
-  template<class T, class U> requires (!@\exposid{is-optional}@<U>) && @\libconcept{three_way_comparable_with}@<T, U>
+  template<class T, class U>
+      requires (!@\exposconcept{is-derived-from-optional}@<U>) && @\libconcept{three_way_comparable_with}@<T, U>
     constexpr compare_three_way_result_t<T, U>
       operator<=>(const optional<T>&, const U&);
 
@@ -4748,7 +4749,8 @@
 
 \indexlibrarymember{operator<=>}{optional}%
 \begin{itemdecl}
-template<class T, class U> requires (!@\exposid{is-optional}@<U>) && @\libconcept{three_way_comparable_with}@<T, U>
+template<class T, class U>
+    requires (!@\exposconcept{is-derived-from-optional}@<U>) && @\libconcept{three_way_comparable_with}@<T, U>
   constexpr compare_three_way_result_t<T, U>
     operator<=>(const optional<T>& x, const U& v);
 \end{itemdecl}