LWG3724 decay-copy should be constrained

Author: burblebee

source/lib-intro.tex

@@ -492,9 +492,11 @@
 \indexlibrary{decay-copy@\tcode{\placeholder{decay-copy}}}%
 \begin{codeblock}
 namespace std {
-  template<class T> constexpr decay_t<T> @\placeholdernc{decay-copy}@(T&& v)
-      noexcept(is_nothrow_convertible_v<T, decay_t<T>>)         // \expos
-    { return std::forward<T>(v); }
+  template<class T>
+    requires @\libconcept{convertible_to}@<T, decay_t<T>>
+      constexpr decay_t<T> @\placeholdernc{decay-copy}@(T&& v)
+        noexcept(is_nothrow_convertible_v<T, decay_t<T>>)       // \expos
+      { return std::forward<T>(v); }
 
   constexpr auto @\placeholdernc{synth-three-way}@ =
     []<class T, class U>(const T& t, const U& u)