enable implicit conversion when List can be constructed from U

Author: Enchufa2

inst/include/Rcpp/vector/ListOf.h

@@ -40,9 +40,14 @@ class ListOf
         std::transform(list.begin(), list.end(), list.begin(), as<T>);
     }
 
-    template <typename U,
-              typename std::enable_if<std::is_convertible<U, SEXP>::value, int>::type = 0>
-    ListOf(const U& data_): ListOf(static_cast<SEXP>(data_)) {}
+    // enabled only when List itself can be constructed from data_, so this
+    // does not compete with a converting operator T() offered by data_'s own
+    // type while preserving every constructor List/Vector accepts
+    // (SEXP, size, Dimension, wrap()-able third-party types, ...)
+    template <typename U, typename = decltype(List(std::declval<const U&>()))>
+    ListOf(const U& data_): list(data_) {
+        std::transform(list.begin(), list.end(), list.begin(), as<T>);
+    }
 
     ListOf(const ListOf& other): list(other.list) {}