alter. to avoid constructor taking range itself

Co-authored-by: Thomas Grützmacher <thomas.gruetzmacher@kit.edu>

accessor/range.hpp

@@ -42,6 +42,29 @@ OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 
 namespace gko {
 namespace acc {
+namespace {
+
+
+/**
+ * the default check_if_valid gives true.
+ *
+ * @tparam Ref  the reference type
+ * @tparam Args  the input type
+ */
+template <typename Ref, typename... Args>
+struct check_if_valid : public std::true_type {};
+
+/**
+ * check_if_valid gives false if the decay type of input is the same type as
+ * Ref.
+ *
+ * @tparam Ref  the reference type
+ */
+template <typename Ref>
+struct check_if_valid<Ref, Ref> : public std::false_type {};
+
+
+}  // namespace
 
 
 template <typename Accessor>
@@ -66,14 +89,16 @@ class range {
      * Creates a new range.
      *
      * @tparam AccessorParam  types of parameters forwarded to the accessor
-     *                        constructor
+     *                        constructor. It does not allow range itself.
      *
      * @param params  parameters forwarded to Accessor constructor.
      */
     template <typename... AccessorParams,
-              std::enable_if_t<(sizeof...(AccessorParams) > 1), bool> = true>
-    GKO_ACC_ATTRIBUTES constexpr explicit range(AccessorParams &&... params)
-        : accessor_{std::forward<AccessorParams>(params)...}
+              std::enable_if_t<
+                  check_if_valid<range, std::decay_t<AccessorParams>...>::value,
+                  int> = 0>
+    GKO_ACC_ATTRIBUTES constexpr explicit range(AccessorParams &&... args)
+        : accessor_{std::forward<AccessorParams>(args)...}
     {}
 
     /**