checking Erasable concept

test/unit.cpp

@@ -10767,6 +10767,24 @@ TEST_CASE("concepts")
 
         // the expression "X()" has the post-condition "X().empty()"
         CHECK(json().empty());
+
+
+        // from http://en.cppreference.com/w/cpp/concept/Container:
+        // T must be Eraseable
+        {
+            // prepare
+            auto m = json::get_allocator();
+            json* p = m.allocate(1);
+            m.construct(p, "foo");
+
+            // required
+            std::allocator_traits<json::allocator_type>::destroy(m, p);
+
+            CHECK(*p == "foo");
+
+            // cleanup
+            m.deallocate(p, 1);
+        }
     }
 
     SECTION("class json")