Fix issue 8411 - add opCast!bool support for Duration.

src/core/time.d

@@ -1104,6 +1104,22 @@ public:
         }
     }
 
+    /++
+        Allow Duration to be used as a boolean.
+        Returns: `true` if this duration is non-zero.
+      +/
+    bool opCast(T : bool)() const nothrow @nogc
+    {
+        return _hnsecs != 0;
+    }
+
+    unittest
+    {
+        auto d = 10.minutes;
+        assert(d);
+        assert(!(d - d));
+        assert(d + d);
+    }
 
     //Temporary hack until bug http://d.puremagic.com/issues/show_bug.cgi?id=5747 is fixed.
     Duration opCast(T)() const nothrow @nogc