Short circuit type for takeExactly(takeExactly(r, n))

std/range.d

@@ -2384,25 +2384,35 @@ is an input range with length. Finally, if $(D R) is a forward range
 auto takeExactly(R)(R range, size_t n)
 if (isInputRange!R && !isRandomAccessRange!R)
 {
-    static struct Result
+    static if (is(typeof(takeExactly(range._input, n)) == R))
     {
-        private R _input;
-        private size_t _n;
-
-        @property bool empty() const { return !_n; }
-        @property auto ref front()
+        // takeExactly(takeExactly(r, n1), n2) has the same type as
+        // takeExactly(r, n1) and simply returns takeExactly(r, n2)
+        range._n = n;
+        return range;
+    }
+    else
+    {
+        static struct Result
         {
-            assert(_n > 0, "front() on an empty " ~ Result.stringof);
-            return _input.front();
-        }
-        void popFront() { _input.popFront(); --_n; }
-        size_t length() const { return _n; }
+            R _input;
+            private size_t _n;
+
+            @property bool empty() const { return !_n; }
+            @property auto ref front()
+            {
+                assert(_n > 0, "front() on an empty " ~ Result.stringof);
+                return _input.front();
+            }
+            void popFront() { _input.popFront(); --_n; }
+            size_t length() const { return _n; }
 
-        static if (isForwardRange!R)
-            auto save() { return this; }
+            static if (isForwardRange!R)
+                auto save() { return this; }
+        }
+
+        return Result(range, n);
     }
-
-    return Result(range, n);
 }
 
 auto takeExactly(R)(R range, size_t n)
@@ -2416,6 +2426,12 @@ unittest
     auto a = [ 1, 2, 3, 4, 5 ];
     auto b = takeExactly(a, 3);
     assert(equal(b, [1, 2, 3]));
+    auto c = takeExactly(b, 2);
+
+    auto d = filter!"a > 0"(a);
+    auto e = takeExactly(d, 3);
+    assert(equal(e, [1, 2, 3]));
+    assert(equal(takeExactly(e, 4), [1, 2, 3, 4]));
     // assert(b.length == 3);
     // assert(b.front == 1);
     // assert(b.back == 3);