fix issue 13314 - BinaryHeap assumes Store has dup property

std/container/binaryheap.d

@@ -198,15 +198,18 @@ Returns $(D true) if the heap is _empty, $(D false) otherwise.
     }
 
 /**
-Returns a duplicate of the heap. The underlying store must also
-support a $(D dup) method.
+Returns a duplicate of the heap. The $(D dup) method is available only if the
+underlying store supports it.
      */
-    @property BinaryHeap dup()
+    static if (is(typeof((Store s) { return s.dup; }(Store.init)) == Store))
     {
-        BinaryHeap result;
-        if (!_payload.refCountedStore.isInitialized) return result;
-        result.assume(_store.dup, length);
-        return result;
+        @property BinaryHeap dup()
+        {
+            BinaryHeap result;
+            if (!_payload.refCountedStore.isInitialized) return result;
+            result.assume(_store.dup, length);
+            return result;
+        }
     }
 
 /**
@@ -473,3 +476,64 @@ unittest // 16072
 
     assert(r.front == 99);
 }
+
+unittest
+{
+    import std.algorithm.comparison : equal;
+    int[] a = [4, 1, 3, 2, 16, 9, 10, 14, 8, 7];
+    auto heap = heapify(a);
+    auto dup = heap.dup();
+    assert(dup.equal([16, 14, 10, 9, 8, 7, 4, 3, 2, 1]));
+}
+
+unittest
+{
+    static struct StructWithoutDup
+    {
+        int[] a;
+        @disable StructWithoutDup dup()
+        {
+            StructWithoutDup d;
+            return d;
+        }
+        alias a this;
+    }
+
+    // Assert Binary heap can be created when Store doesn't have dup
+    // if dup is not used.
+    assert(__traits(compiles, ()
+        {
+            auto s = StructWithoutDup([1,2]);
+            auto h = heapify(s);
+        }));
+
+    // Assert dup can't be used on BinaryHeaps when Store doesn't have dup
+    assert(!__traits(compiles, ()
+        {
+            auto s = StructWithoutDup([1,2]);
+            auto h = heapify(s);
+            h.dup();
+        }));
+}
+
+unittest
+{
+    static struct StructWithDup
+    {
+        int[] a;
+        StructWithDup dup()
+        {
+            StructWithDup d;
+            return d;
+        }
+        alias a this;
+    }
+
+    // Assert dup can be used on BinaryHeaps when Store has dup
+    assert(__traits(compiles, ()
+        {
+            auto s = StructWithDup([1, 2]);
+            auto h = heapify(s);
+            h.dup();
+        }));
+}