Disable length when taking from a lengthless range

Fix #79.

source/tanya/algorithm/iteration.d

@@ -23,6 +23,7 @@ module tanya.algorithm.iteration;
 import tanya.algorithm.comparison;
 import tanya.algorithm.mutation;
 import tanya.range;
+version (unittest) import tanya.test.range;
 
 private struct Take(R, bool exactly)
 {
@@ -73,14 +74,17 @@ private struct Take(R, bool exactly)
         }
         else
         {
-            return length == 0 || this.source.empty;
+            return this.length_ == 0 || this.source.empty;
         }
     }
 
+    static if (exactly || hasLength!R)
+    {
         @property size_t length()
         {
             return this.length_;
         }
+    }
 
     static if (hasAssignableElements!R)
     {
@@ -315,6 +319,19 @@ if (isInputRange!R)
     assert(t.empty);
 }
 
+// length is unknown when taking from a range without length
+@nogc nothrow pure @safe unittest
+{
+    @Empty
+    static struct R
+    {
+        mixin InputRange;
+    }
+    auto actual = take(R(), 100);
+
+    static assert(!hasLength!(typeof(actual)));
+}
+
 /**
  * Takes exactly $(D_PARAM n) elements from $(D_PARAM range).
  *

source/tanya/meta/trait.d

@@ -1493,6 +1493,8 @@ if (F.length == 1)
 }
 
 /**
+ * Determines whether $(D_PARAM T) defines a symbol $(D_PARAM member).
+ *
  * Params:
  *  T      = Aggregate type.
  *  member = Symbol name.

source/tanya/test/range.d

@@ -0,0 +1,59 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+/**
+ * Range generators for tests.
+
+ * Copyright: Eugene Wissner 2018.
+ * License: $(LINK2 https://www.mozilla.org/en-US/MPL/2.0/,
+ *                  Mozilla Public License, v. 2.0).
+ * Authors: $(LINK2 mailto:info@caraus.de, Eugene Wissner)
+ * Source: $(LINK2 https://github.com/caraus-ecms/tanya/blob/master/source/tanya/test/range.d,
+ *                 tanya/test/range.d)
+ */
+module tanya.test.range;
+
+package(tanya) struct Empty
+{
+}
+
+package(tanya) template InputRange()
+{
+    import tanya.meta.metafunction : AliasSeq;
+
+    private alias attributes = AliasSeq!(__traits(getAttributes, typeof(this)));
+
+    static foreach (attribute; attributes)
+    {
+        static if (is(attribute == Empty))
+        {
+            @property bool empty() const @nogc nothrow pure @safe
+            {
+                return true;
+            }
+        }
+    }
+
+    void popFront() @nogc nothrow pure @safe
+    {
+        static foreach (attribute; attributes)
+        {
+            static if (is(attribute == Empty))
+            {
+                assert(false);
+            }
+        }
+    }
+
+    int front() @nogc nothrow pure @safe
+    {
+        static foreach (attribute; attributes)
+        {
+            static if (is(attribute == Empty))
+            {
+                assert(false);
+            }
+        }
+    }
+}