Make joiner (with separator) work with forward ranges and transient r…

…anges.

This is for the variant of joiner that takes a separator.

Restructure algorithm to not call .popFront while we're still using
.front.

Make sure we .save subranges when we're exporting .save, since RoR.save
does not guarantee the state of subranges are also .save'd.

std/algorithm.d

@@ -2665,28 +2665,45 @@ if (isInputRange!RoR && isInputRange!(ElementType!RoR)
         private ElementType!RoR _current;
         private Separator _sep, _currentSep;
 
+        private enum setItem =
+        q{
+            if (!_items.empty)
+            {
+                // If we're exporting .save, we must not consume any of the
+                // subranges, since RoR.save does not guarantee that the states
+                // of the subranges are also saved.
+                static if (isForwardRange!RoR &&
+                           isForwardRange!(ElementType!RoR))
+                    _current = _items.front.save;
+                else
+                    _current = _items.front;
+            }
+        };
+
         private void useSeparator()
         {
-            assert(_currentSep.empty && _current.empty,
+            // Separator must always come after an item.
+            assert(_currentSep.empty && !_items.empty,
                     "joiner: internal error");
+            _items.popFront();
+
+            // If there are no more items, we're done, since separators are not
+            // terminators.
+            if (_items.empty) return;
+
             if (_sep.empty)
             {
                 // Advance to the next range in the
                 // input
-                //_items.popFront();
-                for (;; _items.popFront())
+                while (_items.front.empty)
                 {
+                    _items.popFront();
                     if (_items.empty) return;
-                    if (!_items.front.empty) break;
                 }
-                _current = _items.front;
-                _items.popFront();
+                mixin(setItem);
             }
             else
             {
-                // Must make sure something is coming after the
-                // separator - it's a separator, not a terminator!
-                if (_items.empty) return;
                 _currentSep = _sep.save;
                 assert(!_currentSep.empty);
             }
@@ -2698,34 +2715,24 @@ if (isInputRange!RoR && isInputRange!(ElementType!RoR)
                     "joiner: internal error");
             // Use the input
             if (_items.empty) return;
-            _current = _items.front;
-            _items.popFront();
-            if (!_current.empty)
+            mixin(setItem);
+            if (_current.empty)
             {
-                return;
+                // No data in the current item - toggle to use the separator
+                useSeparator();
             }
-            // No data in the current item - toggle to use the
-            // separator
-            useSeparator();
         };
 
         this(RoR items, Separator sep)
         {
             _items = items;
             _sep = sep;
             mixin(useItem); // _current should be initialized in place
-            // We need the separator if the input has at least two
-            // elements
-            if (_current.empty && _items.empty)
-            {
-                // Vacate the whole thing
-                _currentSep = _currentSep.init;
-            }
         }
 
         @property auto empty()
         {
-            return _current.empty && _currentSep.empty;
+            return _items.empty;
         }
 
         @property ElementType!(ElementType!RoR) front()
@@ -2737,7 +2744,7 @@ if (isInputRange!RoR && isInputRange!(ElementType!RoR)
 
         void popFront()
         {
-            assert(!empty);
+            assert(!_items.empty);
             // Using separator?
             if (!_currentSep.empty)
             {