Coercing ranges to the same type

[Matt Kline via Digitalmars-d-learn]

As it turns out, inputRangeObject does an excellent job at this 
task. The solution then becomes something like:

InputRange!string getEntries(string[] paths, bool recursive)
{
     auto files = paths.filter!(p => p.isFile);

     if (recursive) {
         auto expandedDirs = paths
             .filter!(p => p.isDir)
             .map!(p => dirEntries(p, SpanMode.depth, false))
             .joiner
             .map!(de => de.name);

         return inputRangeObject(chain(files, expandedDirs));
     }
     else {
         foreach (dir; paths.filter!(p => p.isDir))
             stderr.writeln("omitting directory " , dir);

         return inputRangeObject(files);
     }
}