Coercing ranges to the same type

[Matt Kline via Digitalmars-d-learn]

Say I'm trying to expand an array of file and directory paths 
(such as ones given as command line args) into a range of file 
paths I can iterate over. A simplified example might be:

auto 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); // back to strings

         return chain(files, expandedDirs);
     }
     else {
         return files;
     }
}

Even though both return statements return a range of strings, 
this doesn't compile because the result of `chain` is a different 
type than the result of `filter`. Is there some generic range I 
could coerce both ranges to in order to have the same return type 
and make this work? .array is a non-starter since it throws out 
the ranges' laziness.