Redundant function calls in filtered joined range

[FreeSlave]

Consider the following code. It counts the number of 
subdirectories in directories provided via commandline args.

import std.stdio;
import std.algorithm;
import std.file;
import std.range;
import std.exception;

@trusted bool isDirNothrow(string dir) nothrow
{
     bool ok;
     collectException(dir.isDir(), ok);
     return ok;
}

auto subDirs(Range)(Range searchDirs)
if(is(ElementType!Range : string))
{
     return searchDirs
         .filter!(isDirNothrow).map!(function(dir) {
             return dir.dirEntries(SpanMode.shallow)
                 .filter!(isDirNothrow);
         }).cache().joiner;
}

int main(string[] args)
{
     auto r = args[1..$].subDirs;
     writefln("Found %s subdirs", count(r));
     return 0;
}

Using strace I inspect how many stat calls it uses. Like this:

strace -C -e stat path/to/binary

strace reports some duplicated stat calls, specifically on the 
first provided directory and first subdirectory of each 
directories.

How can I avoid redundant stat calls? Adding array() after each 
filter() removes duplicates but it leads to allocations 
obviously. Changing the cache() to array() before joiner also 
solve the issue.

I suspect there's something wrong in combination of filters and 
joiner.