See http://trac.nginx.org/nginx/ticket/97

Given the file structure:

  /home/geira/scratch/foo
  /home/geira/scratch/bar/
  /home/geira/scratch/bar/baz

and the nginx configuration stub:

  location /test/ {
      alias /home/geira/scratch/;
      try_files $uri bar/$uri $uri/baz =404;
  }

let's try some experiments.

$ HEAD http://localhost:8000/test/foo  # expecting /home/geira/scratch/foo (case 1)
200 OK

 *10 http uri: "/test/foo"
...
 *10 http script var: "/test/foo"
 *10 trying to use file: "foo" "/home/geira/scratch/foo"
 *10 try file uri: "/test/foo"
...
 *10 http filename: "/home/geira/scratch/foo"
...
 *10 HTTP/1.1 200 OK

$ HEAD http://localhost:8000/test/bar  # expecting /home/geira/scratch/bar/baz (case 3)
200 OK

 *11 http uri: "/test/bar"
...
 *11 http script var: "/test/bar"
 *11 trying to use file: "bar" "/home/geira/scratch/bar"
 *11 http script copy: "bar/"
 *11 http script var: "/test/bar"
 *11 trying to use file: "bar//test/bar" "/home/geira/scratch/bar//test/bar"
 *11 http script var: "/test/bar"
 *11 http script copy: "/baz"
 *11 trying to use file: "bar/baz" "/home/geira/scratch/bar/baz"
 *11 try file uri: "/test/bar/baz"
...
 *11 http filename: "/home/geira/scratch/bar/baz"
...
 *11 HTTP/1.1 200 OK

$ HEAD http://localhost:8000/test/baz  # expecting /home/geira/scratch/bar/baz (case 2)
404 Not Found

 *12 http uri: "/test/baz"
...
 *12 http script var: "/test/baz"
 *12 trying to use file: "baz" "/home/geira/scratch/baz"
 *12 http script copy: "bar/"
 *12 http script var: "/test/baz"
 *12 trying to use file: "bar//test/baz" "/home/geira/scratch/bar//test/baz"
 *12 http script var: "/test/baz"
 *12 http script copy: "/baz"
 *12 trying to use file: "baz/baz" "/home/geira/scratch/baz/baz"
 *12 trying to use file: "=404" "/home/geira/scratch/=404"
 *12 http finalize request: 404, "/test/baz?" a:1, c:1
 *12 http special response: 404, "/test/baz?"
...
 *12 HTTP/1.1 404 Not Found

-----------------------------

Seeing concatenation with $uri doesn't work except when used as prefix,
let's try adding a hardcoded file path to baz as fallback:

location /test/ {
    alias /home/geira/scratch/;
    try_files $uri bar/$uri $uri/baz bar/baz =404;
}

$ HEAD http://localhost:8000/test/nonexistant  # expecting /home/geira/scratch/bar/baz (case 4)
200 OK

 *13 http uri: "/test/nonexistant"
...
 *13 try files phase: 9
 *13 http script var: "/test/nonexistant"
 *13 trying to use file: "nonexistant" "/home/geira/scratch/nonexistant"
 *13 http script copy: "bar/"
 *13 http script var: "/test/nonexistant"
 *13 trying to use file: "bar//test/nonexistant" "/home/geira/scratch/bar//test/nonexistant"
 *13 http script var: "/test/nonexistant"
 *13 http script copy: "/baz"
 *13 trying to use file: "nonexistant/baz" "/home/geira/scratch/nonexistant/baz"
 *13 trying to use file: "bar/baz" "/home/geira/scratch/bar/baz"
 *13 try file uri: "/test/bar/baz"
...
 *13 HTTP/1.1 200 OK