vim wildmode=longest,list doesn't work when files are on different directories

Vim file name autocompletion doesn't work if files are on different directory levels. Steps to reproduce:

1. set wildmode=longest,list in vimrc
2. vim usa1.txt test/usa2.txt
3. type :b u in EX mode, then press TAB, it is expected to auto complete the word up 开发者_JS百科to :b usa, which is the common header of two file names. But it just doesn't work.

Any solutions? Thanks a lot!

PS: if the above two files are placed on the SAME directory, it works!!!

This is by design. Completion isn't "deep" (that could potentially cost a lot of time, or result in a lot of matches), it only considers the current directory level.