GIT_INDEX_FILE relative paths are relative to the work tree

When I use a relative path in the GIT_INDEX_FILE environment variable, git interprets that path relative to the the work tree. This can be confusing if my cwd is some subdirectory of my project; in that case an index file is created in the project root rather than in my cwd. It can also be confusing if I'm using --git-dir and --work-tree, in which case the path is interpreted relative to the latter.

Is this behavior intentional? If so, I think it would be worth mentioning in the documentation for GIT_INDEX_FILE. But I can't think of a case where I would ever want an index file to end up inside my work tree.

This also comes up in http://stackoverflow.com/a/7645492/823869. Using absolute paths is a workaround.