RE: what's the mystery of file-name variable?

[Drew Adams]

> (setq file-name "test")
> (describe-variable 'file-name)
>  file-name's value is nil
> 
>  Documentation:
>  Not documented as a variable."

It works for me.  Says file-name's value is "test" (and not documented etc.).

Give a recipe starting from emacs -Q.