file-name completion succeeds with final $$

[Drew Adams]

This seems like a bug to me, but, if not, I'd like to understand the
rationale behind it:
 
emacs -Q
 
(try-completion "c:/foo/$" 'read-file-name-internal "c:/foo/")
 
raises an error: "Bad format environment-variable substitution".
That's as I expect.
 
(try-completion "c:/foo/$$" 'read-file-name-internal "c:/foo/")
 
returns "c:/foo/$$", which I didn't expect. I expected it to
return nil, since there is no existing file named `$'.
(`$$' escapes `$', giving a literal `$'.)
 
Same thing, apparently, with anything else in place of `foo' (both
places).
 

In GNU Emacs 22.1.1 (i386-mingw-nt5.1.2600)
 of 2007-06-02 on RELEASE
Windowing system distributor `Microsoft Corp.', version 5.1.2600
configured using `configure --with-gcc (3.4) --cflags -Ic:/gnuwin32/include'