bug#34178: 27.0.50; Tramp not responding correctly to "/scp:slc:" syntax

[Tim Blackman]

When using this nightly Emacs build, I noticed that C-x C-f was not
responding correctly to the normal tramp syntax.

After starting up with 'emacs -Q', I did:

  C-x C-f /scp:slc:<RET>

("slc" is the name of my remote host.)

The message area displayed:

 File not found and directory write-protected

The buffer showing was named "scp:slc:", so it seemed to think I was
talking about a local file.

Note that when I type C-x C-f in the 26.1 pretest version, the default
directory is "~/", but it is "/" in the nightly. Not sure if that is
significant.

Note that, following instructions, I did M-: (setq tramp-verbose 6)
prior to calling C-x C-f, but there did not seem to be a tramp debug
trace generated. I'm guessing that the problem is happening earlier and
the command is deciding to not invoke tramp at all.

- Tim

In GNU Emacs 27.0.50 (build 1, x86_64-apple-darwin14.5.0, NS appkit-1561.60 
Version 10.13.6 (Build 17G4015))
 of 2019-01-23 built on Lime.local
Windowing system distributor 'Apple', version 10.3.1561
System Description:  Mac OS X 10.13.6

Recent messages:
Loading electric...done
Loading emacs-lisp/eldoc...done
Loading cus-start...done
Loading tooltip...done
Loading 
/Users/tblackma/Desktop/Emacs-nightly.app/Contents/Resources/lisp/leim/leim-list.el
 (source)...done
Finding pointers to doc strings...done
For information about GNU Emacs and the GNU system, type C-h C-a.
Loading loadup.el (source)...done
6 (#o6, #x6, ?\C-f)
File not found and directory write-protected

Configured using:
 'configure --with-ns '--enable-locallisppath=/Library/Application
 Support/Emacs/${version}/site-lisp:/Library/Application
 Support/Emacs/site-lisp' --with-modules'

Configured features:
NOTIFY KQUEUE ACL GNUTLS LIBXML2 ZLIB TOOLKIT_SCROLL_BARS NS MODULES
THREADS CANNOT_DUMP

Important settings:
  value of $LANG: en_US.UTF-8
  locale-coding-system: utf-8-unix

Major mode: Fundamental

Minor modes in effect:
  shell-dirtrack-mode: t
  tooltip-mode: t
  global-eldoc-mode: t
  electric-indent-mode: t
  mouse-wheel-mode: t
  tool-bar-mode: t
  menu-bar-mode: t
  file-name-shadow-mode: t
  global-font-lock-mode: t
  font-lock-mode: t
  blink-cursor-mode: t
  auto-composition-mode: t
  auto-encryption-mode: t
  auto-compression-mode: t
  buffer-read-only: t
  line-number-mode: t
  transient-mark-mode: t

Load-path shadows:
None found.

Features:
(shadow sort mail-extr emacsbug message rmc puny dired dired-loaddefs
rfc822 mml easymenu mml-sec epa derived epg epg-config gnus-util rmail
rmail-loaddefs time-date mm-decode mm-bodies mm-encode mail-parse
rfc2231 mailabbrev gmm-utils mailheader sendmail rfc2047 rfc2045
ietf-drums mm-util mail-prsvr mail-utils tramp-loaddefs trampver
tramp-compat shell pcomplete comint ansi-color ring parse-time
format-spec advice auth-source cl-seq eieio eieio-core cl-macs
eieio-loaddefs password-cache json map seq byte-opt gv bytecomp
byte-compile cconv cl-loaddefs cl-lib elec-pair tooltip cus-start eldoc
electric uniquify ediff-hook vc-hooks lisp-float-type mwheel term/ns-win
ns-win ucs-normalize mule-util term/common-win tool-bar dnd fontset
image regexp-opt fringe tabulated-list replace newcomment text-mode
elisp-mode lisp-mode prog-mode register page menu-bar rfn-eshadow
isearch timer select scroll-bar mouse jit-lock font-lock syntax facemenu
font-core term/tty-colors frame cl-generic cham georgian utf-8-lang
misc-lang vietnamese tibetan thai tai-viet lao korean japanese eucjp-ms
cp51932 hebrew greek romanian slovak czech european ethiopic indian
cyrillic chinese composite charscript charprop case-table epa-hook
jka-cmpr-hook help simple abbrev obarray minibuffer cl-preloaded nadvice
loaddefs button faces cus-face macroexp files text-properties overlay
sha1 md5 base64 format env code-pages mule custom widget
hashtable-print-readable backquote threads kqueue cocoa ns multi-tty
make-network-process emacs)

Memory information:
((conses 16 280154 15648)
 (symbols 48 21850 2)
 (strings 32 59698 2519)
 (string-bytes 1 2495898)
 (vectors 16 51938)
 (vector-slots 8 1404615 50944)
 (floats 8 190 60)
 (intervals 56 225 0)
 (buffers 992 12))