From 6e76e11c4200a4d4185e0b7d6cea5164d459737b Mon Sep 17 00:00:00 2001
From: Eli Zaretskii <eliz@gnu.org>
Date: Wed, 12 Dec 2018 18:27:05 +0200
Subject: [PATCH] Fix regression in expand-file-name with drive-relative HOME

* src/fileio.c (get_homedir) [DOS_NT]: Expand drive-relative
$HOME to begin with "X:/".

* test/src/fileio-tests.el (fileio-tests--relative-HOME): Add
testing of drive-relative value of $HOME on MS-Windows and
MS-DOS.
---
 src/fileio.c             | 28 ++++++++++++++++++++++++++++
 test/src/fileio-tests.el |  4 ++++
 2 files changed, 32 insertions(+)

diff --git a/src/fileio.c b/src/fileio.c
index d9795715f9e..687f6ec7452 100644
--- a/src/fileio.c
+++ b/src/fileio.c
@@ -1692,6 +1692,34 @@ get_homedir (void)
       if (!home)
 	return "";
     }
+#ifdef DOS_NT
+  /* If home is a drive-relative directory, expand it.  */
+  if (IS_DRIVE (*home)
+      && IS_DEVICE_SEP (home[1])
+      && !IS_DIRECTORY_SEP (home[2]))
+    {
+# ifdef WINDOWSNT
+      static char hdir[MAX_UTF8_PATH];
+# else
+      static char hdir[MAXPATHLEN];
+# endif
+      if (!getdefdir (c_toupper (*home) - 'A' + 1, hdir))
+	{
+	  hdir[0] = c_toupper (*home);
+	  hdir[1] = ':';
+	  hdir[2] = '/';
+	  hdir[3] = '\0';
+	}
+      if (home[2])
+	{
+	  size_t homelen = strlen (hdir);
+	  if (!IS_DIRECTORY_SEP (hdir[homelen - 1]))
+	    strcat (hdir, "/");
+	  strcat (hdir, home + 2);
+	}
+      home = hdir;
+    }
+#endif
   if (IS_ABSOLUTE_FILE_NAME (home))
     return home;
   if (!emacs_wd)
diff --git a/test/src/fileio-tests.el b/test/src/fileio-tests.el
index b7b78bbda09..a74bcea41f2 100644
--- a/test/src/fileio-tests.el
+++ b/test/src/fileio-tests.el
@@ -102,4 +102,8 @@ Also check that an encoding error can appear in a symlink."
     (setenv "HOME" "a/b/c")
     (should (equal (expand-file-name "~/foo")
                    (expand-file-name "a/b/c/foo")))
+    (when (memq system-type '(ms-dos windows-nt))
+      ;; Test expansion of drive-relative file names.
+      (setenv "HOME" "x:foo")
+      (should (equal (expand-file-name "~/bar") "x:/foo/bar")))
     (setenv "HOME" old-home)))
-- 
2.39.2