patch 8.2.0208: fnamemodify() does not apply ":~" when followed by ":."

Problem:    Fnamemodify() does not apply ":~" when followed by ":.".
Solution:   Don't let a failing ":." cause the ":~" to be skipped. (Yasuhiro
            Matsumoto, closes #5577)

runtime/doc/cmdline.txt

@@ -933,8 +933,7 @@ These modifiers can be given, in this order:
 		directory.
 	:.	Reduce file name to be relative to current directory, if
 		possible.  File name is unmodified if it is not below the
-		current directory, but on MS-Windows the drive is removed if
-		it is the current drive.
+		current directory.
 		For maximum shortness, use ":~:.".
 	:h	Head of the file name (the last component and any separators
 		removed).  Cannot be used with :e, :r or :t.

src/filepath.c

@@ -301,6 +301,7 @@ modify_fname(
     char_u	dirname[MAXPATHL];
     int		c;
     int		has_fullname = 0;
+    int		has_homerelative = 0;
 #ifdef MSWIN
     char_u	*fname_start = *fnamep;
     int		has_shortname = 0;
@@ -412,7 +413,7 @@ repeat:
 	}
 	pbuf = NULL;
 	// Need full path first (use expand_env() to remove a "~/")
-	if (!has_fullname)
+	if (!has_fullname && !has_homerelative)
 	{
 	    if (c == '.' && **fnamep == '~')
 		p = pbuf = expand_env_save(*fnamep);
@@ -428,11 +429,28 @@ repeat:
 	{
 	    if (c == '.')
 	    {
+		size_t	namelen;
+
 		mch_dirname(dirname, MAXPATHL);
-		s = shorten_fname(p, dirname);
-		if (s != NULL)
+		if (has_homerelative)
 		{
-		    *fnamep = s;
+		    s = vim_strsave(dirname);
+		    if (s != NULL)
+		    {
+			home_replace(NULL, s, dirname, MAXPATHL, TRUE);
+			vim_free(s);
+		    }
+		}
+		namelen = STRLEN(dirname);
+
+		// Do not call shorten_fname() here since it removes the prefix
+		// even though the path does not have a prefix.
+		if (fnamencmp(p, dirname, namelen) == 0)
+		{
+		    p += namelen;
+		    while (*p && vim_ispathsep(*p))
+			++p;
+		    *fnamep = p;
 		    if (pbuf != NULL)
 		    {
 			vim_free(*bufp);   // free any allocated file name
@@ -453,6 +471,7 @@ repeat:
 			*fnamep = s;
 			vim_free(*bufp);
 			*bufp = s;
+			has_homerelative = TRUE;
 		    }
 		}
 	    }
@@ -701,6 +720,7 @@ f_chdir(typval_T *argvars, typval_T *rettv)
     rettv->vval.v_string = NULL;
 
     if (argvars[0].v_type != VAR_STRING)
+	// Returning an empty string means it failed.
 	return;
 
     // Return the current directory

src/testdir/test_fnamemodify.vim

@@ -3,8 +3,10 @@
 func Test_fnamemodify()
   let save_home = $HOME
   let save_shell = &shell
+  let save_shellslash = &shellslash
   let $HOME = fnamemodify('.', ':p:h:h')
   set shell=sh
+  set shellslash
 
   call assert_equal('/', fnamemodify('.', ':p')[-1:])
   call assert_equal('r', fnamemodify('.', ':p:h')[-1:])
@@ -28,6 +30,15 @@ func Test_fnamemodify()
   call assert_equal('fb2.tar.gz', fnamemodify('abc.fb2.tar.gz', ':e:e:e:e'))
   call assert_equal('tar', fnamemodify('abc.fb2.tar.gz', ':e:e:r'))
 
+  let cwd = getcwd()
+  call mkdir($HOME . '/XXXXXXXX/a', 'p')
+  call mkdir($HOME . '/XXXXXXXX/b', 'p')
+  call chdir($HOME . '/XXXXXXXX/a/')
+  call assert_equal('foo', fnamemodify($HOME . '/XXXXXXXX/a/foo', ':p:~:.'))
+  call assert_equal('~/XXXXXXXX/b/foo', fnamemodify($HOME . '/XXXXXXXX/b/foo', ':p:~:.'))
+  call chdir(cwd)
+  call delete($HOME . '/XXXXXXXX', 'rf')
+
   call assert_equal('''abc def''', fnamemodify('abc def', ':S'))
   call assert_equal('''abc" "def''', fnamemodify('abc" "def', ':S'))
   call assert_equal('''abc"%"def''', fnamemodify('abc"%"def', ':S'))
@@ -44,6 +55,7 @@ func Test_fnamemodify()
 
   let $HOME = save_home
   let &shell = save_shell
+  let &shellslash = save_shellslash
 endfunc
 
 func Test_fnamemodify_er()

src/version.c

@@ -742,6 +742,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    208,
 /**/
     207,
 /**/