vim-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 vim/vim#5577)
https://github.com/vim/vim/commit/d816cd94d87afb73c505bf1e5cd5e07522482113

3 files changed, 34 insertions, 9 deletions

diff --git a/runtime/doc/cmdline.txt b/runtime/doc/cmdline.txt
index 641cd93386..f2f6ebb2c9 100644
--- a/runtime/doc/cmdline.txt
+++ b/runtime/doc/cmdline.txt
@@ -908,8 +908,7 @@ 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.
diff --git a/src/nvim/eval.c b/src/nvim/eval.c
index dccad5a2d0..70909b46cb 100644
--- a/src/nvim/eval.c
+++ b/src/nvim/eval.c
@@ -10624,12 +10624,13 @@ int modify_fname(char_u *src, bool tilde_file, size_t *usedlen, char_u **fnamep,
   char_u *s, *p, *pbuf;
   char_u dirname[MAXPATHL];
   int c;
-  int has_fullname = 0;
+  bool has_fullname = false;
+  bool has_homerelative = false;
 
 repeat:
   // ":p" - full path/file_name
   if (src[*usedlen] == ':' && src[*usedlen + 1] == 'p') {
-    has_fullname = 1;
+    has_fullname = true;
 
     valid |= VALID_PATH;
     *usedlen += 2;
@@ -10698,7 +10699,7 @@ 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);
       } else {
@@ -10708,14 +10709,26 @@ repeat:
       p = *fnamep;
     }
 
-    has_fullname = 0;
+    has_fullname = false;
 
     if (p != NULL) {
       if (c == '.') {
         os_dirname(dirname, MAXPATHL);
-        s = path_shorten_fname(p, dirname);
-        if (s != NULL) {
-          *fnamep = s;
+        if (has_homerelative) {
+          s = vim_strsave(dirname);
+          home_replace(NULL, s, dirname, MAXPATHL, true);
+          xfree(s);
+        }
+        size_t 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) {
             xfree(*bufp);               // free any allocated file name
             *bufp = pbuf;
@@ -10730,6 +10743,7 @@ repeat:
           *fnamep = s;
           xfree(*bufp);
           *bufp = s;
+          has_homerelative = true;
         }
       }
       xfree(pbuf);
diff --git a/src/nvim/testdir/test_fnamemodify.vim b/src/nvim/testdir/test_fnamemodify.vim
index 411f7ebbb3..e8fad397c3 100644
--- a/src/nvim/testdir/test_fnamemodify.vim
+++ b/src/nvim/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()