From 19715e6e8a7145c8d4da39c9ebc2b09da5be74e7 Mon Sep 17 00:00:00 2001
From: tao <2471314@gmail.com>
Date: Thu, 19 Mar 2026 07:54:19 +0800
Subject: [PATCH] fix(fs): expand drive-relative paths on Windows #37084

Problem:
On windows, if a drive-relative path doesn't contain a slash,
`path_to_absolute` can't split out the relative component, causing
expansion to fails. e.g., `c:` `c:.` `c:..` `c:foo.md`

Solution:
For these cases, we can pass letter and colon to `path_full_dir_name`.
Notably, `..` is included as well.
if that relative path exists, it can be expanded correctly.
---
 src/nvim/path.c                                |  7 +++++--
 test/functional/vimscript/fnamemodify_spec.lua | 17 +++++++++++++++++
 2 files changed, 22 insertions(+), 2 deletions(-)

diff --git a/src/nvim/path.c b/src/nvim/path.c
index 298026d7b3..fe8fe651d8 100644
--- a/src/nvim/path.c
+++ b/src/nvim/path.c
@@ -2380,20 +2380,23 @@ static int path_to_absolute(const char *fname, char *buf, size_t len, int force)
     if (p == NULL) {
       p = strrchr(fname, '\\');
     }
+    if (p == NULL && ASCII_ISALPHA(fname[0]) && fname[1] == ':') {
+      p = fname + 1;
+    }
 #endif
     if (p == NULL && strcmp(fname, "..") == 0) {
       // Handle ".." without path separators.
       p = fname + 2;
     }
     if (p != NULL) {
-      if (vim_ispathsep_nocolon(*p) && strcmp(p + 1, "..") == 0) {
+      if (vim_ispathsep(*p) && strcmp(p + 1, "..") == 0) {
         // For "/path/dir/.." include the "/..".
         p += 3;
       }
       assert(p >= fname);
       memcpy(relative_directory, fname, (size_t)(p - fname + 1));
       relative_directory[p - fname + 1] = NUL;
-      end_of_path = (vim_ispathsep_nocolon(*p) ? p + 1 : p);
+      end_of_path = (vim_ispathsep(*p) ? p + 1 : p);
     } else {
       relative_directory[0] = NUL;
     }
diff --git a/test/functional/vimscript/fnamemodify_spec.lua b/test/functional/vimscript/fnamemodify_spec.lua
index 8b0b242c62..380471320e 100644
--- a/test/functional/vimscript/fnamemodify_spec.lua
+++ b/test/functional/vimscript/fnamemodify_spec.lua
@@ -8,6 +8,7 @@ local getcwd = n.fn.getcwd
 local command = n.command
 local write_file = t.write_file
 local is_os = t.is_os
+local chdir = n.fn.chdir
 
 local function eq_slashconvert(expected, got)
   eq(t.fix_slashes(expected), t.fix_slashes(got))
@@ -16,12 +17,15 @@ end
 describe('fnamemodify()', function()
   setup(function()
     write_file('Xtest-fnamemodify.txt', [[foobar]])
+    t.mkdir('foo')
+    write_file('foo/bar', [[bar]])
   end)
 
   before_each(clear)
 
   teardown(function()
     os.remove('Xtest-fnamemodify.txt')
+    n.rmdir('foo')
   end)
 
   it('handles the root path', function()
@@ -37,6 +41,19 @@ describe('fnamemodify()', function()
       eq(root, fnamemodify([[\]], ':p'))
       eq(root, fnamemodify([[/]], ':p:h'))
       eq(root, fnamemodify([[/]], ':p'))
+
+      local letter_colon = root:sub(1, 2)
+      local old_dir = getcwd() .. '/'
+      local foo_dir = old_dir .. 'foo/'
+      eq(old_dir, fnamemodify(letter_colon, ':p'))
+      eq(old_dir, fnamemodify(letter_colon .. '.', ':p'))
+      eq(old_dir, fnamemodify(letter_colon .. './', ':p'))
+      eq(foo_dir, fnamemodify(letter_colon .. './foo', ':p'))
+      eq(foo_dir, fnamemodify(letter_colon .. 'foo', ':p'))
+      chdir('foo')
+      eq(old_dir, fnamemodify(letter_colon .. '..', ':p'))
+      eq(old_dir, fnamemodify(letter_colon .. '../', ':p'))
+      eq(foo_dir .. 'bar', fnamemodify(letter_colon .. 'bar', ':p'))
     end
   end)