win: vim_FullName(): force backslashes #7287

- Replace obvious cases of '/' literal with PATHSEP. (There are still
  some remaining cases that need closer inspection.)
- Fixup tests: ui/screen_basic

closes #7117
ref https://github.com/neovim/neovim/issues/2471#issuecomment-271193714

src/nvim/path.c

@@ -1690,6 +1690,9 @@ int vim_FullName(const char *fname, char *buf, size_t len, bool force)
 
   if (strlen(fname) > (len - 1)) {
     xstrlcpy(buf, fname, len);  // truncate
+#ifdef WIN32
+    slash_adjust(buf);
+#endif
     return FAIL;
   }
 
@@ -1702,6 +1705,9 @@ int vim_FullName(const char *fname, char *buf, size_t len, bool force)
   if (rv == FAIL) {
     xstrlcpy(buf, fname, len);  // something failed; use the filename
   }
+#ifdef WIN32
+  slash_adjust(buf);
+#endif
   return rv;
 }
 
@@ -2196,11 +2202,11 @@ static int path_get_absolute_path(const char_u *fname, char_u *buf,
 
   // expand it if forced or not an absolute path
   if (force || !path_is_absolute_path(fname)) {
-    if ((p = vim_strrchr(fname, '/')) != NULL) {
+    if ((p = vim_strrchr(fname, PATHSEP)) != NULL) {
       // relative to root
       if (p == fname) {
         // only one path component
-        relative_directory[0] = '/';
+        relative_directory[0] = PATHSEP;
         relative_directory[1] = NUL;
       } else {
         assert(p >= fname);

test/functional/core/path_spec.lua

@@ -0,0 +1,61 @@
+local helpers = require('test.functional.helpers')(after_each)
+local clear = helpers.clear
+local eq = helpers.eq
+local eval = helpers.eval
+local get_pathsep = helpers.get_pathsep
+local command = helpers.command
+
+describe("'%:p' expanding", function()
+  local pathsep
+  local targetdir
+  local expected_path
+
+  local function get_full_path()
+    return eval('expand("%:p")')
+  end
+
+  local function join_path(...)
+    return table.concat({...}, pathsep)
+  end
+
+  before_each(function()
+    clear()
+    pathsep = get_pathsep()
+    targetdir = join_path('test', 'functional', 'fixtures')
+    clear(join_path(targetdir, 'tty-test.c'))
+    expected_path = get_full_path()
+  end)
+
+  it('given a relative path with current directory in the middle #7117', function()
+    clear(join_path(targetdir, '.', 'tty-test.c'))
+    eq(expected_path, get_full_path())
+  end)
+
+  it('given a relative path with current directory #7117', function()
+    clear(join_path('.', targetdir, 'tty-test.c'))
+    eq(expected_path, get_full_path())
+  end)
+
+  it('given a relative path with current directory to a file when changing directory #7117', function()
+    clear(join_path('.', targetdir, 'tty-test.c'))
+    command('cd test')
+    eq(expected_path, get_full_path())
+  end)
+
+  it('given a relative path with directory up the tree to a file #7117', function()
+    clear(join_path(targetdir, '..', 'fixtures', 'tty-test.c'))
+    eq(expected_path, get_full_path())
+  end)
+
+  it('given a different starting directory and a relative path with directory up the tree #7117', function()
+    command('cd test')
+    command('e ' .. join_path('..', targetdir, 'tty-test.c'))
+    eq(expected_path, get_full_path())
+  end)
+
+  it('given a different starting directory and a relative path with current directory and up the tree #7117', function()
+    command('cd test')
+    command('e ' .. join_path('.', '..', targetdir, 'tty-test.c'))
+    eq(expected_path, get_full_path())
+  end)
+end)

test/functional/ui/screen_basic_spec.lua

@@ -5,6 +5,7 @@ local feed, command = helpers.feed, helpers.command
 local insert = helpers.insert
 local eq = helpers.eq
 local eval = helpers.eval
+local iswin = helpers.iswin
 
 describe('screen', function()
   local screen
@@ -120,8 +121,15 @@ describe('Screen', function()
 
     it('has correct default title with named file', function()
       local expected = 'myfile (/mydir) - NVIM'
+      if iswin() then
+        expected = 'myfile (C:\\mydir) - NVIM'
+      end
       command('set title')
+      if iswin() then
+        command('file C:\\mydir\\myfile')
+      else
         command('file /mydir/myfile')
+      end
       screen:expect(function()
         eq(expected, screen.title)
       end)

test/unit/path_spec.lua

@@ -481,6 +481,20 @@ describe('path.c', function()
       eq('/tmp', ffi.string(buffer))
       eq(OK, result)
     end)
+
+    itp('works with a relative path with the current directory prefix #7117', function()
+      local force_expansion = 1
+      local result = vim_FullName('./unit-test-directory/test.file', buffer, length, force_expansion)
+      eq(OK, result)
+      eq(lfs.currentdir() .. '/unit-test-directory/test.file', (ffi.string(buffer)))
+    end)
+
+    itp('works with a relative path with the directory name mentioned twice #7117', function()
+      local force_expansion = 1
+      local result = vim_FullName('unit-test-directory/../unit-test-directory/test.file', buffer, length, force_expansion)
+      eq(OK, result)
+      eq(lfs.currentdir() .. '/unit-test-directory/test.file', (ffi.string(buffer)))
+    end)
   end)
 
   describe('path_fix_case', function()