use posixpath.join when loading template names

CHANGES.rst

@@ -36,6 +36,9 @@ Unreleased
 -   The ``groupby`` filter is case-insensitive by default, matching
     other comparison filters. Added the ``case_sensitive`` parameter to
     control this. :issue:`1463`
+-   Windows drive-relative path segments in template names will not
+    result in ``FileSystemLoader`` and ``PackageLoader`` loading from
+    drive-relative paths. :pr:`1621`
 
 
 Version 3.0.3

src/jinja2/loaders.py

@@ -3,6 +3,7 @@
 """
 import importlib.util
 import os
+import posixpath
 import sys
 import typing as t
 import weakref
@@ -193,7 +194,9 @@ def get_source(
     ) -> t.Tuple[str, str, t.Callable[[], bool]]:
         pieces = split_template_path(template)
         for searchpath in self.searchpath:
-            filename = os.path.join(searchpath, *pieces)
+            # Use posixpath even on Windows to avoid "drive:" or UNC
+            # segments breaking out of the search directory.
+            filename = posixpath.join(searchpath, *pieces)
             f = open_if_exists(filename)
             if f is None:
                 continue
@@ -326,7 +329,9 @@ def __init__(
     def get_source(
         self, environment: "Environment", template: str
     ) -> t.Tuple[str, str, t.Optional[t.Callable[[], bool]]]:
-        p = os.path.join(self._template_root, *split_template_path(template))
+        # Use posixpath even on Windows to avoid "drive:" or UNC
+        # segments breaking out of the search directory.
+        p = posixpath.join(self._template_root, *split_template_path(template))
         up_to_date: t.Optional[t.Callable[[], bool]]
 
         if self._archive is None: