bpo-39899: os.path.expanduser(): don't guess other Windows users' home directories if the basename of the current user's home directory doesn't match their username. (GH-18841)
This makes `ntpath.expanduser()` match `pathlib.Path.expanduser()` in this regard, and is more in line with `posixpath.expanduser()`'s cautious approach.
Also remove the near-duplicate implementation of `expanduser()` in pathlib, and by doing so fix a bug where KeyError could be raised when expanding another user's home directory.
diff --git a/Doc/library/os.path.rst b/Doc/library/os.path.rst
index 251df4d..e2f335e 100644
--- a/Doc/library/os.path.rst
+++ b/Doc/library/os.path.rst
@@ -175,8 +175,8 @@
On Windows, :envvar:`USERPROFILE` will be used if set, otherwise a combination
of :envvar:`HOMEPATH` and :envvar:`HOMEDRIVE` will be used. An initial
- ``~user`` is handled by stripping the last directory component from the created
- user path derived above.
+ ``~user`` is handled by checking that the last directory component of the current
+ user's home directory matches :envvar:`USERNAME`, and replacing it if so.
If the expansion fails or if the path does not begin with a tilde, the path is
returned unchanged.