typing: attempt to remove @overloads in the platform module for stdlib methods

This is mostly successful, as examining util.pyi, posix.pyi, and windows.pyi
after a pytype run shows that the type overloads for `oslink`, `readlink`,
`removedirs`, `rename`, `split`, and `unlink` have been removed.  (Some of these
still have an @overload, but the differences are the variable names, not the
types.)  However, @overloads remain for `abspath` and `normpath` for some
reason.

It's useful to redefine these methods for the type checking phase because in
addition to excluding str and PathLike variants, some of these functions have
optional args in stdlib that aren't implemented in the custom implementation on
Windows, and we want the type checking to flag that instead of assuming it's an
allowable overload everywhere.

One last quirk I noticed that I can't explain- `pycompat.TYPE_CHECKING` is
always False, so the conditionals need to check `typing.TYPE_CHECKING` directly.
I tried dropping the custom code for assigning `pycompat.TYPE_CHECKING` and
simply did `from typing import TYPE_CHECKING` directly in pycompat.py, and used
`pycompat.TYPE_CHECKING` for the conditional here... and pytype complained that
`pycompat` doesn't have the `TYPE_CHECKING` variable.

mercurial/posix.py

@@ -17,10 +17,12 b' import select'
 import stat
 import sys
 import tempfile
+import typing
 import unicodedata
 
 from typing import (
     Any,
+    AnyStr,
     Iterable,
     Iterator,
     List,
@@ -67,6 +69,38 b' readlink = os.readlink'
 unlink = os.unlink
 rename = os.rename
 removedirs = os.removedirs
+
+if typing.TYPE_CHECKING:
+    # Replace the various overloads that come along with aliasing stdlib methods
+    # with the narrow definition that we care about in the type checking phase
+    # only.  This ensures that both Windows and POSIX see only the definition
+    # that is actually available.
+    #
+    # Note that if we check pycompat.TYPE_CHECKING here, it is always False, and
+    # the methods aren't replaced.
+
+    def normpath(path: bytes) -> bytes:
+        raise NotImplementedError
+
+    def abspath(path: AnyStr) -> AnyStr:
+        raise NotImplementedError
+
+    def oslink(src: bytes, dst: bytes) -> None:
+        raise NotImplementedError
+
+    def readlink(path: bytes) -> bytes:
+        raise NotImplementedError
+
+    def unlink(path: bytes) -> None:
+        raise NotImplementedError
+
+    def rename(src: bytes, dst: bytes) -> None:
+        raise NotImplementedError
+
+    def removedirs(name: bytes) -> None:
+        raise NotImplementedError
+
+
 expandglobs: bool = False
 
 umask: int = os.umask(0)

mercurial/windows.py

@@ -14,6 +14,7 b' import re'
 import stat
 import string
 import sys
+import typing
 import winreg  # pytype: disable=import-error
 
 from typing import (
@@ -27,6 +28,7 b' from typing import ('
     Optional,
     Pattern,
     Sequence,
+    Tuple,
     Union,
 )
 
@@ -59,6 +61,18 b' split = os.path.split'
 testpid = win32.testpid
 unlink = win32.unlink
 
+if typing.TYPE_CHECKING:
+    # Replace the various overloads that come along with aliasing stdlib methods
+    # with the narrow definition that we care about in the type checking phase
+    # only.  This ensures that both Windows and POSIX see only the definition
+    # that is actually available.
+    #
+    # Note that if we check pycompat.TYPE_CHECKING here, it is always False, and
+    # the methods aren't replaced.
+    def split(p: bytes) -> Tuple[bytes, bytes]:
+        raise NotImplementedError
+
+
 umask: int = 0o022