pythonGH-113528: Deoptimise pathlib._abc.PathBase._make_child_relpath() (python#113532)

Call straight through to `joinpath()` in `PathBase._make_child_relpath()`.
Move optimised/caching code to `pathlib.Path._make_child_relpath()`

Author: barneygale

Lib/pathlib/__init__.py

@@ -405,6 +405,22 @@ def _make_child_entry(self, entry):
         path._tail_cached = self._tail + [entry.name]
         return path
 
+    def _make_child_relpath(self, name):
+        path_str = str(self)
+        tail = self._tail
+        if tail:
+            path_str = f'{path_str}{self.pathmod.sep}{name}'
+        elif path_str != '.':
+            path_str = f'{path_str}{name}'
+        else:
+            path_str = name
+        path = self.with_segments(path_str)
+        path._str = path_str
+        path._drv = self.drive
+        path._root = self.root
+        path._tail_cached = tail + [name]
+        return path
+
     def glob(self, pattern, *, case_sensitive=None, follow_symlinks=None):
         """Iterate over this subtree and yield all existing files (of any
         kind, including directories) matching the given relative pattern.

Lib/pathlib/_abc.py

@@ -753,20 +753,7 @@ def _make_child_entry(self, entry):
         return entry
 
     def _make_child_relpath(self, name):
-        path_str = str(self)
-        tail = self._tail
-        if tail:
-            path_str = f'{path_str}{self.pathmod.sep}{name}'
-        elif path_str != '.':
-            path_str = f'{path_str}{name}'
-        else:
-            path_str = name
-        path = self.with_segments(path_str)
-        path._str = path_str
-        path._drv = self.drive
-        path._root = self.root
-        path._tail_cached = tail + [name]
-        return path
+        return self.joinpath(name)
 
     def glob(self, pattern, *, case_sensitive=None, follow_symlinks=None):
         """Iterate over this subtree and yield all existing files (of any