Using the slash "/" operator between a PurePath and a str should have an inferred type of PurePath #1242

Environment data

• VS Code version: 1.35.1
• Extension version (available under the Extensions sidebar): 2019.5.18875
• OS and version: Ubuntu 18.04
• Python version (& distribution if applicable, e.g. Anaconda): Anaconda Python 3.6
• Type of virtual environment used (N/A | venv | virtualenv | conda | ...): conda
• Relevant/affected Python packages and their versions: standard library pathlib

Expected behaviour

Using the / operator on a pathlib.PurePath and a str should infer a type of pathlib.PurePath.

Given the following Python fragment:

import pathlib

root = pathlib.Path("/some/directory")
subdir = root / "subdir"
child = subdir / "file.txt"

root should have an infered type of Path ✔️
subdir should have an an infered type of PurePath ✔️
child should have an an infered type of PurePath ❌

Actual behaviour

root has an infered type of Path ✔️
subdir has an infered type of PurePath ✔️
child has an infered type of str ❌

Steps to reproduce:

1. Copy the code above into a new Python file in VS code.
2. Hover over child, or trigger autocomplete with its methods (the completion is for str).

Logs

Output for Python in the Output panel (View→Output, change the drop-down the upper-right of the Output panel to Python)

Starting Microsoft Python language server.
##########Linting Output - mypy##########
##########Linting Output - flake8##########
[Info  - 9:15:03 AM] Analysis cache path: /home/user/.cache/Microsoft/Python Language Server
[Info  - 9:15:03 AM] GetCurrentSearchPaths /home/user/anaconda3/bin/python 
[Info  - 9:15:03 AM] Interpreter search paths:
[Info  - 9:15:03 AM]     /home/user/anaconda3/lib/python3.6
[Info  - 9:15:03 AM]     /home/user/anaconda3/lib/python3.6/lib-dynload
[Info  - 9:15:03 AM]     /home/user/anaconda3/lib/python3.6/site-packages
[Info  - 9:15:03 AM] User search paths:
[Info  - 9:15:04 AM] Microsoft Python Language Server version 0.3.16.0
[Info  - 9:15:04 AM] Initializing for /home/user/anaconda3/bin/python