nested isinstances lose type information

[Akuli]

Bug Report

To Reproduce

x = object()
if isinstance(x, (int, str)):
    reveal_type(x)      # Union[int, str]
    if isinstance(x, int):
        pass
    reveal_type(x)      # object

Expected Behavior

both types revealed as Union[int, str]

Actual Behavior

object from second type

Your Environment

• Mypy version used: latest master (0.820+dev.72ac2cee910ca61bfdf449217328b6d10b45d378)
• Mypy command-line flags: none
• Mypy configuration options from mypy.ini (and other config files): none
• Python version used: 3.7.3
• Operating system and version: debian 10 based linux distro

[JukkaL]

It looks like we should perhaps maintain a stack of narrowed down types (though I might be wrong).

[erictraut]

This is another example of join vs union. It should probably be tagged as such.