Use of sys.version_info in ternary operator with straddling code is not handled correctly #5550

When if statements are used, mypy is able to recognize the use of the sys.version_info and handle the types correctly for straddling code that needs to work with both Python 3 and 2. Unfortunately, mypy is not able to handle the same case with a ternary operator.

Can this bug be fixed?

Here's the code that demonstrates the issue:

def ensure_native_str(str_or_bytes):  # type: (Union[Text, bytes]) -> str
    return (
        str_or_bytes if isinstance(str_or_bytes, str) else
        str_or_bytes.decode('utf-8') if six.PY3 else
        str_or_bytes.encode('utf-8')
    )

When processed by mypy it returns:

# using mypy --python-version 3.7 /path/to/file
...: error: "bytes" has no attribute "encode"; maybe "decode"?
# using mypy --python-version 2.7 /path/to/file
...: error: Incompatible return value type (got "unicode", expected "str")

When the same function is changed into this:

def ensure_native_str(str_or_bytes):  # type: (Union[Text, bytes]) -> str
    if isinstance(str_or_bytes, str):
        return str_or_bytes
    if six.PY3:
        return str_or_bytes.decode('utf-8')
    return str_or_bytes.encode('utf-8')

it passes mypy checks with both --python-version 3.7 and --python-version 2.7.
This means that the branches in straddling code were correctly handled and the types inferred appropriately. With the ternary operator above, unfortunately, that did not happen.

This was tested with both mypy-0.620 and mypy-0.560.
Python version that mypy was run with was 3.6.
When six.PY3 is replaced with the explicit sys.version_info[0] == 3 the behavior is the same.
The if statement version passes the checks, the ternary operator version doesn't.