Bug with match.group().count in loop on v1.15.0

[kmurphy4]

Bug Report

On v1.15.0, with mypy --warn-unreachable ... successfully finds no errors with

import re
re.sub("x", lambda m: m.group().count("x") * "x", "x")

but reports an error with

import re
for _ in re.findall("x", "x"):
    re.sub("x", lambda m: m.group().count("x") * "x", "x")  # E: Argument 1 to "count" of "bytes" has incompatible type "str"; expected "Buffer | SupportsIndex"  [arg-type]

for reference, both examples pass with v1.14.1 and with v1.15.0 without --warn-unreachable.