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.

[sterliakov]

Minimal repro to avoid re dependency, playground:

from typing import overload, Any, AnyStr, Callable, Generic

def takes_str(s: str) -> str:
    return s

class Match(Generic[AnyStr]):
    def group(self, group: int, /) -> AnyStr: ...
    
@overload
def sub(
    pattern: str, 
    repl: Callable[[Match[str]], str]
) -> str: ...
@overload
def sub(
    pattern: bytes,
    repl: Callable[[Match[bytes]], bytes]
) -> bytes: ...

text: str
while True:
    text = sub("", lambda m: takes_str(m.group(1)))  # E: Argument 1 to "takes_str" has incompatible type "bytes"; expected "str"  [arg-type]

[sterliakov]

Bisects to #18433, but only exposed by it - accepting this loop for the second time results in this error.