email.utils.format_datetime rejects custom zero-offset tzinfo with usegmt=True

[augusto-rehfeldt]

Bug report

Bug description:

Documented behaviour: The format_datetime docstring states: "If usegmt is True, dt must be an aware datetime with an offset of zero. In this case 'GMT' will be rendered instead of the normal +0000 required by RFC2822."

Expected: Return a date string ending in ' GMT' without raising ValueError for an aware zero-offset datetime.

Actual: Raises ValueError: usegmt option requires a UTC datetime.

import datetime as d
import email.utils

class ZeroOffset(d.tzinfo):
    def utcoffset(self, dt): return d.timedelta(0)
    def dst(self, dt): return d.timedelta(0)
    def tzname(self, dt): return "UTC"
    def __repr__(self): return "ZeroOffset()"

dt = d.datetime(1, 1, 1, tzinfo=ZeroOffset())
if dt.tzinfo is None or dt.utcoffset() != d.timedelta(0):
    print("REFUTATION REJECTED: input is not aware with zero offset")
else:
    expected = (
        f"{('Mon','Tue','Wed','Thu','Fri','Sat','Sun')[dt.weekday()]}, "
        f"{dt.day:02d} {'Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec'.split()[dt.month-1]} "
        f"{dt.year:04d} {dt.hour:02d}:{dt.minute:02d}:{dt.second:02d} GMT"
    )
    try:
        actual = ("result", email.utils.format_datetime(dt, usegmt=True))
    except Exception as e:
        actual = ("exception", type(e).__name__, str(e))
    if actual != ("result", expected):
        print("REFUTATION CONFIRMED:", repr(dt), "actual:", actual, "expected:", repr(expected))
    else:
        print("REFUTATION REJECTED: result matches documented expectation")

Output on Python 3.14.6 (Windows-11-10.0.26220-SP0), standard library email.utils:

REFUTATION CONFIRMED: datetime.datetime(1, 1, 1, 0, 0, tzinfo=ZeroOffset()) actual: ('exception', 'ValueError', 'usegmt option requires a UTC datetime') expected: 'Mon, 01 Jan 0001 00:00:00 GMT'

This report was found and written by an automated property-testing tool I run (bugforge). The reproducer above was executed and its output is pasted unedited; no person reviewed the report before it was filed. The search script is in https://github.com/augusto-rehfeldt/bugforge-results/tree/main/email.utils-20261003-021105-c3

CPython versions tested on:

3.14

Operating systems tested on:

Windows

[ZeroIntensity]

See #158748 (comment).