UnstructuredHeader drops encoded-word Base64 padding defects

[augusto-rehfeldt]

Bug report

Bug description:

Documented behaviour: BaseHeader documentation in email.headerregistry: "Defects should be added to the list instead. The standard header parsers register defects for RFC compliance issues, for obsolete RFC syntax, and for unrecoverable parsing errors."

Expected: The Subject header's defects includes InvalidBase64PaddingDefect.

Actual: The parser's all_defects includes InvalidBase64PaddingDefect, but the Subject header's defects does not.

from email.headerregistry import HeaderRegistry, UnstructuredHeader
from email.errors import InvalidBase64PaddingDefect

value = '=?utf-8?b?YQ?='
if not value.isascii() or '\r' in value or '\n' in value:
    print('REFUTATION REJECTED: input is not unfolded ASCII')
else:
    expected = any(isinstance(d, InvalidBase64PaddingDefect)
                   for d in UnstructuredHeader.value_parser(value).all_defects)
    actual = any(isinstance(d, InvalidBase64PaddingDefect)
                 for d in HeaderRegistry()('Subject', value).defects)
    if expected and not actual:
        print('REFUTATION CONFIRMED:', repr(value),
              'actual:', actual, 'expected:', expected)
    else:
        print('REFUTATION REJECTED: parser prerequisite absent or defect preserved',
              repr(value), 'actual:', actual, 'expected:', expected)

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

REFUTATION CONFIRMED: '=?utf-8?b?YQ?=' actual: False expected: True

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.headerregistry-20261003-021509-c2

CPython versions tested on:

3.14

Operating systems tested on:

Windows

[ZeroIntensity]

See #158748 (comment).