Windows code page codec cannot encode with code pages that require dwFlags=0

Bug report

On Windows, codecs.code_page_encode() fails with OSError: [WinError 87] The parameter is incorrect for every code page that WideCharToMultiByte() requires to be called with dwFlags = 0 and lpUsedDefaultChar = NULL: 50220, 50221, 50222, 50225, 50227, 50229, 52936 (HZ-GB2312), 54936 (GB18030), 57002-57011 (ISCII), 65000 and 42. Decoding these code pages works.

>>> import codecs
>>> codecs.code_page_decode(50220, b'\x1b$B$"\x1b(B')
('あ', 8)
>>> codecs.code_page_encode(50220, 'あ')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
OSError: [WinError 87] The parameter is incorrect

encode_code_page_flags() returns WC_NO_BEST_FIT_CHARS, and encode_code_page_strict() passes a non-NULL lpUsedDefaultChar, for every code page except CP_UTF8 and CP_UTF7. decode_code_page_strict() retries with flags = 0 when MultiByteToWideChar() fails with ERROR_INVALID_FLAGS; the encoder has no such fallback. With dwFlags = 0 and lpUsedDefaultChar = NULL the conversion works:

>>> import ctypes
>>> buf = ctypes.create_string_buffer(64)
>>> n = ctypes.windll.kernel32.WideCharToMultiByte(50220, 0, 'あ', 1, buf, 64, None, None)
>>> buf.raw[:n]
b'\x1b$B$"\x1b(B'

Note that usedDefaultChar is how the encoder detects unencodable characters, so dropping it needs a replacement (for example decoding the result back and comparing).

cp51932, cp51936 and cp51950 fail in both directions; those code pages are not supported by MultiByteToWideChar()/WideCharToMultiByte() at all.

Linked PRs

• gh-155018

Commencez par codecs.code_page_encode() et les helpers encode_code_page_flags() et encode_code_page_strict(), puis comparez leur comportement avec decode_code_page_strict(). Reproduisez l’échec pour la page de codes 50220 et vérifiez que les pages de codes Windows listées s’encodent correctement, tandis que la détection des caractères non encodables reste couverte. Le PR lié gh-155018 indique que le travail est déjà en cours.