Omitting defaults does not work with array-like structs

[chuckwondo]

Description

When setting both omit_defaults and array_like to True, defaults are not omitted.

For example, without array_like=True:

class Position(
    msgspec.Struct,
    frozen=True,
    forbid_unknown_fields=True,
    omit_defaults=True,
):
    longitude: float
    latitude: float
    altitude: float = 0.0

Performing a roundtrip works as expected:

>>> pos = msgspec.json.Decoder(Position).decode('{ "longitude": 1.0, "latitude": 2.0 }')
>>> pos
Position(longitude=1.0, latitude=2.0, altitude=0.0)
>>> msgspec.json.encode(pos)
b'{"longitude":1.0,"latitude":2.0}'

However, adding array_like=True to the Position definition above causes rountripping to fail:

>>> pos = msgspec.json.Decoder(Position).decode('[1.0,2.0]')
>>> pos
Position(longitude=1.0, latitude=2.0, altitude=0.0)
>>> msgspec.json.encode(pos)
b'[1.0,2.0,0.0]'

[ipseitas]

Hi there!

Just noticed this yesterday as well. From what I can see ,this doesn't only affect the json encoding. This is an example of MsgPack acting the same way:

from msgspec.msgpack import decode, encode
from msgspec.structs import Struct

class User(Struct, tag=1, array_like=True):
    username: str
    is_admin: bool = False

class Comment(Struct, tag=2, array_like=True):
    user: User
    content: str
    likes: int = 0
    is_highlighted: bool = False

username = "ipseitas"
content = "Lovin' this lib, please add donations!"

comment = Comment(User(username), content)
commentlike = (2, (1, username), content)

>>> encode(comment)
... b"\x95\x02\x93\x01\xa8ipseitas\xc2\xd9&Lovin' this lib, please add donations!\x00\xc2"

>>> encode(commentlike)
... b"\x93\x02\x92\x01\xa8ipseitas\xd9&Lovin' this lib, please add donations!"

>>> decode(encode(commentlike), type=Comment) == comment
... True

Implementing fits the theme of being efficient that your package shines at. If this helps you deliver, please add donation #542. I'll gladly transfer some of the time your lib saved me and am sure others would as well :)