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Class N(int): pass; N(5) + 1 returns 6, N(5).bit_length() works,
isinstance(N(5), int) is True, {N(5): "a"}[5] returns "a", and
S("hi").upper() returns "HI". Same for str/list/dict/bytes
subclasses.
Builtin types stay as *object.BuiltinFunc; instead the subclass
instance carries a BuiltinValue payload built at construction time.
After dunder dispatch, binary/unary/compare/contains/getitem paths
unbox both operands so the type-specific Go switches handle them
transparently. Operator results are plain builtin types unless the
class overrides the dunder, matching CPython.
object.Class gets BuiltinBase + BuiltinMRO; object.Instance gets
BuiltinValue. __build_class__ detects builtin-type bases. Class call
path auto-constructs BuiltinValue from args when no user __init__.
isinstance/issubclass walk the BuiltinBase relation. getAttr falls
back to BuiltinValue so inherited methods work. instance hashes/eq/
repr/str/truthy fall through to the payload when no override.
Spec at notes/Spec/1500/1544_goipy_v0103_builtin_subclass_arith.md.
Fixture 349.
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Summary
class N(int): pass; N(5) + 1returns 6,N(5).bit_length()works,isinstance(N(5), int)is True,{N(5): "a"}[5]returns "a", andS("hi").upper()returns "HI". Same for str / list / dict / bytes subclasses.*object.BuiltinFunc. Subclass instances carry aBuiltinValuepayload constructed at instantiation; after every dunder dispatch the binary/unary/compare/contains/getitem paths callunboxBuiltinso the type-specific Go switches handle the unboxed operands. Results are plain builtin types unless the class overrides the dunder — matches CPython.notes/Spec/1500/1544_goipy_v0103_builtin_subclass_arith.md. v0.1.4 lands the typing fallout.Test plan
go build ./...go test ./vm/...internal/testdata/349_builtin_subclass_arith.py) matches CPython 3.14 byte-for-byte