parity(0047): JSON null is None, undefined prints Undefined in containers

[rmorozov]

Requested by Ruslan · project thread

Before: {"x": null} from a JSON binding gave json.x is defined false and printed empty. {{ [nope, 1] }}, {{ dict(a=nope) }} and {{ nope|pprint }} printed None or none where Python prints Undefined.

After: a JSON null is present and is None (json.x is defined true, prints None). Undefined inside a container and under pprint prints Undefined. Item 3 of the task was already done by 0019.

How: new non-pure virtual IMapItemAccessor::EmptyIsNone() (default false). The three JSON accessors return true, and GenericMapAdapter::GetItem then turns an empty Value from a present item into None. Reflected struct fields keep reading as undefined, so the map(attribute=, default=) convention is unchanged. HasValue could not carry this, since a reflected struct reports every declared field as present. The MakeUndefined guard stays for the same reason. The ValueRenderer repr and PrettyPrinter now print Undefined.

Tests: removes 3 lines from test/parity/divergences/undefined.txt; NullMemberIsNone in the boost, nlohmann and rapid binding tests (rapid built locally with the patched pinned RapidJSON); 4 unit rows that encoded none for undefined in forloop_test.cpp and filters_test.cpp.

Follow-up filed: 0173 (pprint prints none/true where Python prints None/True).

Roles: verifier, 1 round, no blocking findings (clang ASan+UBSan, nlohmann config, 80-template Python oracle check, preflight.sh --perf all clean). Red pushes: none so far.

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https://claude.ai/code/session_011DkJzm2CJsYbt4dmQT5uYZ