FA-4388 / Mapping semantics / Member archive

Map setdefault keeps existing none · case 03

The operation returns a result or retained state that violates this contract: Insert the default only for an absent key; return [selected value,map].

Member previewVariant 3 · 3 implementations · 4 checks per implementation

Case scope

A controlled failure with recorded boundary checks.

Why this case matters

A controlled local-runtime regression for collection APIs, language semantics, or ownership wrappers. Fixtures include boundary and interaction cases.

One recorded failure

Sample boundary fixture

This sample comes from the broken implementation of a controlled reproducer.

Boundary fixture	Actual	Expected	Outcome
existing None	`["d", {"a": "d"}]`	`[null, {"a": null}]`	Failed

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