FA-4749 / Scoped mappings / Member archive

Chainmap setdefault observes outer binding · case 04

The operation returns a result or retained state that violates this contract: setdefault returns an existing binding from any scope without inserting into the first scope; only an absent key is inserted.

Member previewVariant 4 · 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
outer binding remains outer	`[2, [{"a": 2}, {"a": 1}]]`	`[1, [{}, {"a": 1}]]`	Failed

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