fix: preserve Map state when reassigning NaN

[maxazure]

Setting an existing Map entry from NaN to NaN currently creates a new Map and, when nested, a new parent object even though the value is unchanged.

Treat two NaN values as equal in the Map.set change check. Keep the existing +0/-0 behavior and missing-key handling.

Added seven cases across the existing nine configurations, including both auto-freeze modes. Before the fix, 18 cases failed and 45 passed; afterward, all 63 passed. The tests cover root and nested structural sharing, NaN ↔ number changes, missing keys, and both signed-zero directions.

Validation on Node 25.2.1 / Yarn 1.22.22:

• yarn test: 3,899 source tests and 3,348 production-build tests passed, with eight existing skips in each suite; build and Flow passed.
• yarn coverage, yarn test:perf, and git diff --check passed.
• Source type checking with --skipLibCheck --moduleResolution node passed. Default tsc --noEmit has the same dependency-declaration errors on the unmodified baseline. Whole-file Prettier differences are also identical to baseline; the added code is formatted.

Prepared with AI assistance, including code, tests, and a separate AI review of the final diff. No human review is claimed.

[Copilot]

🟢 Approval recommended

The five-line fix correctly implements SameValueZero equality matching the existing proxy.ts NaN pattern, and is fully covered by correct, comprehensive new tests.

0 open findings

What changed in this PR

This PR fixes a structural-sharing bug in Immer's Map support: setting an existing Map entry from NaN to NaN previously produced a new Map (and, when nested, a new parent object) because the change check relied on !==, and NaN !== NaN is always true. The fix adds a NaN-equality guard to the Map.set change-detection condition so reassigning NaN to NaN is treated as a no-op, while preserving existing +0/-0 and missing-key handling.

Changes:

• In mapset.ts, extract current and treat two NaN values as equal in the set change check (current !== value && !(Number.isNaN(current) && Number.isNaN(value))), giving SameValueZero semantics consistent with the pattern already used in proxy.ts:208.
• Add seven test cases covering root/nested NaN→NaN sharing, NaN↔number changes, missing-key insertion, and both signed-zero directions.

File	Description
src/​plugins/​mapset.ts	Adds a NaN-aware guard to the Map.set change check so identical NaN reassignment no longer marks the Map as changed.
__tests__/​base.js	Adds tests for NaN→NaN sharing (root and nested), NaN↔number changes, missing-key NaN insertion, and +0/-0 preservation.

🧠 Review effort: Balanced

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