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Feb 13, 2026
The `Matcher` singleton's `matchers` array is accessed without synchronization. Under Swift Testing's parallel execution, concurrent `register()` writes and `comparator()` reads from multiple test `init()` methods cause data races that crash the test process. Observed crash sites: - `Matcher.register<A>(_:)` — concurrent appends to the array - `closure #1 in Parameter.eraseToGenericValue()` — concurrent reads via `Matcher.comparator(for:)` while another thread appends Add an NSLock to synchronize all matchers array access. The lock is applied at the leaf level (append and snapshot-read) to avoid re-entrancy in the Sequence comparator path which calls `comparator(by:)` → `comparator(for: T.Element.self)` → `comparator(by:)`.
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