ICU 78.1's u_unescapeAt dereferences the offset and calls the callback without checks, and u_unescape reads the source without one, so a null one faults: throw std::invalid_argument there instead of NotImplemented_nid_no_patch. A negative u_unescape capacity is clamped by _appendUChars and fails every room check, so nothing is written and the full length is returned; only a capacity whose signed difference with the output length overflows, which ICU leaves undefined, still throws.
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