n exceeds the maximum representable length
(2^60 - 1 on 64-bit, 2^28 - 1 on 32-bit).
void _validateLength(size_t n) @safeValidate that a length value fits in the tagged-length representation.
n exceeds the maximum representable length
(2^60 - 1 on 64-bit, 2^28 - 1 on 32-bit).