There are only two changes to two files where an explicit conversion as per Rcpp::as<double> was called for. A Debian archive rebuild had triggered one of those fails to build from source bug reports as the compiler version seems to be more finicky now than when version 0.3.5 was uploaded in November. No other changes were made.
Thanks to CRANberries, there is also a diff to the previous release 0.3.5. Full changelog details, examples and more details about this package are at my RQuantLib page.