`STRING_ELT` as LHS value

[eddelbuettel]

Luke Tierney emailed me:

I happened to come across the fact that in barrier.cpp you have

// [[Rcpp::register]] void set_string_elt(SEXP x, int i, SEXP value) { STRING_ELT(x, i) = value; }

You can't to that -- using STRING_ELT(x, i) as a LHS breaks the write
barrier and also would mess up reference counting. You have to use
SET_VECTOR_ELT.

Correction forthcoming. Appears to only affect barrier.cpp.