expose_stan_functions fails on functions postfixed with `_lp`

Summary:

expose_stan_functions does not work with functions that modify target, which worked previously

Description:

If you try, there is a compiler error

Reproducible Steps:

expose_stan_functions this

functions {
  void foo_lp() {
    target += normal_lpdf(0.5 | 0, 1);
  }
}

Current Output:

In file included from file7d8f62dde969.cpp:1:
In file included from /usr/local/lib/R/site-library/rstan/include/exporter.h:1:
In file included from /usr/local/lib/R/site-library/Rcpp/include/RcppCommon.h:168:
In file included from /usr/local/lib/R/site-library/Rcpp/include/Rcpp/as.h:25:
/usr/local/lib/R/site-library/Rcpp/include/Rcpp/internal/Exporter.h:31:28: error: no matching constructor for initialization of 'stan::math::accumulator<double>'
                    Exporter( SEXP x ) : t(x){}
                                         ^ ~
/usr/local/lib/R/site-library/Rcpp/include/Rcpp/as.h:87:41: note: in instantiation of member function 'Rcpp::traits::Exporter<stan::math::accumulator<double> >::Exporter' requested here
            ::Rcpp::traits::Exporter<T> exporter(x);
                                        ^
/usr/local/lib/R/site-library/Rcpp/include/Rcpp/as.h:152:26: note: in instantiation of function template specialization 'Rcpp::internal::as<stan::math::accumulator<double> >' requested here
        return internal::as<T>(x, typename traits::r_type_traits<T>::r_category());
                         ^
/usr/local/lib/R/site-library/Rcpp/include/Rcpp/InputParameter.h:46:49: note: in instantiation of function template specialization 'Rcpp::as<stan::math::accumulator<double> >' requested here
        ReferenceInputParameter(SEXP x_) : obj( as<T>(x_) ){}
                                                ^
file7d8f62dde969.cpp:75:77: note: in instantiation of member function 'Rcpp::ReferenceInputParameter<stan::math::accumulator<double> >::ReferenceInputParameter' requested here
    Rcpp::traits::input_parameter< stan::math::accumulator<double>& >::type lp_accum__(lp_accum__SEXP);
                                                                            ^
/usr/local/lib/R/site-library/StanHeaders/include/stan/math/prim/mat/fun/accumulator.hpp:25:7: note: candidate constructor (the implicit copy constructor) not viable: cannot convert argument of incomplete type 'SEXP' (aka 'SEXPREC *') to 'const stan::math::accumulator<double>' for 1st argument
class accumulator {
      ^
/usr/local/lib/R/site-library/StanHeaders/include/stan/math/prim/mat/fun/accumulator.hpp:33:3: note: candidate constructor not viable: requires 0 arguments, but 1 was provided
  accumulator() : buf_() {}
  ^

and the relevant part of the generated C++ file is

void foo_lp(double& lp__, stan::math::accumulator<double>& lp_accum__, std::ostream* pstream__);
RcppExport SEXP sourceCpp_1_foo_lp(SEXP lp__SEXP, SEXP lp_accum__SEXP, SEXP pstream__SEXP) {
BEGIN_RCPP
    Rcpp::RNGScope rcpp_rngScope_gen;
    Rcpp::traits::input_parameter< double& >::type lp__(lp__SEXP);
    Rcpp::traits::input_parameter< stan::math::accumulator<double>& >::type lp_accum__(lp_accum__SEXP);
    Rcpp::traits::input_parameter< std::ostream* >::type pstream__(pstream__SEXP);
    foo_lp(lp__, lp_accum__, pstream__);
    return R_NilValue;
END_RCPP
}

Expected Output:

None

RStan Version:

develop (2.18) branch from GitHub

R Version:

3.5

Operating System:

Debian

Start at the expose_stan_functions entry point and inspect the generated C++ wrapper for the foo_lp example, especially conversion of lp_accum__ through Rcpp. Reproduce the compiler error with the supplied Stan function and compare the generated wrapper with the expected no-error behavior. Done means functions that modify target and end in _lp compile through expose_stan_functions.