Target should be reserved keyword

[VMatthijs]

Summary:

Given its central role in the language, target should be a reserved keyword and should not be free to use as a variable.

Description:

target should only be usable to access (increment or read off) the log-density. We should not be overloading this critical key word.

Reproducible Steps:

Currently, the following is a valid Stan program

functions {
  vector target(vector y, vector theta, real[] x_r, int[] x_i) {
    vector[2] deltas;
    deltas[1] = y[1] - theta[1] - x_r[1];
    return deltas;
  }
}

It shouldn't be.

Current Output:

Parser does not complain.

Expected Output:

Parser complains about illegal use of target.

Additional Information:

(I would expect the same of get_lp, but I guess that's been deprecated.)

Current Version:

v2.18.0

[mcol]

Fixed in stanc3.