Element-wise (vectorised) power operations do not work

Summary:

Element-wise (vectorised) power operations do not work

Description:

According to the stan functions reference, it should be possible to use .^ to do various element-wise power operations. However, rstudio_stanc(), rstanc(), and the alike come up with parsing errors in many cases.

Reproducible Steps:

The following stan program illustrates the problem. Any one of the commented out assignments for z in the transformed parameters block should be valid according to the stan functions reference, but the first four are coming up with different parser errors for me.

data {
  int<lower=0> N;
  vector[N] y;
}
parameters {
  real x;
}
transformed parameters{
  vector[N] z;

  // According to the Stan Function Reference (SFR), RHS should evaluate to vector for
  // all these, but parser says RHS is real.
  //z = x .^ y;
  //z = y .^ x;
  //z = y .^ 2.0;

  //This should work according to the SFR but parser says ; is missing???
  //z = y .^ y;

  //element-wise multiplication and division seem to work
  //z = x ./ y; //no parser error
  //z = y ./ x; //no parser error -- though should be the same as y/x?
  //z = y .* y; //no parser error
}
model {
  y ~ normal(z, 1);
}

Current Output:

For z = x .^ y; I get the following error

Dimension mismatch in assignment; variable name = z, type = vector; right-hand side type = real.
Illegal statement beginning with non-void expression parsed as
  z
Not a legal assignment, sampling, or function statement.  Note that
  * Assignment statements only allow variables (with optional indexes) on the left;
  * Sampling statements allow arbitrary value-denoting expressions on the left.
  * Functions used as statements must be declared to have void returns

 error in 'modelad434da0cc83_TestScript' at line 11, column 2
  -------------------------------------------------
     9: transformed parameters{
    10:   vector[N] z;
    11:   z = x .^ y;
         ^
    12:   //z = y .^ x;
  -------------------------------------------------

PARSER EXPECTED: <one of the following:
  a variable declaration, beginning with type
      (int, real, vector, row_vector, matrix, unit_vector,
       simplex, ordered, positive_ordered,
       corr_matrix, cov_matrix,
       cholesky_corr, cholesky_cov
  or a <statement>
  or '}' to close variable declarations and definitions>
Error in stanc(filename, allow_undefined = TRUE) : 
  failed to parse Stan model 'TestScript' due to the above error.

RStan Version:

2.21.2

R Version:

4.0.3 (2020-10-10)

Operating System:

OS X 10.13.6

Start by running the supplied Stan program through rstanc() and rstudio_stanc(), focusing on each commented .^ assignment in the transformed parameters block. Trace the parser behavior for these expressions and verify that the valid element-wise power forms parse without the reported dimension or statement errors.