Re: BUG #16149: Prepared COPY queries always report 0 parameters when described

The following bug has been logged on the website:

Bug reference:      16149
Logged by:          Steven Fackler
Email address:      sfackler@gmail.com
PostgreSQL version: 12.1
Operating system:   Debian Buster
Description:       

When a Postgres backend describes a prepared `COPY ... TO STDOUT query`, it
always reports 0 query parameters regardless of how many are actually
present

        PGresult *result = PQprepare(conn, "a", "COPY (SELECT $1::TEXT) TO STDOUT",
0, NULL);

        result = PQexecPrepared(conn, "a", 0, NULL, NULL, NULL, 0);
When run, it prints the following:

```
nparams: 0
error: there is no parameter $1
```

If you change the query to just the inner `SELECT $1::TEXT`, the number of
parameters is correctly reported, but interestingly the error message
changes:

```
nparams: 1
error: bind message supplies 0 parameters, but prepared statement "a"
requires 1
```

From some quick Googling, I did see this StackOverflow post[1] stating that
COPY queries don't support parameters,

but if that's the case it seems like
an error should be reported at the preparation stage.

I also don't see
anything about that in the documentation for COPY[2], though I may have
missed it!

I see the same behavior on Postgres 11.1 as well, if that's
relevant.