BUG #16958: "Invalid reference to FROM-clause entry for table" when qualifying columns in "on conflict .. where"

The following bug has been logged on the website:

Bug reference:      16958
Logged by:          Lukas Eder
Email address:      lukas.eder@gmail.com
PostgreSQL version: 13.2
Operating system:   Docker on Windows
Description:

I'm using Docker on Windows:

-------------------------------------------------------------------
select version();
-------------------------------------------------------------------
PostgreSQL 13.2 (Debian 13.2-1.pgdg100+1) on x86_64-pc-linux-gnu, compiled
by gcc (Debian 8.3.0-6) 8.3.0, 64-bit

Consider the following SQL script:
-------------------------------------------------------------------
create table t (
  a int,
  b int,
  c int
);

create unique index i on t(a)
where t.b is null;

insert into t (a, b)
values (
  1, 
  1
)
on conflict (a)
where t.b is null -- Error here
do update
set
  c = 1
returning t.a, t.b, t.c;

drop table t;
-------------------------------------------------------------------

There's an error reported:

SQL Error [42P01]: ERROR: invalid reference to FROM-clause entry for table
"t"
  Hint: There is an entry for table "t", but it cannot be referenced from
this part of the query.
  Position: 71

Notice the qualification of the t.b column in the "on conflict .. where"
clause. I don't understand why b cannot be qualified at this location. It
can be qualified in the index definition, looks like a bug to me.