generated column cast from timestamptz to timestamp not OK.

CREATE TABLE test_g (
a timestamptz,
b timestamp GENERATED ALWAYS AS (a::timestamp) STORED
);
then an error occurred.

> ERROR: 42P17: generation expression is not immutable
> LOCATION: cookDefault, heap.c:2768
>

However the following 2 commands is ok.

CREATE TABLE test_i (
> a int,
> b bigint GENERATED ALWAYS AS (a::bigint) STORED
> );
>

> CREATE TABLE test_c (
> a varchar,
> b text GENERATED ALWAYS AS (a::text) STORED
> );
>

I didn't get it. timestamptz changes then timestamp also changes. timestamp
is part of timestamptz...
Even if column timestamptz is some value that is constantly changing (like
now() ), it changes/updates then just in the mean time captures timestamp
to column b.