generated column cast from timestamptz to timestamp not OK.

[alias <postgres.rocks@xxxxxxxxx>]

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.