Converting to identity columns with domains on PK columns

Domains on Postgres are really strange to me. Am I creating a domain which is
exactly equal to integer, right ?

create domain i32 as integer;
create domain T50 as varchar(50);

Create table MyTable(
ID I32 not null primary key,
Description T50);

Then, after inserts and updates done to that table, I want to convert that
primary key to a identity column.

alter table MyTable alter ID add generated always as identity;

ERROR: identity column type must be smallint, integer, or bigint

So, What do I need do to create this identity column ?
Why Postgres consider different I32 and integer ?

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