9.1 - rewrite less alter table?

hi
perhaps I misunderstood something from commits, but I assumed that in
9.1 this operation shouldn't rewrite the table:

CREATE TABLE test ( x varchar(16) );
insert into test select i::text from generate_series(1,1000000) i;
alter table test alter column x set data type varchar(32);

but it does.

In commit log I see information about "binary coercible" (which doesn't
mean much to me) - so I assumed varchars() can work this way.

What am I missing?

Best regards,

depesz

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