"Named" column default expression

Hello,

I just noticed that Postgres allows the following syntax:

create table foo
(
     id integer constraint id_default_value default 42
);

But as far as I can tell the "constraint id_default_value" part seems to be only syntactical sugar as this is stored
nowhere.At least I couldn't find it going through the catalog tables and neither pg_dump -s or pgAdmin are showing that
namein the generated SQL source for the table. 

It's not important, I'm just curious why the syntax is accepted (I never saw a default value as a constraint) and if
thereis a way to retrieve that information once the table is created. 

Thanks
Thomas