Using types in an inappropriate way causes crash of backend

Boris Boehlen (boris(at)i3(dot)informatik(dot)rwth-aachen(dot)de) reports a bug with a severity of 1
The lower the number the more severe it is.

Short Description
Using types in an inappropriate way causes crash of backend

Long Description
I tried to create a type to achieve the same as the "CREATE DOMAIN"
command of SQL2. After declaring a type `f' which should be the same
as `int4' I created table using this type. When tried to insert a value
into the table I got the following error:

ERROR: Attribute 'a' is of type 'f' but expression is of type 'int4'
You will need to rewrite or cast the expression

Then I tried the next insert statement and the backend crashed.

BTW: How can I achieve the same as using "CREATE DOMAIN" without
extending PSQL?

Sample Code
create type f (input = int4in, output=int4out, internallength=4);
create table z (a f);
insert into z values(1);
insert into z values('1');

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