Routine resolution with built-in data types as source

If the source type is a built-in data type, the distinct type does not inherit any built-in casts provided for the built-in type, but it does inherit any user-defined casts that are defined on the source type. For example, suppose you create the following distinct data type and table:
CREATE DISTINCT TYPE inches AS FLOAT;
CREATE TABLE test(col1 inches);
INSERT INTO test VALUES (2.5::FLOAT::inches);
An SQL user might execute the following sample query: 
SELECT 4.8 + col1 FROM test;

Although the source data type of the col1 argument has a built-in cast function to convert from FLOAT to DECIMAL (the 4.8 is DECIMAL), this query fails because the distinct type inches does not inherit the built-in cast.

You must use explicit casts in the SQL query. The following queries succeed: 
SELECT 4.8 + col1::INT from test;
SELECT 4.8::FLOAT::inches + col1 FROM test;