BUG #15426: A prior column in a default expression of an xmltable()call does not exist

The following bug has been logged on the website:

Bug reference:      15426
Logged by:          Ivan Ustûžanin
Email address:      profic@gmail.com
PostgreSQL version: 10.5
Operating system:   Any
Description:

Executing
SELECT * FROM xmltable(
    '.' PASSING '<r/>'
    COLUMNS
    c1 Text PATH 'name()',
    c2 Text DEFAULT c1
);
complains that column c1 does not exist.
However the documentation states that it is possible to use prior columns in
default expressions of later columns.