BUG #14385: Operator IS changed priority in 9.5

The following bug has been logged on the website:

Bug reference: 14385
Logged by: Strahinja Kustudic
Email address: strahinjak@nordeus.com
PostgreSQL version: 9.5.4
Operating system: CentOS 6
Description:

If we create a test table like this:

test-# create table test (i integer, b boolean);

And insert the following data:

test-# insert into test (i) values (5), (null);
test-# update test set b = i is not null;

When we run the following select on Postgres 9.1-9.4:
test-# select
test-# 'aaa' || i is not null || 'bbb' as v1,
test-# 'aaa' || b || 'bbb' as v2,
test-# 'aaa' || i is not null || 'bbb' = 'aaa' || b || 'bbb' as v1_eq_v2
test-# from test;
v1 | v2 | v1_eq_v2
-------------+-------------+----------
aaatruebbb | aaatruebbb | t
aaafalsebbb | aaafalsebbb | t
(2 rows)

but if we run the same query on 9.5 we get a different output:

test-# select
test-# 'aaa' || i is not null || 'bbb' as v1,
test-# 'aaa' || b || 'bbb' as v2,
test-# 'aaa' || i is not null || 'bbb' = 'aaa' || b || 'bbb' as v1_eq_v2
test-# from test;
v1 | v2 | v1_eq_v2
----------+-------------+----------
truebbb | aaatruebbb | f
falsebbb | aaafalsebbb | f
(2 rows)

As you can see 9.5 removes the first 'aaa' string from concatenation. We
guess it's because of the operator IS priority has changed in 9.5, because
if we put 'i is not null' between parenthesis, it works like before 9.5:

test-# select
test-# 'aaa' || (i is not null) || 'bbb' as v1,
test-# 'aaa' || b || 'bbb' as v2,
test-# 'aaa' || (i is not null) || 'bbb' = 'aaa' || b || 'bbb' as
v1_eq_v2
test-# from test;
v1 | v2 | v1_eq_v2
-------------+-------------+----------
aaatruebbb | aaatruebbb | t
aaafalsebbb | aaafalsebbb | t
(2 rows)

We couldn't find any documentation changes about this, so is this a bug, or
was it intentional?

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