BUG #14717: NULL=NULL is true for composite types

The following bug has been logged on the website:

Bug reference: 14717
Logged by: Paolo Guagliardo
Email address: paolo(dot)guagliardo(at)ed(dot)ac(dot)uk
PostgreSQL version: 9.6.3
Operating system: Scientific Linux release 7.3 (Nitrogen)
Description:

Comparisons between row types with NULLs behave as expected:

SELECT (1,NULL) = (1,NULL);

?column?
----------

(1 row)

However, if we now create a composite type:

CREATE TYPE pair AS (a int, b int);

then we get

SELECT (1,NULL)::pair = (1,NULL)::pair ;

?column?
----------
t
(1 row)

while the answer should be NULL (unknown) as before.

Other operations (<>,>,<,>=,<=) are affected as well.