BUG #12462: NULLIF changes the argument type

The following bug has been logged on the website:

Bug reference:      12462
Logged by:          Thomas Neumann
Email address:      tneumann@users.sourceforge.net
PostgreSQL version: 9.4.0
Operating system:   Linux 3.16.0
Description:

The SQL standard in Section 6.11, Syntax rule 1 c) says that

""
NULLIF (V1, V2) is equivalent to the following <case specification>:
CASE WHEN
V1=V2 THEN
NULL ELSE V1
END
""

which is currently not the case in Postgres. Postgres promotes V1 to the
type of V2, which can lead to behavior changes.

Example query as illustration: It should produce 0,0,0 (and does on SQL
Server and DB2), but PostgreSQL promotes the type and produces 0,0.5,0

select 1/2,nullif(1,2.3)/2,case when 1=2.3 then NULL else 1 end/2