Re: Issue with custom operator in simple case

AFAICS, we have the following problem constructions:

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a IS DISTINCT FROM b  
a IS NOT DISTINCT FROM b  
a IN (...)  
a NOT IN (...)  
CASE a WHEN b THEN ... ELSE d END  
NULLIF(a, b)  
JOIN USING / NATURAL JOIN 

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As a quick recap, the following options are available to us.

1) Add the "USING operator" clause.
Rejected due to:
* Non-standard syntax.
* ruleutils.c could not safely convert this back to the source text.
* In case "IS DISTINCT FROM" on composite types, using only the eq operator is maybe not enough.

2) Using default btree opclass/opfamily operators.

Rejected due to:
 * Adding yet another way of selecting an operator to the existingfunc_select_candidate and opfamily 

   seems too complicated and not safe.
 * May not work in some cases.

3) CVE-2018-1058 revert.
Rejected due to obvious reasons. 

In my humble opinion, the best way to solve an issue is (1). Whether it's even worth it. Although it 

uses non-standard syntax, it does not break the existing one.