segfault in 9.5alpha - plpgsql function, implicit cast and IMMUTABLE cast function

SELECT $1::text::date

$$;

DROP CAST IF EXISTS (integer AS date);

CREATE CAST (integer AS date) WITH FUNCTION to_date(integer) AS IMPLICIT;

CREATE OR REPLACE FUNCTION newcrash(INTEGER) returns DATE LANGUAGE plpgsql AS $$ BEGIN

RETURN $1;

END$$;

SELECT newcrash(20150202);

SELECT newcrash(20150203);

============ cut ===============

It doesn't crash the first time, but does consistently crash the second. Given that if I remove IMMUTABLE from the function definition it doesn't fail, it implies that there's a problem with the mechanism used to cache function results - although the fact that the second function call doesn't have to be the same value does suggest it's a problem with the code that searches that result cache, rather than the section that retrieves it. 

I tried cutting out the implicit CAST altogether and doing

RETURN to_date($1);

but this doesn't fail, which implies also that it's something related to the implicit cast.

If I DECLARE a local DATE variable and SELECT INTO that (rather than just using RETURN $1), it crashes at that point too.

Geoff