BUG #14414: SPI_ERROR_CONNECT on stable plpgsql function used for domain check

The following bug has been logged on the website:

Bug reference: 14414
Logged by: Marcos Castedo
Email address: marcos(dot)castedo(at)anachronics(dot)com
PostgreSQL version: 9.6.1
Operating system: Debian stretch
Description:

I'm trying to do a domain using stable plpgsql function for value check.
This isn't working on 9.6.x on plpgsql using assignment from base data type,
but it works fine on 9.2.x.

Test case:

BEGIN;

--Only fails on stable function, immutable works fine
CREATE OR REPLACE FUNCTION x_domain_test_check()
RETURNS boolean AS $$
BEGIN
RETURN true;
END;
$$ LANGUAGE plpgsql STABLE;

CREATE DOMAIN x_domain_test AS integer CHECK(x_domain_test_check());

DO $$
DECLARE
v_test x_domain_test;
BEGIN
v_test := 1;
END;
$$;

ROLLBACK;

Output on PostgreSQL 9.2.17 (expected)
DO
ROLLBACK

Output on PostgreSQL 9.6.1 (error)

ERROR: SPI_connect failed: SPI_ERROR_CONNECT
CONTEXT: PL/pgSQL function inline_code_block line 5 at assignment
ROLLBACK