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The following bug has been logged on the website:

Bug reference:      18463
Logged by:          Drew Kimball
Email address:      drewk@cockroachlabs.com
PostgreSQL version: 16.3
Operating system:   macOS
Description:        

Hello,

I believe there may be a bug related to stored procedures with
polymorphic-typed OUT parameters:

CREATE PROCEDURE p(INOUT x ANYELEMENT) LANGUAGE SQL AS $$ SELECT x; $$;
CALL p(1);

The above example results in an error message "cannot display a value of
type anyelement", but I would expect it to succeed and output "1". This also
reproduces with the following stored procedures:

CREATE PROCEDURE p(INOUT x ANYELEMENT) LANGUAGE SQL AS $$ SELECT 1; $$;
CREATE PROCEDURE p(x ANYELEMENT, OUT y ANYELEMENT) LANGUAGE SQL AS $$ SELECT
x; $$;
CREATE PROCEDURE p(x ANYARRAY, OUT y ANYELEMENT) LANGUAGE SQL AS $$ SELECT
x[1]; $$;

Interestingly, this doesn't seem to reproduce when the OUT param has type
ANYARRAY. The following example succeeds:

CREATE PROCEDURE p(INOUT x ANYARRAY) LANGUAGE SQL AS $$ SELECT x; $$;
CALL p(ARRAY[1, 2, 3]);