failed to find conversion function from "unknown" to text

Dear Gurus,

Version: 8.0.0rc4

The scene below might look funny but it's essential for our project:
  [local]:tir=# select 'a' as asdf;   asdf  ------   a  (1 row)
  [local]:tir=# select case 'a' when 'a' then 1 else 2 end as asdf;  ERROR:  failed to find conversion function from
"unknown"to text
 

This worked up to v7.4.6, and couldn't see it in the "Migration" part of the 
HISTORY file.

Question: is there a way to tell the server to convert unknown to something 
(text or varchar, don't really care), or to write such a "conversion function"?

A similar conversion for name to varchar conversion would also be highly 
desirable for easier migration. Right now I'm writing wrapper functions for 
all our functions where params might be names.

If it's written in the doc, could you please tell me the page?

TIA,
-- 
G.