CAST Within EXCLUSION constraint - Mailing list pgsql-hackers

Hackers,

I am trying to do something like this:
   CREATE TYPE source AS ENUM(       'fred', 'wilma', 'barney', 'betty'   );
   CREATE EXTENSION btree_gist;
   CREATE TABLE things (       source source NOT NULL,       within tstzrange NOT NULL,       EXCLUDE USING gist
(sourceWITH =, within WITH &&)   ); 

Alas, enums are not supported by btree_gist:
   try.sql:13: ERROR:  data type source has no default operator class for access method "gist"   HINT:  You must
specifyan operator class for the index or define a default operator class for the data type. 

Well, maybe I can cast it? But no, changing the EXCLUDE line to
       EXCLUDE USING gist (source::text WITH =, within WITH &&)

Yields a syntax error:
   try.sql:13: ERROR:  syntax error at or near "::"   LINE 4:     EXCLUDE USING gist (source::text WITH =, within WITH
&&)

So that's out. Why shouldn't :: be allowed?

No problem, I can use CAST(), right? So I try:
       EXCLUDE USING gist (CAST(source AS text) WITH =, within WITH &&)

Not so much:
   try.sql:13: ERROR:  functions in index expression must be marked IMMUTABLE

I guess it's because locale settings might change, and therefore change the text representation? Seems unlikely,
though.

I guess I can create my own IMMUTABLE function over the ENUM:
   CREATE FUNCTION source_to_text(       source   ) RETURNS TEXT LANGUAGE sql STRICT IMMUTABLE AS $$       SELECT
$1::text;  $$; 

So this works:
   EXCLUDE USING gist (source_to_text(source) WITH =, within WITH &&)

So I guess that’s good enough for now. But should :: really be a syntax error in index expressions?

Thanks,

David