plpgsql doesn't coerce boolean expressions to boolean

Following up this gripe
http://archives.postgresql.org/pgsql-sql/2003-09/msg00044.php
I've realized that plpgsql just assumes that the test expression
of an IF, WHILE, or EXIT statement is a boolean expression. It
doesn't take any measures to ensure this is the case or convert
the value if it's not the case. This seems pretty bogus to me.

However ... with the code as it stands, for pass-by-reference datatypes
any nonnull value will appear TRUE, while for pass-by-value datatypes
any nonzero value will appear TRUE. I fear that people may actually be
depending on these behaviors, particularly the latter one which is
pretty reasonable if you're accustomed to C. So while I'd like to throw
an error if the argument isn't boolean, I'm afraid of breaking people's
function definitions.

Here are some possible responses, roughly in order of difficulty
to implement:

1. Leave well enough alone (and perhaps document the behavior).

2. Throw an error if the expression doesn't return boolean.

3. Try to convert nonbooleans to boolean using plpgsql's usual method
for cross-type coercion, ie run the type's output proc to get a
string and feed it to bool's input proc. (This seems unlikely to
avoid throwing an error in very many cases, but it'd be the most
consistent with other parts of plpgsql.)

4. Use the parser's coerce_to_boolean procedure, so that nonbooleans
will be accepted in exactly the same cases where they'd be accepted
in a boolean-requiring SQL construct (such as CASE). (By default,
none are, so this isn't really different from #2. But people could
create casts to boolean to override this behavior in a controlled
fashion.)

Any opinions about what to do?

regards, tom lane