[Chicken-hackers] [PATCH] Disjoint return types with undefined (or *?) mess up "not" typechecks

[Peter Bex]

Hi all,

While debugging the other scrutinizer issue with empty lists, I found
this one:

(define (some-proc x y) (if (string->number y) (set-cdr! x x) x))
(assert (null? (some-proc (list) "invalid number syntax")))

This fails, because there's an early return in simplify in simplify-type
in scrutinizer.scm which gets triggered whenever it encounters a
"undefined" type.  This means that a type of '(or * undefined) gets
simplified to 'undefined, which means it matches the '(not null)
specialisation for null (this is implicit, since it's defined as
a predicate for the null type).  Of course that's invalid as the
string->number check is false for Y = "invalid number syntax",
so SOME-PROC will simply return the X value it is passed, which
is null.

The attached patch is a big one, but it simply removes the early
returns; the rest is just due to the indentation change.

I wasn't 100% sure whether the early return on * is bad too, but
it seems safer to omit any early returns.

Cheers,
Peter
-- 
http://sjamaan.ath.cx
--
"The process of preparing programs for a digital computer
 is especially attractive, not only because it can be economically
 and scientifically rewarding, but also because it can be an aesthetic
 experience much like composing poetry or music."
                                                        -- Donald Knuth

0001-Do-not-return-early-when-encountering-undefined-or-t.patch
Description: Text document