Ambiguous predicate in local closure

[macrologist]

The following is a minimal example of a behavior that seems curious to me.

(defpackage #:ambiguous-pred-in-local-closure
  (:import-from
   #:common-lisp
   #:describe
   #:disassemble)
  (:use #:coalton #:coalton-prelude)
  (:local-nicknames
   (#:optional #:coalton-library/optional)
   (#:iter #:coalton-library/iterator)))

(named-readtables:in-readtable coalton:coalton)

(in-package #:ambiguous-pred-in-local-closure)

(coalton-toplevel

  (define-class (Moo :a :b :c (:a -> :b :c))
    (moo-size (:a -> :b))
    (moo-find (:a -> :b -> (Optional :c))))

  (declare filled-moos ((Num :b) (Ord :b) (Moo :a :b :c) => :a -> List :b))
  (define (filled-moos moo)
    (let ((filled?
            (fn (i) (optional:some? (moo-find moo i)))))
      (filter filled? (iter:up-to (moo-size moo))))))

This fails to compile, erroring with an ambiguous predicate condition at the call to moo-find inside of filled?.

Is this surprising?

[eliaslfox]

This is a bug. I'm working on a fix.

[macrologist]

Oh: obviously that example is slightly wrong. There should be a collect in there, or the List should be an iter:Iterator. But the issue stands.