This should work:
Cause: CollReduce only supports IReduce, not IReduceInit so when reduce calls into it, it falls back to trying to obtain a seq representation which fails.
Proposed: Extend CollReduce to IReduceInit and in the non-init arity, cast to IReduce. Also, now that CollReduce supports both IReduceInit and Iterable, a coll that implements both makes the path through CollReduce nondeterministic. transduce does an explicit check that prefers IReduceInit - the patch copies that approach to reduce as well.
Another consequence of this change is that since PersistentVector implements IReduce but throws on the non-init path, there are some test breakages. To address this,
CLJ-1619 (which implements the non-init reduce) must be applied first.
CLJ-1619 being applied first