non-terminating instances involving FDs
GHC accepts the following (tc109) with -fglasgow-exts:
class P a class R a b | b->a instance (P a,R a b) => P [b]
However as pointed out in the FD-CHR paper (example 15), adding the instance
instance R [[a]] [a]
will cause non-termination. Such instances (context contains variables not in the head, even if determined by variables in the head) should be forbidden without -fallow-undecidable-instances. (This is the second part of the Bound Variable Condition in the above paper). Similar cases are tc118 and tc125.