Ticket #4200 (closed bug: fixed)
UndecidableInstances not needed for equality constraints
|Reported by:||simonpj||Owned by:|
|Type of failure:||None/Unknown||Difficulty:|
|Test Case:||indexed-types/should_compile/T4200||Blocked By:|
instance (Op c, In c ~ Int) => Op (D2 c) where ...
Without -XUndecidableInstaces we get
undec.hs:21:0: Constraint is no smaller than the instance head in the constraint: In c ~ Int (Use -XUndecidableInstances to permit this)
But the undecidable-instance check is only needed for class constraints I think, so GHC is being over conservative. Let's fix this.