; let { matches = lookupFamInstEnvUnify inst_envs fam tys'
; conflicts = [ conflictingFamInst
- | match@(_, conflictingFamInst) <- matches
- , conflicting fam tys' tycon match
+ | match@((conflictingFamInst, _), _) <- matches
+ , conflicting tycon match
]
}
; unless (null conflicts) $
conflictInstErr famInst (head conflicts)
}
where
- -- In the case of data/newtype instances, any overlap is a conflict (as
- -- these instances imply injective type mappings).
- conflicting _ _ tycon _ | isAlgTyCon tycon = True
- conflicting fam tys' tycon (subst, cFamInst) | otherwise =
- panic "FamInst.checkForConflicts: overlap check for indexed synonyms is still missing"
+ -- * In the case of data family instances, any overlap is fundamentally a
+ -- conflict (as these instances imply injective type mappings).
+ -- * In the case of type family instances, overlap is admitted as long as
+ -- the right-hand sides of the overlapping rules coincide under the
+ -- overlap substitution. We require that they are syntactically equal;
+ -- anything else would be difficult to test for at this stage.
+ conflicting tycon1 ((famInst2, _), subst)
+ | isAlgTyCon tycon1 = True
+ | otherwise = not (rhs1 `tcEqType` rhs2)
+ where
+ tycon2 = famInstTyCon famInst2
+ rhs1 = substTy subst $ synTyConType tycon1
+ rhs2 = substTy subst $ synTyConType tycon2
conflictInstErr famInst conflictingFamInst
= addFamInstLoc famInst $