-- The givens can include methods
-- See Note [Pruning the givens in an implication constraint]
- -- If there are no 'givens', then it's safe to
+ -- If there are no 'givens' *and* the refinement is empty
+ -- (the refinement is like more givens), then it's safe to
-- partition the 'wanteds' by their qtvs, thereby trimming irreds
-- See Note [Freeness and implications]
- ; irreds' <- if null givens'
+ ; irreds' <- if null givens' && isEmptyRefinement reft
then do
{ let qtv_set = mkVarSet qtvs
(frees, real_irreds) = partition (isFreeWrtTyVars qtv_set) irreds
; wanteds <- newDictBndrsO orig (substTheta tenv theta)
; (irreds, _) <- topCheckLoop doc wanteds
- ; let (dicts, non_dicts) = partition isDict irreds
- -- Exclude implication consraints
- ; addNoInstanceErrs non_dicts -- I'm not sure if these can really happen
+ -- Insist that the context of a derived instance declaration
+ -- consists of constraints of form (C a b), where a,b are
+ -- type variables
+ -- NB: the caller will further check the tv_dicts for
+ -- legal instance-declaration form
+ ; let (tv_dicts, non_tv_dicts) = partition isTyVarDict irreds
+ ; addNoInstanceErrs non_tv_dicts
; let rev_env = zipTopTvSubst tvs (mkTyVarTys tyvars)
- simpl_theta = substTheta rev_env (map dictPred dicts)
- -- This reverse-mapping is a Royal Pain,
- -- but the result should mention TyVars not TcTyVars
+ simpl_theta = substTheta rev_env (map dictPred tv_dicts)
+ -- This reverse-mapping is a pain, but the result
+ -- should mention the original TyVars not TcTyVars
; return simpl_theta }
where