= TyConApp tc1 (zipWith boxyLub ts1 ts2)
go (TyVarTy tv1) ty2 -- This is the whole point;
- | isTcTyVar tv1, isMetaTyVar tv1 -- choose ty2 if ty2 is a box
- = ty2
+ | isTcTyVar tv1, isBoxyTyVar tv1 -- choose ty2 if ty2 is a box
+ = orig_ty2
-- Look inside type synonyms, but only if the naive version fails
go ty1 ty2 | Just ty1' <- tcView ty1 = go ty1' ty2
= -- Expand synonyms; ignore FTVs
uUnfilledVar False swapped tv1 details1 nb2 ps_ty2 ty2'
-uUnfilledVar outer swapped tv1 details1 nb2 ps_ty2 ty2@(TyVarTy tv2)
- -- Same type variable => no-op
- | tv1 == tv2
- = returnM ()
+uUnfilledVar outer swapped tv1 details1 nb2 ps_ty2 (TyVarTy tv2)
+ | tv1 == tv2 -- Same type variable => no-op (but watch out for the boxy case)
+ = case details1 of
+ MetaTv BoxTv ref1 -- A boxy type variable meets itself;
+ -- this is box-meets-box, so fill in with a tau-type
+ -> do { tau_tv <- tcInstTyVar tv1
+ ; updateMeta tv1 ref1 (mkTyVarTy tau_tv) }
+ other -> returnM () -- No-op
-- Distinct type variables
| otherwise
-- tv1 is an un-filled-in meta type variable (maybe boxy, maybe tau)
-- ty2 is not a type variable
+uMetaVar swapped tv1 BoxTv ref1 nb2 ps_ty2 non_var_ty2
+ = -- tv1 is a BoxTv. So we must unbox ty2, to ensure
+ -- that any boxes in ty2 are filled with monotypes
+ --
+ -- It should not be the case that tv1 occurs in ty2
+ -- (i.e. no occurs check should be needed), but if perchance
+ -- it does, the unbox operation will fill it, and the DEBUG
+ -- checks for that.
+ do { final_ty <- unBox ps_ty2
+#ifdef DEBUG
+ ; meta_details <- readMutVar ref1
+ ; case meta_details of
+ Indirect ty -> WARN( True, ppr tv1 <+> ppr ty )
+ return () -- This really should *not* happen
+ Flexi -> return ()
+#endif
+ ; checkUpdateMeta swapped tv1 ref1 final_ty }
+
uMetaVar swapped tv1 info1 ref1 nb2 ps_ty2 non_var_ty2
- = do { final_ty <- case info1 of
- BoxTv -> unBox ps_ty2 -- No occurs check
- other -> checkTauTvUpdate tv1 ps_ty2 -- Occurs check + monotype check
+ = do { final_ty <- checkTauTvUpdate tv1 ps_ty2 -- Occurs check + monotype check
; checkUpdateMeta swapped tv1 ref1 final_ty }
----------------