diff options
Diffstat (limited to 'compiler/GHC/Tc/Utils/Env.hs')
| -rw-r--r-- | compiler/GHC/Tc/Utils/Env.hs | 34 |
1 files changed, 33 insertions, 1 deletions
diff --git a/compiler/GHC/Tc/Utils/Env.hs b/compiler/GHC/Tc/Utils/Env.hs index 199fb57cc6..1b13d82869 100644 --- a/compiler/GHC/Tc/Utils/Env.hs +++ b/compiler/GHC/Tc/Utils/Env.hs @@ -102,6 +102,7 @@ import GHC.Core.ConLike import GHC.Core.TyCon import GHC.Core.Type import GHC.Core.Coercion.Axiom +import GHC.Core.Coercion import GHC.Core.Class import GHC.Unit.Module @@ -663,10 +664,41 @@ tcCheckUsage name id_mult thing_inside ; wrapper <- case actual_u of Bottom -> return idHsWrapper Zero -> tcSubMult (UsageEnvironmentOf name) Many id_mult - MUsage m -> tcSubMult (UsageEnvironmentOf name) m id_mult + MUsage m -> do { m <- zonkTcType m + ; m <- promote_mult m + ; tcSubMult (UsageEnvironmentOf name) m id_mult } ; tcEmitBindingUsage (deleteUE uenv name) ; return wrapper } + -- This is gross. The problem is in test case typecheck/should_compile/T18998: + -- f :: a %1-> Id n a -> Id n a + -- f x (MkId _) = MkId x + -- where MkId is a GADT constructor. Multiplicity polymorphism of constructors + -- invents a new multiplicity variable p[2] for the application MkId x. This + -- variable is at level 2, bumped because of the GADT pattern-match (MkId _). + -- We eventually unify the variable with One, due to the call to tcSubMult in + -- tcCheckUsage. But by then, we're at TcLevel 1, and so the level-check + -- fails. + -- + -- What to do? If we did inference "for real", the sub-multiplicity constraint + -- would end up in the implication of the GADT pattern-match, and all would + -- be well. But we don't have a real sub-multiplicity constraint to put in + -- the implication. (Multiplicity inference works outside the usual generate- + -- constraints-and-solve scheme.) Here, where the multiplicity arrives, we + -- must promote all multiplicity variables to reflect this outer TcLevel. + -- It's reminiscent of floating a constraint, really, so promotion is + -- appropriate. The promoteTcType function works only on types of kind TYPE rr, + -- so we can't use it here. Thus, this dirtiness. + -- + -- It works nicely in practice. + (promote_mult, _, _, _) = mapTyCo mapper + mapper = TyCoMapper { tcm_tyvar = \ () tv -> do { _ <- promoteTyVar tv + ; zonkTcTyVar tv } + , tcm_covar = \ () cv -> return (mkCoVarCo cv) + , tcm_hole = \ () h -> return (mkHoleCo h) + , tcm_tycobinder = \ () tcv _flag -> return ((), tcv) + , tcm_tycon = return } + {- ********************************************************************* * * The TcBinderStack |