Introduce ConcreteTv metavariables

  This patch introduces a new kind of metavariable, by adding the
  constructor `ConcreteTv` to `MetaInfo`. A metavariable with
  `ConcreteTv` `MetaInfo`, henceforth a concrete metavariable, can only
  be unified with a type that is concrete (that is, a type that answers
  `True` to `GHC.Core.Type.isConcrete`).

  This solves the problem of dangling metavariables in `Concrete#`
  constraints: instead of emitting `Concrete# ty`, which contains a
  secret existential metavariable, we simply emit a primitive equality
  constraint `ty ~# concrete_tv` where `concrete_tv` is a fresh concrete
  metavariable.

  This means we can avoid all the complexity of canonicalising
  `Concrete#` constraints, as we can just re-use the existing machinery
  for `~#`.

  To finish things up, this patch then removes the `Concrete#` special
  predicate, and instead introduces the special predicate `IsRefl#`
  which enforces that a coercion is reflexive.
  Such a constraint is needed because the canonicaliser is quite happy
  to rewrite an equality constraint such as `ty ~# concrete_tv`, but
  such a rewriting is not handled by the rest of the compiler currently,
  as we need to make use of the resulting coercion, as outlined in the
  FixedRuntimeRep plan.

  The big upside of this approach (on top of simplifying the code)
  is that we can now selectively implement PHASE 2 of FixedRuntimeRep,
  by changing individual calls of `hasFixedRuntimeRep_MustBeRefl` to
  `hasFixedRuntimeRep` and making use of the obtained coercion.

1 files changed, 26 insertions, 4 deletions

diff --git a/compiler/GHC/Tc/Utils/TcType.hs b/compiler/GHC/Tc/Utils/TcType.hs
index 807ad0ab56..b01f72b185 100644
--- a/compiler/GHC/Tc/Utils/TcType.hs
+++ b/compiler/GHC/Tc/Utils/TcType.hs
@@ -41,6 +41,7 @@ module GHC.Tc.Utils.TcType (
   MetaDetails(Flexi, Indirect), MetaInfo(..), skolemSkolInfo,
   isImmutableTyVar, isSkolemTyVar, isMetaTyVar,  isMetaTyVarTy, isTyVarTy,
   tcIsTcTyVar, isTyVarTyVar, isOverlappableTyVar,  isTyConableTyVar,
+  isConcreteTyVar,
   isAmbiguousTyVar, isCycleBreakerTyVar, metaTyVarRef, metaTyVarInfo,
   isFlexi, isIndirect, isRuntimeUnkSkol,
   metaTyVarTcLevel, setMetaTyVarTcLevel, metaTyVarTcLevel_maybe,
@@ -566,21 +567,27 @@ data MetaDetails
   | Indirect TcType
 
 data MetaInfo
-   = TauTv         -- This MetaTv is an ordinary unification variable
+   = TauTv         -- ^ This MetaTv is an ordinary unification variable
                    -- A TauTv is always filled in with a tau-type, which
                    -- never contains any ForAlls.
 
-   | TyVarTv       -- A variant of TauTv, except that it should not be
+   | TyVarTv       -- ^ A variant of TauTv, except that it should not be
                    --   unified with a type, only with a type variable
                    -- See Note [TyVarTv] in GHC.Tc.Utils.TcMType
 
-   | RuntimeUnkTv  -- A unification variable used in the GHCi debugger.
+   | RuntimeUnkTv  -- ^ A unification variable used in the GHCi debugger.
                    -- It /is/ allowed to unify with a polytype, unlike TauTv
 
    | CycleBreakerTv  -- Used to fix occurs-check problems in Givens
                      -- See Note [Type variable cycles] in
                      -- GHC.Tc.Solver.Canonical
 
+   | ConcreteTv
+        -- ^ A unification variable that can only be unified
+        -- with a concrete type, in the sense of
+        -- Note [Concrete types] in GHC.Tc.Utils.Concrete.
+        -- See Note [ConcreteTv] in GHC.Tc.Utils.Concrete.
+
 instance Outputable MetaDetails where
   ppr Flexi         = text "Flexi"
   ppr (Indirect ty) = text "Indirect" <+> ppr ty
@@ -590,6 +597,7 @@ instance Outputable MetaInfo where
   ppr TyVarTv        = text "tyv"
   ppr RuntimeUnkTv   = text "rutv"
   ppr CycleBreakerTv = text "cbv"
+  ppr ConcreteTv     = text "conc"
 
 {- *********************************************************************
 *                                                                      *
@@ -1081,6 +1089,18 @@ isCycleBreakerTyVar tv
   | otherwise
   = False
 
+-- | Is this type variable a concrete type variable, i.e.
+-- it is a metavariable with 'ConcreteTv' 'MetaInfo'?
+--
+-- Works with both 'TyVar' and 'TcTyVar'.
+isConcreteTyVar :: TcTyVar -> Bool
+isConcreteTyVar tv
+  | isTcTyVar tv
+  , MetaTv { mtv_info = ConcreteTv } <- tcTyVarDetails tv
+  = True
+  | otherwise
+  = False
+
 isMetaTyVarTy :: TcType -> Bool
 isMetaTyVarTy (TyVarTy tv) = isMetaTyVar tv
 isMetaTyVarTy _            = False
@@ -1892,7 +1912,9 @@ isImprovementPred ty
       ClassPred cls _    -> classHasFds cls
       IrredPred {}       -> True -- Might have equalities after reduction?
       ForAllPred {}      -> False
-      SpecialPred {}     -> False
+      SpecialPred s      ->
+        case s of
+          IsReflPrimPred {} -> False
 
 {- Note [Expanding superclasses]
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~