Re-do rubbish literals

As #19882 pointed out, we were simply doing rubbish literals wrong.
(I'll refrain from explaining the wrong-ness here -- see the ticket.)

This patch fixes it by adding a Type (of kind RuntimeRep) as field of
LitRubbish, rather than [PrimRep].

The Note [Rubbish literals] in GHC.Types.Literal explains the details.

18 files changed, 271 insertions, 128 deletions

diff --git a/compiler/GHC/Builtin/Types/Prim.hs b/compiler/GHC/Builtin/Types/Prim.hs
index eaeda97f69..1125655398 100644
--- a/compiler/GHC/Builtin/Types/Prim.hs
+++ b/compiler/GHC/Builtin/Types/Prim.hs
@@ -13,7 +13,8 @@ Wired-in knowledge about primitive types
 module GHC.Builtin.Types.Prim(
         mkPrimTyConName, -- For implicit parameters in GHC.Builtin.Types only
 
-        mkTemplateKindVars, mkTemplateTyVars, mkTemplateTyVarsFrom,
+        mkTemplateKindVar, mkTemplateKindVars,
+        mkTemplateTyVars, mkTemplateTyVarsFrom,
         mkTemplateKiTyVars, mkTemplateKiTyVar,
 
         mkTemplateTyConBinders, mkTemplateKindTyConBinders,
diff --git a/compiler/GHC/Core/Make.hs b/compiler/GHC/Core/Make.hs
index 129120139b..e0897670fc 100644
--- a/compiler/GHC/Core/Make.hs
+++ b/compiler/GHC/Core/Make.hs
@@ -13,6 +13,7 @@ module GHC.Core.Make (
         sortQuantVars, castBottomExpr,
 
         -- * Constructing boxed literals
+        mkLitRubbish,
         mkWordExpr,
         mkIntExpr, mkIntExprInt, mkUncheckedIntExpr,
         mkIntegerExpr, mkNaturalExpr,
@@ -243,6 +244,23 @@ castBottomExpr e res_ty
   where
     e_ty = exprType e
 
+mkLitRubbish :: Type -> Maybe CoreExpr
+-- Make a rubbish-literal CoreExpr of the given type.
+-- Fail (returning Nothing) if
+--    * the RuntimeRep of the Type is not monomorphic;
+--    * the type is (a ~# b), the type of coercion
+-- See INVARIANT 1 and 2 of item (2) in Note [Rubbish literals]
+-- in GHC.Types.Literal
+mkLitRubbish ty
+  | not (noFreeVarsOfType rep)
+  = Nothing   -- Satisfy INVARIANT 1
+  | isCoVarType ty
+  = Nothing   -- Satisfy INVARIANT 2
+  | otherwise
+  = Just (Lit (LitRubbish rep) `mkTyApps` [ty])
+  where
+    rep  = getRuntimeRep ty
+
 {-
 ************************************************************************
 *                                                                      *
diff --git a/compiler/GHC/Core/Opt/Specialise.hs b/compiler/GHC/Core/Opt/Specialise.hs
index 68b16ea753..1f0b6fb2a0 100644
--- a/compiler/GHC/Core/Opt/Specialise.hs
+++ b/compiler/GHC/Core/Opt/Specialise.hs
@@ -27,6 +27,7 @@ import GHC.Core.Opt.Monad
 import qualified GHC.Core.Subst as Core
 import GHC.Core.Unfold.Make
 import GHC.Core
+import GHC.Core.Make      ( mkLitRubbish )
 import GHC.Core.Unify     ( tcMatchTy )
 import GHC.Core.Rules
 import GHC.Core.Utils     ( exprIsTrivial, getIdFromTrivialExpr_maybe
@@ -47,10 +48,8 @@ import GHC.Types.Unique.Supply
 import GHC.Types.Unique.DFM
 import GHC.Types.Name
 import GHC.Types.Tickish
-import GHC.Types.RepType  ( typeMonoPrimRep_maybe )
 import GHC.Types.Id.Make  ( voidArgId, voidPrimId )
 import GHC.Types.Var      ( isLocalVar )
-import GHC.Types.Literal  ( mkLitRubbish )
 import GHC.Types.Var.Set
 import GHC.Types.Var.Env
 import GHC.Types.Id
@@ -2346,8 +2345,8 @@ specHeader env (bndr : bndrs) (UnspecArg : args)
              -- C.f. GHC.Core.Opt.WorkWrap.Utils.mk_absent_let
              (mb_spec_bndr, spec_arg)
                 | isDeadBinder bndr
-                , Just reps <- typeMonoPrimRep_maybe bndr_ty
-                = (Nothing, mkTyApps (Lit (mkLitRubbish reps)) [bndr_ty])
+                , Just lit_expr <- mkLitRubbish bndr_ty
+                = (Nothing, lit_expr)
                 | otherwise
                 = (Just bndr', varToCoreExpr bndr')
 
diff --git a/compiler/GHC/Core/Opt/WorkWrap/Utils.hs b/compiler/GHC/Core/Opt/WorkWrap/Utils.hs
index 16234d09fa..127e684938 100644
--- a/compiler/GHC/Core/Opt/WorkWrap/Utils.hs
+++ b/compiler/GHC/Core/Opt/WorkWrap/Utils.hs
@@ -28,16 +28,14 @@ import GHC.Core.DataCon
 import GHC.Types.Demand
 import GHC.Types.Cpr
 import GHC.Core.Make     ( mkAbsentErrorApp, mkCoreUbxTup, mkCoreApp, mkCoreLet
-                         , mkWildValBinder )
+                         , mkWildValBinder, mkLitRubbish )
 import GHC.Types.Id.Make ( voidArgId, voidPrimId )
 import GHC.Builtin.Types ( tupleDataCon )
-import GHC.Types.Literal ( mkLitRubbish )
 import GHC.Types.Var.Env ( mkInScopeSet )
 import GHC.Types.Var.Set ( VarSet )
 import GHC.Core.Type
 import GHC.Core.Multiplicity
 import GHC.Core.Predicate ( isClassPred )
-import GHC.Types.RepType  ( isVoidTy, typeMonoPrimRep_maybe )
 import GHC.Core.Coercion
 import GHC.Core.FamInstEnv
 import GHC.Types.Basic       ( Boxity(..) )
@@ -60,6 +58,8 @@ import Control.Applicative ( (<|>) )
 import Control.Monad ( zipWithM )
 import Data.List ( unzip4 )
 
+import GHC.Types.RepType
+
 {-
 ************************************************************************
 *                                                                      *
@@ -458,7 +458,10 @@ mkWWargs :: TCvSubst            -- Freshening substitution to apply to the type
 
 mkWWargs subst fun_ty demands
   | null demands
-  = return ([], nop_fn, nop_fn, substTy subst fun_ty)
+  = return ([], nop_fn, nop_fn, substTyUnchecked subst fun_ty)
+    -- I got an ASSERT failure here with `substTy`, and I was
+    -- disinclined to pursue it since this code is about to be
+    -- deleted by Sebastian
 
   | (dmd:demands') <- demands
   , Just (mult, arg_ty, fun_ty') <- splitFunTy_maybe fun_ty
@@ -1011,23 +1014,19 @@ mk_absent_let :: WwOpts -> Id -> Maybe (CoreExpr -> CoreExpr)
 mk_absent_let opts arg
   -- The lifted case: Bind 'absentError' for a nice panic message if we are
   -- wrong (like we were in #11126). See (1) in Note [Absent fillers]
-  | Just [LiftedRep] <- mb_mono_prim_reps
+  | not (isUnliftedType arg_ty)
   , not (isStrictDmd (idDemandInfo arg)) -- See (2) in Note [Absent fillers]
   = Just (Let (NonRec arg panic_rhs))
 
-  -- The default case for mono rep: Bind @RUBBISH[prim_reps] \@arg_ty@
+  -- The default case for mono rep: Bind `RUBBISH[rr] arg_ty`
   -- See Note [Absent fillers], the main part
-  | Just prim_reps <- mb_mono_prim_reps
-  = Just (bindNonRec arg (mkTyApps (Lit (mkLitRubbish prim_reps)) [arg_ty]))
-
-  -- Catch all: Either @arg_ty@ wasn't of form @TYPE rep@ or @rep@ wasn't mono rep.
-  -- See (3) in Note [Absent fillers]
-  | Nothing <- mb_mono_prim_reps
-  = warnPprTrace True (text "No absent value for" <+> ppr arg_ty) $
-    Nothing
+  | Just lit_expr <- mkLitRubbish arg_ty
+  = Just (bindNonRec arg lit_expr)
+
+  | otherwise
+  = Nothing
   where
     arg_ty = idType arg
-    mb_mono_prim_reps = typeMonoPrimRep_maybe arg_ty
 
     panic_rhs = mkAbsentErrorApp arg_ty msg
 
@@ -1179,7 +1178,7 @@ they are *dead code*) and they are probably discarded after the next run of the
 Simplifier (when they are in fact *unreachable code*). Yet, we have to come up
 with "filler" values that we bind the absent arg Ids to.
 
-That is exactly what Note [Rubbish values] are for: A convenient way to
+That is exactly what Note [Rubbish literals] are for: A convenient way to
 conjure filler values at any type (and any representation or levity!).
 
 Needless to say, there are some wrinkles:
@@ -1187,7 +1186,7 @@ Needless to say, there are some wrinkles:
   1. In case we have a absent, /lazy/, and /lifted/ arg, we use an error-thunk
      instead. If absence analysis was wrong (e.g., #11126) and the binding
      in fact is used, then we get a nice panic message instead of undefined
-     runtime behavior (See Modes of failure from Note [Rubbish values]).
+     runtime behavior (See Modes of failure from Note [Rubbish literals]).
 
      Obviously, we can't use an error-thunk if the value is of unlifted rep
      (like 'Int#' or 'MutVar#'), because we'd immediately evaluate the panic.
diff --git a/compiler/GHC/Core/TyCon.hs b/compiler/GHC/Core/TyCon.hs
index 07b583f92b..675206b1a7 100644
--- a/compiler/GHC/Core/TyCon.hs
+++ b/compiler/GHC/Core/TyCon.hs
@@ -1435,7 +1435,7 @@ tyConRepModOcc tc_module tc_occ = (rep_module, mkTyConRepOcc tc_occ)
 ************************************************************************
 
 Note [rep swamp]
-
+~~~~~~~~~~~~~~~~
 GHC has a rich selection of types that represent "primitive types" of
 one kind or another.  Each of them makes a different set of
 distinctions, and mostly the differences are for good reasons,
diff --git a/compiler/GHC/CoreToIface.hs b/compiler/GHC/CoreToIface.hs
index 117020c5fc..a1c8d0fe78 100644
--- a/compiler/GHC/CoreToIface.hs
+++ b/compiler/GHC/CoreToIface.hs
@@ -50,6 +50,7 @@ import GHC.Driver.Ppr
 import GHC.Iface.Syntax
 import GHC.Core.DataCon
 import GHC.Types.Id
+import GHC.Types.Literal
 import GHC.Types.Id.Info
 import GHC.StgToCmm.Types
 import GHC.Core
@@ -539,6 +540,7 @@ toIfUnfolding _ NoUnfolding = Nothing
 
 toIfaceExpr :: CoreExpr -> IfaceExpr
 toIfaceExpr (Var v)         = toIfaceVar v
+toIfaceExpr (Lit (LitRubbish r)) = IfaceLitRubbish (toIfaceType r)
 toIfaceExpr (Lit l)         = IfaceLit l
 toIfaceExpr (Type ty)       = IfaceType (toIfaceType ty)
 toIfaceExpr (Coercion co)   = IfaceCo   (toIfaceCoercion co)
@@ -581,7 +583,9 @@ toIfaceAlt (Alt c bs r) = IfaceAlt (toIfaceCon c) (map getOccFS bs) (toIfaceExpr
 ---------------------
 toIfaceCon :: AltCon -> IfaceConAlt
 toIfaceCon (DataAlt dc) = IfaceDataAlt (getName dc)
-toIfaceCon (LitAlt l)   = IfaceLitAlt l
+toIfaceCon (LitAlt l)   = assertPpr (not (isLitRubbish l)) (ppr l) $
+                          -- assert: see Note [Rubbish literals] wrinkle (b)
+                          IfaceLitAlt l
 toIfaceCon DEFAULT      = IfaceDefault
 
 ---------------------
diff --git a/compiler/GHC/CoreToStg.hs b/compiler/GHC/CoreToStg.hs
index a868fa2de3..bf20cc4286 100644
--- a/compiler/GHC/CoreToStg.hs
+++ b/compiler/GHC/CoreToStg.hs
@@ -395,9 +395,11 @@ coreToStgExpr (Coercion _)
 
 coreToStgExpr expr@(App _ _)
   = case app_head of
-      Var f               -> coreToStgApp f args ticks -- Regular application
-      Lit l@LitRubbish{}  -> return (StgLit l) -- LitRubbish
-      _                   -> pprPanic "coreToStgExpr - Invalid app head:" (ppr expr)
+      Var f -> coreToStgApp f args ticks -- Regular application
+      Lit l | isLitRubbish l             -- If there is LitRubbish at the head,
+            -> return (StgLit l)         --    discard the arguments
+
+      _     -> pprPanic "coreToStgExpr - Invalid app head:" (ppr expr)
     where
       (app_head, args, ticks) = myCollectArgs expr
 coreToStgExpr expr@(Lam _ _)
diff --git a/compiler/GHC/Iface/Rename.hs b/compiler/GHC/Iface/Rename.hs
index 04b227c50e..7f51a8f933 100644
--- a/compiler/GHC/Iface/Rename.hs
+++ b/compiler/GHC/Iface/Rename.hs
@@ -646,8 +646,9 @@ rnIfaceExpr (IfaceLet (IfaceRec pairs) body)
                <*> rnIfaceExpr body
 rnIfaceExpr (IfaceCast expr co)
     = IfaceCast <$> rnIfaceExpr expr <*> rnIfaceCo co
-rnIfaceExpr (IfaceLit lit) = pure (IfaceLit lit)
-rnIfaceExpr (IfaceFCall cc ty) = IfaceFCall cc <$> rnIfaceType ty
+rnIfaceExpr (IfaceLit lit)           = pure (IfaceLit lit)
+rnIfaceExpr (IfaceLitRubbish rep)    = IfaceLitRubbish <$> rnIfaceType rep
+rnIfaceExpr (IfaceFCall cc ty)       = IfaceFCall cc <$> rnIfaceType ty
 rnIfaceExpr (IfaceTick tickish expr) = IfaceTick tickish <$> rnIfaceExpr expr
 
 rnIfaceBndrs :: Rename [IfaceBndr]
diff --git a/compiler/GHC/Iface/Syntax.hs b/compiler/GHC/Iface/Syntax.hs
index da54049413..d1ecf388cd 100644
--- a/compiler/GHC/Iface/Syntax.hs
+++ b/compiler/GHC/Iface/Syntax.hs
@@ -556,6 +556,8 @@ data IfaceExpr
   | IfaceLet    IfaceBinding  IfaceExpr
   | IfaceCast   IfaceExpr IfaceCoercion
   | IfaceLit    Literal
+  | IfaceLitRubbish IfaceType -- See GHC.Types.Literal
+                              --   Note [Rubbish literals] item (6)
   | IfaceFCall  ForeignCall IfaceType
   | IfaceTick   IfaceTickish IfaceExpr    -- from Tick tickish E
 
@@ -1368,6 +1370,7 @@ pprIfaceExpr :: (SDoc -> SDoc) -> IfaceExpr -> SDoc
 pprIfaceExpr _       (IfaceLcl v)       = ppr v
 pprIfaceExpr _       (IfaceExt v)       = ppr v
 pprIfaceExpr _       (IfaceLit l)       = ppr l
+pprIfaceExpr _       (IfaceLitRubbish r) = text "RUBBISH" <> parens (ppr r)
 pprIfaceExpr _       (IfaceFCall cc ty) = braces (ppr cc <+> ppr ty)
 pprIfaceExpr _       (IfaceType ty)     = char '@' <> pprParendIfaceType ty
 pprIfaceExpr _       (IfaceCo co)       = text "@~" <> pprParendIfaceCoercion co
@@ -2352,6 +2355,9 @@ instance Binary IfaceExpr where
         putByte bh 13
         put_ bh a
         put_ bh b
+    put_ bh (IfaceLitRubbish r) = do
+        putByte bh 14
+        put_ bh r
     get bh = do
         h <- getByte bh
         case h of
@@ -2394,6 +2400,8 @@ instance Binary IfaceExpr where
             13 -> do a <- get bh
                      b <- get bh
                      return (IfaceECase a b)
+            14 -> do r <- get bh
+                     return (IfaceLitRubbish r)
             _ -> panic ("get IfaceExpr " ++ show h)
 
 instance Binary IfaceTickish where
@@ -2618,6 +2626,7 @@ instance NFData IfaceExpr where
     IfaceLet bind e -> rnf bind `seq` rnf e
     IfaceCast e co -> rnf e `seq` rnf co
     IfaceLit l -> l `seq` () -- FIXME
+    IfaceLitRubbish r -> rnf r `seq` ()
     IfaceFCall fc ty -> fc `seq` rnf ty
     IfaceTick tick e -> rnf tick `seq` rnf e
 
diff --git a/compiler/GHC/IfaceToCore.hs b/compiler/GHC/IfaceToCore.hs
index e72c931ae6..de65e43ccd 100644
--- a/compiler/GHC/IfaceToCore.hs
+++ b/compiler/GHC/IfaceToCore.hs
@@ -1454,6 +1454,10 @@ tcIfaceExpr (IfaceLcl name)
 tcIfaceExpr (IfaceExt gbl)
   = Var <$> tcIfaceExtId gbl
 
+tcIfaceExpr (IfaceLitRubbish rep)
+  = do rep' <- tcIfaceType rep
+       return (Lit (LitRubbish rep'))
+
 tcIfaceExpr (IfaceLit lit)
   = do lit' <- tcIfaceLit lit
        return (Lit lit')
diff --git a/compiler/GHC/Stg/Unarise.hs b/compiler/GHC/Stg/Unarise.hs
index 6b41063f9b..b3ae7957d9 100644
--- a/compiler/GHC/Stg/Unarise.hs
+++ b/compiler/GHC/Stg/Unarise.hs
@@ -231,7 +231,7 @@ STG programs after unarisation have these invariants:
     This means that it's safe to wrap `StgArg`s of DataCon applications with
     `GHC.StgToCmm.Env.NonVoid`, for example.
 
-  * Similar to unboxed tuples, Note [Rubbish values] of TupleRep may only
+  * Similar to unboxed tuples, Note [Rubbish literals] of TupleRep may only
     appear in return position.
 
   * Alt binders (binders in patterns) are always non-void.
@@ -390,7 +390,7 @@ unariseExpr rho (StgCase scrut bndr alt_ty alts)
   , Just args' <- unariseMulti_maybe rho dc args ty_args
   = elimCase rho args' bndr alt_ty alts
 
-  -- See (3) of Note [Rubbish values] in GHC.Types.Literal
+  -- See (3) of Note [Rubbish literals] in GHC.Types.Literal
   | StgLit lit <- scrut
   , Just args' <- unariseRubbish_maybe lit
   = elimCase rho args' bndr alt_ty alts
@@ -427,19 +427,18 @@ unariseMulti_maybe rho dc args ty_args
 
 -- Doesn't return void args.
 unariseRubbish_maybe :: Literal -> Maybe [OutStgArg]
-unariseRubbish_maybe lit
-  | LitRubbish preps <- lit
-  , [prep] <- preps
+unariseRubbish_maybe (LitRubbish rep)
+  | [prep] <- preps
   , not (isVoidRep prep)
-  -- Single, non-void PrimRep. Nothing to do!
-  = Nothing
+  = Nothing   -- Single, non-void PrimRep. Nothing to do!
 
-  | LitRubbish preps <- lit
-  -- Multiple reps, possibly with VoidRep. Eliminate!
-  = Just [ StgLitArg (LitRubbish [prep]) | prep <- preps, not (isVoidRep prep) ]
+  | otherwise -- Multiple reps, possibly with VoidRep. Eliminate via elimCase
+  = Just [ StgLitArg (LitRubbish (primRepToType prep))
+         | prep <- preps, not (isVoidRep prep) ]
+  where
+    preps = runtimeRepPrimRep (text "unariseRubbish_maybe") rep
 
-  | otherwise
-  = Nothing
+unariseRubbish_maybe _ = Nothing
 
 --------------------------------------------------------------------------------
 
diff --git a/compiler/GHC/StgToCmm/Lit.hs b/compiler/GHC/StgToCmm/Lit.hs
index 8e2e000753..11de674618 100644
--- a/compiler/GHC/StgToCmm/Lit.hs
+++ b/compiler/GHC/StgToCmm/Lit.hs
@@ -23,6 +23,7 @@ import GHC.Cmm.CLabel
 import GHC.Cmm.Utils
 
 import GHC.Types.Literal
+import GHC.Types.RepType( runtimeRepPrimRep )
 import GHC.Builtin.Types ( unitDataConId )
 import GHC.Core.TyCon
 import GHC.Utils.Misc
@@ -49,8 +50,8 @@ cgLit :: Literal -> FCode CmmExpr
 cgLit (LitString s) =
   CmmLit <$> newByteStringCLit s
  -- not unpackFS; we want the UTF-8 byte stream.
-cgLit (LitRubbish preps) =
-  case expectOnly "cgLit:Rubbish" preps of -- Note [Post-unarisation invariants]
+cgLit (LitRubbish rep) =
+  case expectOnly "cgLit" prim_reps of -- Note [Post-unarisation invariants]
     VoidRep     -> panic "cgLit:VoidRep"   -- dito
     LiftedRep   -> idInfoToAmode <$> getCgIdInfo unitDataConId
     UnliftedRep -> idInfoToAmode <$> getCgIdInfo unitDataConId
@@ -60,7 +61,9 @@ cgLit (LitRubbish preps) =
       let elem_lit = mkSimpleLit platform (num_rep_lit (primElemRepToPrimRep elem))
       pure (CmmLit (CmmVec (replicate n elem_lit)))
     prep        -> cgLit (num_rep_lit prep)
-    where
+  where
+      prim_reps = runtimeRepPrimRep (text "cgLit") rep
+
       num_rep_lit IntRep    = mkLitIntUnchecked 0
       num_rep_lit Int8Rep   = mkLitInt8Unchecked 0
       num_rep_lit Int16Rep  = mkLitInt16Unchecked 0
@@ -74,6 +77,7 @@ cgLit (LitRubbish preps) =
       num_rep_lit FloatRep  = LitFloat 0
       num_rep_lit DoubleRep = LitDouble 0
       num_rep_lit other     = pprPanic "num_rep_lit: Not a num lit" (ppr other)
+
 cgLit other_lit = do
   platform <- getPlatform
   pure (CmmLit (mkSimpleLit platform other_lit))
diff --git a/compiler/GHC/Types/Literal.hs b/compiler/GHC/Types/Literal.hs
index d7958edb8f..6160a450bb 100644
--- a/compiler/GHC/Types/Literal.hs
+++ b/compiler/GHC/Types/Literal.hs
@@ -33,7 +33,6 @@ module GHC.Types.Literal
         , mkLitChar, mkLitString
         , mkLitInteger, mkLitNatural
         , mkLitNumber, mkLitNumberWrap
-        , mkLitRubbish
 
         -- ** Operations on Literals
         , literalType
@@ -52,7 +51,7 @@ module GHC.Types.Literal
         , isZeroLit, isOneLit
         , litFitsInChar
         , litValue, mapLitValue
-        , isLitValue_maybe
+        , isLitValue_maybe, isLitRubbish
 
         -- ** Coercions
         , narrowInt8Lit, narrowInt16Lit, narrowInt32Lit
@@ -68,7 +67,6 @@ import GHC.Prelude
 import GHC.Builtin.Types.Prim
 import {-# SOURCE #-} GHC.Builtin.Types
 import GHC.Core.Type
-import GHC.Core.TyCon
 import GHC.Utils.Outputable
 import GHC.Data.FastString
 import GHC.Types.Basic
@@ -131,13 +129,15 @@ data Literal
                                 -- that can be represented as a Literal. Create
                                 -- with 'nullAddrLit'
 
-  | LitRubbish [PrimRep]        -- ^ A nonsense value of the given
-                                -- representation. See Note [Rubbish values].
+  | LitRubbish Type             -- ^ A nonsense value of the given
+                                -- representation. See Note [Rubbish literals].
+                                --
+                                -- The Type argument, rr, is of kind RuntimeRep.
+                                -- The type of the literal is forall (a:TYPE rr). a
+                                --
+                                -- INVARIANT: the Type has no free variables
+                                --    and so substitution etc can ignore it
                                 --
-                                -- The @[PrimRep]@ of a 'Type' can be obtained
-                                -- from 'typeMonoPrimRep_maybe'. The field
-                                -- becomes empty or singleton post-unarisation,
-                                -- see Note [Post-unarisation invariants].
 
   | LitFloat   Rational         -- ^ @Float#@. Create with 'mkLitFloat'
   | LitDouble  Rational         -- ^ @Double#@. Create with 'mkLitDouble'
@@ -199,7 +199,6 @@ instance Binary LitNumType where
 {-
 Note [BigNum literals]
 ~~~~~~~~~~~~~~~~~~~~~~
-
 GHC supports 2 kinds of arbitrary precision integers (a.k.a BigNum):
 
    * Natural: natural represented as a Word# or as a BigNat
@@ -213,7 +212,6 @@ are replaced with expression to build them at runtime from machine literals
 
 Note [String literals]
 ~~~~~~~~~~~~~~~~~~~~~~
-
 String literals are UTF-8 encoded and stored into ByteStrings in the following
 ASTs: Haskell, Core, Stg, Cmm. TH can also emit ByteString based string literals
 with the BytesPrimL constructor (see #14741).
@@ -242,7 +240,9 @@ instance Binary Literal where
         = do putByte bh 6
              put_ bh nt
              put_ bh i
-    put_ bh (LitRubbish b) = do putByte bh 7; put_ bh b
+    put_ _ (LitRubbish b) = pprPanic "Binary LitRubbish" (ppr b)
+     -- We use IfaceLitRubbish; see Note [Rubbish literals], item (6)
+
     get bh = do
             h <- getByte bh
             case h of
@@ -268,9 +268,6 @@ instance Binary Literal where
                     nt <- get bh
                     i  <- get bh
                     return (LitNumber nt i)
-              7 -> do
-                    b <- get bh
-                    return (LitRubbish b)
               _ -> pprPanic "Binary:Literal" (int (fromIntegral h))
 
 instance Outputable Literal where
@@ -552,11 +549,9 @@ mkLitNatural :: Integer -> Literal
 mkLitNatural x = assertPpr (inNaturalRange x) (integer x)
                     (LitNumber LitNumNatural x)
 
--- | Create a rubbish literal of the given representation.
--- The representation of a 'Type' can be obtained via 'typeMonoPrimRep_maybe'.
--- See Note [Rubbish values].
-mkLitRubbish :: [PrimRep] -> Literal
-mkLitRubbish = LitRubbish
+isLitRubbish :: Literal -> Bool
+isLitRubbish (LitRubbish {}) = True
+isLitRubbish _               = False
 
 inNaturalRange :: Integer -> Bool
 inNaturalRange x = x >= 0
@@ -821,10 +816,12 @@ literalType (LitNumber lt _)  = case lt of
    LitNumWord16  -> word16PrimTy
    LitNumWord32  -> word32PrimTy
    LitNumWord64  -> word64PrimTy
-literalType (LitRubbish preps) = mkForAllTy a Inferred (mkTyVarTy a)
+
+-- LitRubbish: see Note [Rubbish literals]
+literalType (LitRubbish rep)
+  = mkForAllTy a Inferred (mkTyVarTy a)
   where
-    -- See Note [Rubbish values]
-    a = head $ mkTemplateTyVars [tYPE (primRepsToRuntimeRep preps)]
+    a = mkTemplateKindVar (tYPE rep)
 
 {-
         Comparison
@@ -840,7 +837,7 @@ cmpLit (LitDouble    a)     (LitDouble     b)     = a `compare` b
 cmpLit (LitLabel     a _ _) (LitLabel      b _ _) = a `lexicalCompareFS` b
 cmpLit (LitNumber nt1 a)    (LitNumber nt2  b)
   = (nt1 `compare` nt2) `mappend` (a `compare` b)
-cmpLit (LitRubbish b1)      (LitRubbish b2)       = b1 `compare` b2
+cmpLit (LitRubbish b1)      (LitRubbish b2)       = b1 `nonDetCmpType` b2
 cmpLit lit1 lit2
   | isTrue# (dataToTag# lit1 <# dataToTag# lit2) = LT
   | otherwise                                    = GT
@@ -876,8 +873,8 @@ pprLiteral add_par (LitLabel l mb fod) =
     where b = case mb of
               Nothing -> pprHsString l
               Just x  -> doubleQuotes (text (unpackFS l ++ '@':show x))
-pprLiteral _       (LitRubbish reps)
-  = text "RUBBISH" <> ppr reps
+pprLiteral _       (LitRubbish rep)
+  = text "RUBBISH" <> parens (ppr rep)
 
 pprIntegerVal :: (SDoc -> SDoc) -> Integer -> SDoc
 -- See Note [Printing of literals in Core].
@@ -921,75 +918,159 @@ LitInteger      -1                 (-1)
 LitLabel        "__label" ...      ("__label" ...)
 LitRubbish      "RUBBISH[...]"
 
-Note [Rubbish values]
-~~~~~~~~~~~~~~~~~~~~~
+Note [Rubbish literals]
+~~~~~~~~~~~~~~~~~~~~~~~
 Sometimes, we need to cough up a rubbish value of a certain type that is used
 in place of dead code we thus aim to eliminate. The value of a dead occurrence
 has no effect on the dynamic semantics of the program, so we can pick any value
 of the same representation.
+
 Exploiting the results of absence analysis in worker/wrapper is a scenario where
-we need such a rubbish value, see Note [Absent fillers] for examples.
+we need such a rubbish value, see examples in Note [Absent fillers] in
+GHC.Core.Opt.WorkWrap.Utils.
 
 It's completely undefined what the *value* of a rubbish value is, e.g., we could
 pick @0#@ for @Int#@ or @42#@; it mustn't matter where it's inserted into a Core
 program. We embed these rubbish values in the 'LitRubbish' case of the 'Literal'
 data type. Here are the moving parts:
 
-  1. Source Haskell: No way to produce rubbish lits in source syntax. Purely
-     an IR feature.
-
-  2. Core: 'LitRubbish' carries a @[PrimRep]@ which represents the monomorphic
-     'RuntimeRep' of the type it is substituting for.
-     We have it that @RUBBISH[IntRep]@ has type @forall (a :: TYPE IntRep). a@,
-     and the type application @RUBBISH[IntRep] \@Int# :: Int#@ represents
-     a rubbish value of type @Int#@. Rubbish lits are completely opaque in Core.
-     In general, @RUBBISH[preps] :: forall (a :: TYPE rep). a@, where @rep@
-     is the 'RuntimeRep' corresponding to @preps :: [PrimRep]@
-     (via 'primRepsToRuntimeRep'). See 'literalType'.
-     Why not encode a 'RuntimeRep' via a @Type@? Thus
-     > data Literal = ... | LitRubbish Type | ...
-     Because
-       * We have to provide an Eq and Ord instance and @Type@ has none
-       * The encoded @Type@ might be polymorphic and we can only emit code for
-         monomorphic 'RuntimeRep's anyway.
-
-  3. STG: The type app in @RUBBISH[IntRep] \@Int# :: Int#@ is erased and we get
-     the (untyped) 'StgLit' @RUBBISH[IntRep] :: Int#@ in STG.
-     It's treated mostly opaque, with the exception of the Unariser, where we
-     take apart a case scrutinisation on, or arg occurrence of, e.g.,
-     @RUBBISH[IntRep,DoubleRep]@ (which may stand in for @(# Int#, Double# #)@)
-     into its sub-parts @RUBBISH[IntRep]@ and @RUBBISH[DoubleRep]@, similar to
-     unboxed tuples. @RUBBISH[VoidRep]@ is erased.
-     See 'unariseRubbish_maybe' and also Note [Post-unarisation invariants].
-
-  4. Cmm: We translate 'LitRubbish' to their actual rubbish value in 'cgLit'.
-     The particulars are boring, and only matter when debugging illicit use of
-     a rubbish value; see Modes of failure below.
-
-  5. Bytecode: In GHC.ByteCode.Asm we just lower it as a 0 literal, because it's
-     all boxed to the host GC anyway.
-
-Why not lower LitRubbish in CoreToStg? Because it enables us to use RubbishLit
-when unarising unboxed sums in the future, and it allows rubbish values of e.g.
-VecRep, for which we can't cough up dummy values in STG.
+1. Source Haskell: No way to produce rubbish lits in source syntax. Purely
+   an IR feature.
+
+2. Core: 'LitRubbish' carries a `Type` of kind RuntimeRep,
+   describing the runtime representaion of the literal (is it a
+   pointer, an unboxed Double#, or whatever).
+
+   We have it that `RUBBISH[rr]` has type `forall (a :: TYPE rr). a`.
+   See the `LitRubbish` case of `literalType`.
+
+   The function GHC.Core.Make.mkLitRubbish makes a Core rubbish literal of
+   a given type.  It obeys the following invariants:
+
+   INVARIANT 1: 'rr' has no free variables. Main reason: we don't need to run
+   substitutions and free variable finders over Literal. The rules around
+   levity/runtime-rep polymorphism naturally uphold this invariant.
+
+   INVARIANT 2: we never make a rubbish literal of type (a ~# b). Reason:
+   see Note [Core type and coercion invariant] in GHC.Core.  We can't substitute
+   a LitRubbish inside a coercion, so it's best not to make one. They are zero
+   width anyway, so passing absent ones around costs nothing.  If we wanted
+   an absent filler of type (a ~# b) we should use (Coercion (UnivCo ...)),
+   but it doesn't seem worth making a new UnivCoProvenance for this purpose.
+
+   This is sad, though: see #18983.
+
+3. STG: The type app in `RUBBISH[IntRep] @Int# :: Int#` is erased and we get
+   the (untyped) 'StgLit' `RUBBISH[IntRep] :: Int#` in STG.
+
+   It's treated mostly opaque, with the exception of the Unariser, where we
+   take apart a case scrutinisation on, or arg occurrence of, e.g.,
+   `RUBBISH[TupleRep[IntRep,DoubleRep]]` (which may stand in for `(# Int#, Double# #)`)
+   into its sub-parts `RUBBISH[IntRep]` and `RUBBISH[DoubleRep]`, similar to
+   unboxed tuples. `RUBBISH[VoidRep]` is erased.
+   See 'unariseRubbish_maybe' and also Note [Post-unarisation invariants].
+
+4. Cmm: We translate 'LitRubbish' to their actual rubbish value in 'cgLit'.
+   The particulars are boring, and only matter when debugging illicit use of
+   a rubbish value; see Modes of failure below.
+
+5. Bytecode: In GHC.ByteCode.Asm we just lower it as a 0 literal, because it's
+   all boxed to the host GC anyway.
+
+6. IfaceSyn: `Literal` is part of `IfaceSyn`, but `Type` really isn't.  So in
+   the passage from Core to Iface I put LitRubbish into its owns IfaceExpr data
+   constructor, IfaceLitRubbish. The remaining constructors of Literal are
+   fine as IfaceSyn.
+
+Wrinkles
+
+a) Why do we put the `Type` (of kind RuntimeRep) inside the literal?  Could
+   we not instead /apply/ the literal to that RuntimeRep?  Alas no, becuase
+   then LitRubbish :: forall (rr::RuntimeRep) (a::TYPE rr). a
+   and that's am ill-formed type because its kind is `TYPE rr`, which escapes
+   the binding site of `rr`. Annoying.
+
+b) A rubbish literal is not bottom, and replies True to exprOkForSpeculation.
+   For unboxed types there is no bottom anyway.  If we have
+       let (x::Int#) = RUBBISH[IntRep] @Int#
+   we want to convert that to a case!  We want to leave it as a let, and
+   probably discard it as dead code soon after because x is unused.
+
+c) We can see a rubbish literal at the head of an application chain.
+   Most obviously, pretty much every rubbish literal is the head of a
+   type application e.g. `RUBBISH[IntRep] @Int#`.  But see also
+   Note [How a rubbish literal can be the head of an application]
+
+c) Literal is in Ord, because (and only because) we use Ord on AltCon when
+   building a TypeMap. Annoying.  We use `nonDetCmpType` here; the
+   non-determinism won't matter because it's only used in TrieMap.
+   Moreover, rubbish literals should not appear in patterns anyway.
+
+d) Why not lower LitRubbish in CoreToStg? Because it enables us to use
+   RubbishLit when unarising unboxed sums in the future, and it allows
+   rubbish values of e.g.  VecRep, for which we can't cough up dummy
+   values in STG.
 
 Modes of failure
 ----------------
 Suppose there is a bug in GHC, and a rubbish value is used after all. That is
 undefined behavior, of course, but let us list a few examples for failure modes:
 
- a) For an value of unboxed numeric type like @Int#@, we just use a silly
+ a) For an value of unboxed numeric type like `Int#`, we just use a silly
     value like 42#. The error might propoagate indefinitely, hence we better
     pick a rather unique literal. Same for Word, Floats, Char and VecRep.
  b) For AddrRep (like String lits), we mit a null pointer, resulting in a
     definitive segfault when accessed.
  c) For boxed values, unlifted or not, we use a pointer to a fixed closure,
-    like @()@, so that the GC has a pointer to follow.
+    like `()`, so that the GC has a pointer to follow.
     If we use that pointer as an 'Array#', we will likely access fields of the
     array that don't exist, and a seg-fault is likely, but not guaranteed.
-    If we use that pointer as @Either Int Bool@, we might try to access the
+    If we use that pointer as `Either Int Bool`, we might try to access the
     'Int' field of the 'Left' constructor (which has the same ConTag as '()'),
     which doesn't exists. In the best case, we'll find an invalid pointer in its
     position and get a seg-fault, in the worst case the error manifests only one
     or two indirections later.
- -}
+
+Note [How a rubbish literal can be the head of an application]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+Consider this (#19824):
+
+    h :: T3 -> Int -> blah
+    h _ (I# n) = ...
+
+    f :: (T1 -> T2 -> T3) -> T4 -> blah
+    f g x = ....(h (g n s) x)...
+
+Demand analysis finds that h does not use its first argument, and w/w's h to
+
+    {-# INLINE h #-}
+    h a b = case b of I# n -> $wh n
+
+Demand analysis also finds that f does not use its first arg,
+so the worker for f look like
+
+    $wf x = let g = RUBBISH in
+            ....(h (g n s) x)...
+
+Now we inline g to get:
+
+    $wf x = ....(h (RUBBISH n s) x)...
+
+And lo, until we inline `h`, we have that application of
+RUBBISH in $wf's RHS.  But surely `h` will inline? Not if the
+arguments look boring.  Well, RUBBISH doesn't look boring.  But it
+could be a bit more complicated like
+   f g x = let t = ...(g n s)...
+           in ...(h t x)...
+
+and now the call looks more boring.  Anyway, the point is that we
+might reasonably see RUBBISH at the head of an application chain.
+
+It would be fine to rewrite
+  RUBBISH @(ta->tb->tr) a b  --->   RUBBISH @tr
+but we don't currently do so.
+
+It is NOT ok to discard the entire continuation:
+  case RUBBISH @ty of DEFAULT -> blah
+does not return RUBBISH!
+-}
diff --git a/compiler/GHC/Types/RepType.hs b/compiler/GHC/Types/RepType.hs
index 8e4384c602..ec2c604e61 100644
--- a/compiler/GHC/Types/RepType.hs
+++ b/compiler/GHC/Types/RepType.hs
@@ -339,7 +339,7 @@ needed and how many bits are required. The data type GHC.Core.TyCon.PrimRep
 enumerates all the possibilities.
 
 data PrimRep
-  = VoidRep
+  = VoidRep       -- See Note [VoidRep]
   | LiftedRep     -- ^ Lifted pointer
   | UnliftedRep   -- ^ Unlifted pointer
   | Int8Rep       -- ^ Signed, 8-bit value
@@ -550,6 +550,7 @@ runtimeRepMonoPrimRep_maybe rr_ty
 
 -- | Take a type of kind RuntimeRep and extract the list of 'PrimRep' that
 -- it encodes. See also Note [Getting from RuntimeRep to PrimRep]
+-- The [PrimRep] is the final runtime representation /after/ unarisation
 runtimeRepPrimRep :: HasDebugCallStack => SDoc -> Type -> [PrimRep]
 runtimeRepPrimRep doc rr_ty
   | Just rr_ty' <- coreView rr_ty
diff --git a/testsuite/tests/stranal/should_compile/T18982.stderr b/testsuite/tests/stranal/should_compile/T18982.stderr
index 310eed5cc3..19f25be15c 100644
--- a/testsuite/tests/stranal/should_compile/T18982.stderr
+++ b/testsuite/tests/stranal/should_compile/T18982.stderr
@@ -1,6 +1,6 @@
 
 ==================== Tidy Core ====================
-Result size of Tidy Core = {terms: 315, types: 214, coercions: 2, joins: 0/0}
+Result size of Tidy Core = {terms: 311, types: 214, coercions: 4, joins: 0/0}
 
 -- RHS size: {terms: 8, types: 9, coercions: 1, joins: 0/0}
 T18982.$WExGADT :: forall e. (e ~ Int) => e %1 -> Int %1 -> ExGADT Int
@@ -210,21 +210,21 @@ T18982.$tc'ExGADT2 = GHC.Types.TrNameS T18982.$tc'ExGADT3
 T18982.$tc'ExGADT :: GHC.Types.TyCon
 T18982.$tc'ExGADT = GHC.Types.TyCon 8468257409157161049## 5503123603717080600## T18982.$trModule T18982.$tc'ExGADT2 1# T18982.$tc'ExGADT1
 
--- RHS size: {terms: 13, types: 15, coercions: 0, joins: 0/0}
-T18982.$wi :: forall {a} {e}. e -> GHC.Prim.Int# -> GHC.Prim.Int#
-T18982.$wi = \ (@a) (@e) (ww :: e) (ww1 :: GHC.Prim.Int#) -> case RUBBISH[] @(a GHC.Prim.~# Int) of ww2 { __DEFAULT -> case ww of { __DEFAULT -> GHC.Prim.+# ww1 1# } }
+-- RHS size: {terms: 11, types: 10, coercions: 0, joins: 0/0}
+T18982.$wi :: forall {a} {e}. (a GHC.Prim.~# Int) -> e -> GHC.Prim.Int# -> GHC.Prim.Int#
+T18982.$wi = \ (@a) (@e) (ww :: a GHC.Prim.~# Int) (ww1 :: e) (ww2 :: GHC.Prim.Int#) -> case ww1 of { __DEFAULT -> GHC.Prim.+# ww2 1# }
 
--- RHS size: {terms: 15, types: 22, coercions: 0, joins: 0/0}
+-- RHS size: {terms: 15, types: 22, coercions: 1, joins: 0/0}
 i :: forall a. ExGADT a -> Int
-i = \ (@a) (w :: ExGADT a) -> case w of { ExGADT @e ww1 ww2 ww3 ww4 -> case ww4 of { GHC.Types.I# ww6 -> case T18982.$wi @a @e ww3 ww6 of ww7 { __DEFAULT -> GHC.Types.I# ww7 } } }
+i = \ (@a) (w :: ExGADT a) -> case w of { ExGADT @e ww ww1 ww2 ww3 -> case ww3 of { GHC.Types.I# ww4 -> case T18982.$wi @a @e @~(ww :: a GHC.Prim.~# Int) ww2 ww4 of ww5 { __DEFAULT -> GHC.Types.I# ww5 } } }
 
--- RHS size: {terms: 8, types: 12, coercions: 0, joins: 0/0}
-T18982.$wh :: forall {a}. GHC.Prim.Int# -> GHC.Prim.Int#
-T18982.$wh = \ (@a) (ww :: GHC.Prim.Int#) -> case RUBBISH[] @(a GHC.Prim.~# Int) of ww1 { __DEFAULT -> GHC.Prim.+# ww 1# }
+-- RHS size: {terms: 6, types: 7, coercions: 0, joins: 0/0}
+T18982.$wh :: forall {a}. (a GHC.Prim.~# Int) -> GHC.Prim.Int# -> GHC.Prim.Int#
+T18982.$wh = \ (@a) (ww :: a GHC.Prim.~# Int) (ww1 :: GHC.Prim.Int#) -> GHC.Prim.+# ww1 1#
 
--- RHS size: {terms: 14, types: 15, coercions: 0, joins: 0/0}
+-- RHS size: {terms: 14, types: 15, coercions: 1, joins: 0/0}
 h :: forall a. GADT a -> Int
-h = \ (@a) (w :: GADT a) -> case w of { GADT ww1 ww2 -> case ww2 of { GHC.Types.I# ww4 -> case T18982.$wh @a ww4 of ww5 { __DEFAULT -> GHC.Types.I# ww5 } } }
+h = \ (@a) (w :: GADT a) -> case w of { GADT ww ww1 -> case ww1 of { GHC.Types.I# ww2 -> case T18982.$wh @a @~(ww :: a GHC.Prim.~# Int) ww2 of ww3 { __DEFAULT -> GHC.Types.I# ww3 } } }
 
 -- RHS size: {terms: 9, types: 4, coercions: 0, joins: 0/0}
 T18982.$wg :: forall {e}. e -> GHC.Prim.Int# -> GHC.Prim.Int#
@@ -232,7 +232,7 @@ T18982.$wg = \ (@e) (ww :: e) (ww1 :: GHC.Prim.Int#) -> case ww of { __DEFAULT -
 
 -- RHS size: {terms: 14, types: 11, coercions: 0, joins: 0/0}
 g :: Ex Int -> Int
-g = \ (w :: Ex Int) -> case w of { Ex @e ww1 ww2 -> case ww2 of { GHC.Types.I# ww4 -> case T18982.$wg @e ww1 ww4 of ww5 { __DEFAULT -> GHC.Types.I# ww5 } } }
+g = \ (w :: Ex Int) -> case w of { Ex @e ww ww1 -> case ww1 of { GHC.Types.I# ww2 -> case T18982.$wg @e ww ww2 of ww3 { __DEFAULT -> GHC.Types.I# ww3 } } }
 
 -- RHS size: {terms: 4, types: 1, coercions: 0, joins: 0/0}
 T18982.$wf :: GHC.Prim.Int# -> GHC.Prim.Int#
@@ -240,7 +240,7 @@ T18982.$wf = \ (ww :: GHC.Prim.Int#) -> GHC.Prim.+# ww 1#
 
 -- RHS size: {terms: 13, types: 8, coercions: 0, joins: 0/0}
 f :: Box Int -> Int
-f = \ (w :: Box Int) -> case w of { Box ww1 -> case ww1 of { GHC.Types.I# ww3 -> case T18982.$wf ww3 of ww4 { __DEFAULT -> GHC.Types.I# ww4 } } }
+f = \ (w :: Box Int) -> case w of { Box ww -> case ww of { GHC.Types.I# ww1 -> case T18982.$wf ww1 of ww2 { __DEFAULT -> GHC.Types.I# ww2 } } }
 
 
 
diff --git a/testsuite/tests/stranal/should_compile/T19882a.hs b/testsuite/tests/stranal/should_compile/T19882a.hs
new file mode 100644
index 0000000000..65d920dfc9
--- /dev/null
+++ b/testsuite/tests/stranal/should_compile/T19882a.hs
@@ -0,0 +1,10 @@
+{-# LANGUAGE UnboxedTuples, MagicHash #-}
+
+module T19882a where
+
+import GHC.Exts
+
+f1 :: (# State# RealWorld, Int, Int #) -> Bool -> Int
+f1 x True  = 1
+f1 x False = f1 x True
+
diff --git a/testsuite/tests/stranal/should_compile/T19882b.hs b/testsuite/tests/stranal/should_compile/T19882b.hs
new file mode 100644
index 0000000000..455bd016c1
--- /dev/null
+++ b/testsuite/tests/stranal/should_compile/T19882b.hs
@@ -0,0 +1,9 @@
+{-# LANGUAGE UnboxedTuples, MagicHash #-}
+
+module T19882b where
+
+import GHC.Exts
+
+f2 :: (# State# RealWorld, Int #) -> Bool -> Int
+f2 x True  = 1
+f2 x False = f2 x True
diff --git a/testsuite/tests/stranal/should_compile/all.T b/testsuite/tests/stranal/should_compile/all.T
index 61e9c58dee..f5ebbf289a 100644
--- a/testsuite/tests/stranal/should_compile/all.T
+++ b/testsuite/tests/stranal/should_compile/all.T
@@ -67,3 +67,5 @@ test('T18982',  [ grep_errmsg(r'\$w. .*Int#$') ], compile, ['-dppr-cols=1000 -dd
 
 test('T19180', normal, compile, [''])
 test('T19849', normal, compile, [''])
+test('T19882a', normal, compile, [''])
+test('T19882b', normal, compile, [''])