Use HsOuterExplicit in instance sigs in deriving-generated code

Issue #18914 revealed that `GeneralizedNewtypeDeriving` would generate code
that mentions unbound type variables, which is dangerously fragile. The
problem (and fix) is described in the new `Wrinkle: Use HsOuterExplicit`
in `Note [GND and QuantifiedConstraints]`. The gist of it: make sure to
put the top-level `forall`s in `deriving`-generated instance signatures in an
`HsOuterExplicit` to ensure that they scope over the bodies of methods
correctly. A side effect of this process is that it will expand any type
synonyms in the instance signature, which will surface any `forall`s that
are hidden underneath type synonyms (such as in the test case for #18914).

While I was in town, I also performed some maintenance on `NewHsTypeX`, which
powers `GeneralizedNewtypeDeriving`:

* I renamed `NewHsTypeX` to `HsCoreTy`, which more accurately describes its
  intended purpose (#15706). I also made `HsCoreTy` a type synonym instead of
  a newtype, as making it a distinct data type wasn't buying us much.
* To make sure that mistakes similar to #18914 do not occur later, I added an
  additional validity check when renaming `HsCoreTy`s that complains if an
  `HsCoreTy`s contains an out-of-scope type variable. See the new
  `Note [Renaming HsCoreTys]` in `GHC.Rename.HsType` for the details.

Fixes #15706. Fixes #18914. Bumps the `haddock` submodule.

1 files changed, 10 insertions, 19 deletions

diff --git a/compiler/GHC/Hs/Type.hs b/compiler/GHC/Hs/Type.hs
index dde27857ec..1b1e56b314 100644
--- a/compiler/GHC/Hs/Type.hs
+++ b/compiler/GHC/Hs/Type.hs
@@ -24,7 +24,7 @@ module GHC.Hs.Type (
         HsArrow(..), arrowToHsType,
         hsLinear, hsUnrestricted, isUnrestricted,
 
-        HsType(..), NewHsTypeX(..), LHsType, HsKind, LHsKind,
+        HsType(..), HsCoreTy, LHsType, HsKind, LHsKind,
         HsForAllTelescope(..), HsTyVarBndr(..), LHsTyVarBndr,
         LHsQTyVars(..),
         HsOuterTyVarBndrs(..), HsOuterFamEqnTyVarBndrs, HsOuterSigTyVarBndrs,
@@ -1040,12 +1040,6 @@ data HsType pass
 
       -- For details on above see note [Api annotations] in GHC.Parser.Annotation
 
-  -- | HsCoreTy (XCoreTy pass) Type -- An escape hatch for tunnelling a *closed*
-  --                                -- Core Type through HsSyn.
-  --     -- ^ - 'GHC.Parser.Annotation.AnnKeywordId' : None
-
-      -- For details on above see note [Api annotations] in GHC.Parser.Annotation
-
   | HsExplicitListTy       -- A promoted explicit list
         (XExplicitListTy pass)
         PromotionFlag      -- whether explicitly promoted, for pretty printer
@@ -1078,16 +1072,13 @@ data HsType pass
   | XHsType
       (XXType pass)
 
-data NewHsTypeX
-  = NHsCoreTy Type -- An escape hatch for tunnelling a *closed*
-                   -- Core Type through HsSyn.
-                   -- See also Note [Typechecking NHsCoreTys] in
-                   -- GHC.Tc.Gen.HsType.
-    deriving Data
-      -- ^ - 'GHC.Parser.Annotation.AnnKeywordId' : None
-
-instance Outputable NewHsTypeX where
-  ppr (NHsCoreTy ty) = ppr ty
+-- An escape hatch for tunnelling a Core 'Type' through 'HsType'.
+-- For more details on how this works, see:
+--
+-- * @Note [Renaming HsCoreTys]@ in "GHC.Rename.HsType"
+--
+-- * @Note [Typechecking HsCoreTys]@ in "GHC.Tc.Gen.HsType"
+type HsCoreTy = Type
 
 type instance XForAllTy        (GhcPass _) = NoExtField
 type instance XQualTy          (GhcPass _) = NoExtField
@@ -1125,7 +1116,7 @@ type instance XTyLit           (GhcPass _) = NoExtField
 
 type instance XWildCardTy      (GhcPass _) = NoExtField
 
-type instance XXType         (GhcPass _) = NewHsTypeX
+type instance XXType         (GhcPass _) = HsCoreTy
 
 
 -- Note [Literal source text] in GHC.Types.Basic for SourceText fields in
@@ -2256,7 +2247,7 @@ hsTypeNeedsParens p = go_hs_ty
     go_hs_ty (HsOpTy{})               = p >= opPrec
     go_hs_ty (HsParTy{})              = False
     go_hs_ty (HsDocTy _ (L _ t) _)    = go_hs_ty t
-    go_hs_ty (XHsType (NHsCoreTy ty)) = go_core_ty ty
+    go_hs_ty (XHsType ty)             = go_core_ty ty
 
     go_core_ty (TyVarTy{})    = False
     go_core_ty (AppTy{})      = p >= appPrec