Use HsTyPats in associated type family defaults

Associated type family default declarations behave strangely in a
couple of ways:

1. If one tries to bind the type variables with an explicit `forall`,
   the `forall`'d part will simply be ignored. (#16110)
2. One cannot use visible kind application syntax on the left-hand
   sides of associated default equations, unlike every other form
   of type family equation. (#16356)

Both of these issues have a common solution. Instead of using
`LHsQTyVars` to represent the left-hand side arguments of an
associated default equation, we instead use `HsTyPats`, which is what
other forms of type family equations use. In particular, here are
some highlights of this patch:

* `FamEqn` is no longer parameterized by a `pats` type variable, as
  the `feqn_pats` field is now always `HsTyPats`.
* The new design for `FamEqn` in chronicled in
  `Note [Type family instance declarations in HsSyn]`.
* `TyFamDefltEqn` now becomes the same thing as `TyFamInstEqn`. This
  means that many of `TyFamDefltEqn`'s code paths can now reuse the
  code paths for `TyFamInstEqn`, resulting in substantial
  simplifications to various parts of the code dealing with
  associated type family defaults.

Fixes #16110 and #16356.

1 files changed, 6 insertions, 15 deletions

diff --git a/compiler/hieFile/HieAst.hs b/compiler/hieFile/HieAst.hs
index d86077ea27..84e5a627d8 100644
--- a/compiler/hieFile/HieAst.hs
+++ b/compiler/hieFile/HieAst.hs
@@ -333,7 +333,7 @@ instance HasLoc a => HasLoc [a] where
   loc [] = noSrcSpan
   loc xs = foldl1' combineSrcSpans $ map loc xs
 
-instance (HasLoc a, HasLoc b) => HasLoc (FamEqn s a b) where
+instance HasLoc a => HasLoc (FamEqn s a) where
   loc (FamEqn _ a Nothing b _ c) = foldl1' combineSrcSpans [loc a, loc b, loc c]
   loc (FamEqn _ a (Just tvs) b _ c) = foldl1' combineSrcSpans
                                               [loc a, loc tvs, loc b, loc c]
@@ -1149,18 +1149,12 @@ instance ToHie (LTyClDecl GhcRn) where
         , toHie $ fmap (BC InstanceBind ModuleScope) meths
         , toHie typs
         , concatMapM (pure . locOnly . getLoc) deftyps
-        , toHie $ map (go . unLoc) deftyps
+        , toHie deftyps
         ]
         where
           context_scope = mkLScope context
           rhs_scope = foldl1' combineScopes $ map mkScope
             [ loc deps, loc sigs, loc (bagToList meths), loc typs, loc deftyps]
-
-          go :: TyFamDefltEqn GhcRn
-             -> FamEqn GhcRn (TScoped (LHsQTyVars GhcRn)) (LHsType GhcRn)
-          go (FamEqn a var bndrs pat b rhs) =
-             FamEqn a var bndrs (TS (ResolvedScopes [mkLScope rhs]) pat) b rhs
-          go (XFamEqn NoExt) = XFamEqn NoExt
       XTyClDecl _ -> []
 
 instance ToHie (LFamilyDecl GhcRn) where
@@ -1206,15 +1200,12 @@ instance ToHie (Located (FunDep (Located Name))) where
     , toHie $ map (C Use) rhs
     ]
 
-instance (ToHie pats, ToHie rhs, HasLoc pats, HasLoc rhs)
-    => ToHie (TScoped (FamEqn GhcRn pats rhs)) where
+instance (ToHie rhs, HasLoc rhs)
+    => ToHie (TScoped (FamEqn GhcRn rhs)) where
   toHie (TS _ f) = toHie f
 
-instance ( ToHie pats
-         , ToHie rhs
-         , HasLoc pats
-         , HasLoc rhs
-         ) => ToHie (FamEqn GhcRn pats rhs) where
+instance (ToHie rhs, HasLoc rhs)
+    => ToHie (FamEqn GhcRn rhs) where
   toHie fe@(FamEqn _ var tybndrs pats _ rhs) = concatM $
     [ toHie $ C (Decl InstDec $ getRealSpan $ loc fe) var
     , toHie $ fmap (tvScopes (ResolvedScopes []) scope) tybndrs