Introduce and use ConGadtSigBody (preparatory refactor for #18389)wip/T18389-task-zero

This patch removes the `con_g_args :: HsConDeclGADTDetails pass` and
`con_res_ty :: LHsType pass` fields of `ConDeclGADT` in favor of a unified
`con_body :: ConGadtSigBody pass` field. There are two major differences
between `HsConDeclGADTDetails` and `ConGadtSigBody`:

1. `HsConDeclGADTDetails` only contains the argument type, while
   `ConGadtSigBody` contains both the argument and result types.
2. The `PrefixConGADT` constructor of `ConGadtSigBody` now uses a new
   `PrefixConGadtSigBody` data type. `PrefixConGadtSigBody` closely mirrors the
   structure of `HsType`, but with minor, data constructor–specific tweaks.
   This will become vital in a future patch which implements nested `forall`s
   and contexts in prefix GADT constructor types (see #18389).

Besides the refactoring in the GHC API (and some minor changes in
GHC AST–related test cases) this does not introduce any user-visible
changes in behavior.

1 files changed, 17 insertions, 11 deletions

diff --git a/compiler/GHC/Parser/PostProcess/Haddock.hs b/compiler/GHC/Parser/PostProcess/Haddock.hs
index 271d9db30f..2ce0dd5274 100644
--- a/compiler/GHC/Parser/PostProcess/Haddock.hs
+++ b/compiler/GHC/Parser/PostProcess/Haddock.hs
@@ -696,22 +696,14 @@ instance HasHaddock (LocatedA (ConDecl GhcPs)) where
   addHaddock (L l_con_decl con_decl) =
     extendHdkA (locA l_con_decl) $
     case con_decl of
-      ConDeclGADT { con_g_ext, con_names, con_bndrs, con_mb_cxt, con_g_args, con_res_ty } -> do
+      ConDeclGADT { con_g_ext, con_names, con_bndrs, con_mb_cxt, con_body } -> do
         -- discardHasInnerDocs is ok because we don't need this info for GADTs.
         con_doc' <- discardHasInnerDocs $ getConDoc (getLocA (head con_names))
-        con_g_args' <-
-          case con_g_args of
-            PrefixConGADT ts -> PrefixConGADT <$> addHaddock ts
-            RecConGADT (L l_rec flds) arr -> do
-              -- discardHasInnerDocs is ok because we don't need this info for GADTs.
-              flds' <- traverse (discardHasInnerDocs . addHaddockConDeclField) flds
-              pure $ RecConGADT (L l_rec flds') arr
-        con_res_ty' <- addHaddock con_res_ty
+        con_body' <- addHaddock con_body
         pure $ L l_con_decl $
           ConDeclGADT { con_g_ext, con_names, con_bndrs, con_mb_cxt,
                         con_doc = lexLHsDocString <$> con_doc',
-                        con_g_args = con_g_args',
-                        con_res_ty = con_res_ty' }
+                        con_body = con_body' }
       ConDeclH98 { con_ext, con_name, con_forall, con_ex_tvs, con_mb_cxt, con_args } ->
         addConTrailingDoc (srcSpanEnd $ locA l_con_decl) $
         case con_args of
@@ -738,6 +730,20 @@ instance HasHaddock (LocatedA (ConDecl GhcPs)) where
                            con_doc = lexLHsDocString <$> con_doc',
                            con_args = RecCon (L l_rec flds') }
 
+instance HasHaddock (ConGadtSigBody GhcPs) where
+  addHaddock (PrefixConGADT body) = PrefixConGADT <$> addHaddock body
+  addHaddock (RecConGADT (L l_rec flds) arr res_ty) = do
+    -- discardHasInnerDocs is ok because we don't need this info for GADTs.
+    flds'   <- traverse (discardHasInnerDocs . addHaddockConDeclField) flds
+    res_ty' <- addHaddock res_ty
+    pure $ RecConGADT (L l_rec flds') arr res_ty'
+
+instance HasHaddock (PrefixConGadtSigBody GhcPs) where
+  addHaddock (PCGSAnonArg arg_ty body) =
+    PCGSAnonArg <$> addHaddock arg_ty <*> addHaddock body
+  addHaddock (PCGSRes res_ty) =
+    PCGSRes <$> addHaddock res_ty
+
 -- Keep track of documentation comments on the data constructor or any of its
 -- fields.
 --