Split HsConDecl{H98,GADT}Detailswip/T18844-alternate

Haskell98 and GADT constructors both use `HsConDeclDetails`, which includes
`InfixCon`. But `InfixCon` is never used for GADT constructors, which results
in an awkward unrepresentable state. This removes the unrepresentable state by:

* Renaming the existing `HsConDeclDetails` synonym to `HsConDeclH98Details`,
  which emphasizes the fact that it is now only used for Haskell98-style data
  constructors, and
* Creating a new `HsConDeclGADTDetails` data type with `PrefixConGADT` and
  `RecConGADT` constructors that closely resemble `PrefixCon` and `InfixCon`
  in `HsConDeclH98Details`. The key difference is that `HsConDeclGADTDetails`
  lacks any way to represent infix constructors.

The rest of the patch is refactoring to accommodate the new structure of
`HsConDecl{H98,GADT}Details`. Some highlights:

* The `getConArgs` and `hsConDeclArgTys` functions have been removed, as
  there is no way to implement these functions uniformly for all
  `ConDecl`s. For the most part, their previous call sites now
  pattern match on the `ConDecl`s directly and do different things for
  `ConDeclH98`s and `ConDeclGADT`s.

  I did introduce one new function to make the transition easier:
  `getRecConArgs_maybe`, which extracts the arguments from a `RecCon(GADT)`.
  This is still possible since `RecCon(GADT)`s still use the same representation
  in both `HsConDeclH98Details` and `HsConDeclGADTDetails`, and since the
  pattern that `getRecConArgs_maybe` implements is used in several places,
  I thought it worthwhile to factor it out into its own function.
* Previously, the `con_args` fields in `ConDeclH98` and `ConDeclGADT` were
  both of type `HsConDeclDetails`. Now, the former is of type
  `HsConDeclH98Details`, and the latter is of type `HsConDeclGADTDetails`,
  which are distinct types. As a result, I had to rename the `con_args` field
  in `ConDeclGADT` to `con_g_args` to make it typecheck.

  A consequence of all this is that the `con_args` field is now partial, so
  using `con_args` as a top-level field selector is dangerous. (Indeed, Haddock
  was using `con_args` at the top-level, which caused it to crash at runtime
  before I noticed what was wrong!) I decided to add a disclaimer in the 9.2.1
  release notes to advertise this pitfall.

Fixes #18844. Bumps the `haddock` submodule.

1 files changed, 5 insertions, 5 deletions

diff --git a/compiler/GHC/Parser/PostProcess.hs b/compiler/GHC/Parser/PostProcess.hs
index 4812486d19..a9cc67ad6c 100644
--- a/compiler/GHC/Parser/PostProcess.hs
+++ b/compiler/GHC/Parser/PostProcess.hs
@@ -608,7 +608,7 @@ recordPatSynErr loc pat =
     addFatalError $ Error (ErrRecordSyntaxInPatSynDecl pat) [] loc
 
 mkConDeclH98 :: Located RdrName -> Maybe [LHsTyVarBndr Specificity GhcPs]
-                -> Maybe (LHsContext GhcPs) -> HsConDeclDetails GhcPs
+                -> Maybe (LHsContext GhcPs) -> HsConDeclH98Details GhcPs
                 -> ConDecl GhcPs
 
 mkConDeclH98 name mb_forall mb_cxt args
@@ -633,17 +633,17 @@ mkGadtDecl :: [Located RdrName]
 mkGadtDecl names ty = do
   let (args, res_ty, anns)
         | L _ (HsFunTy _ _w (L loc (HsRecTy _ rf)) res_ty) <- body_ty
-        = (RecCon (L loc rf), res_ty, [])
+        = (RecConGADT (L loc rf), res_ty, [])
         | otherwise
         = let (arg_types, res_type, anns) = splitHsFunType body_ty
-          in (PrefixCon arg_types, res_type, anns)
+          in (PrefixConGADT arg_types, res_type, anns)
 
   pure ( ConDeclGADT { con_g_ext  = noExtField
                      , con_names  = names
                      , con_forall = L (getLoc ty) $ isJust mtvs
                      , con_qvars  = fromMaybe [] mtvs
                      , con_mb_cxt = mcxt
-                     , con_args   = args
+                     , con_g_args = args
                      , con_res_ty = res_ty
                      , con_doc    = Nothing }
        , anns )
@@ -1615,7 +1615,7 @@ dataConBuilderCon :: DataConBuilder -> Located RdrName
 dataConBuilderCon (PrefixDataConBuilder _ dc) = dc
 dataConBuilderCon (InfixDataConBuilder _ dc _) = dc
 
-dataConBuilderDetails :: DataConBuilder -> HsConDeclDetails GhcPs
+dataConBuilderDetails :: DataConBuilder -> HsConDeclH98Details GhcPs
 
 -- Detect when the record syntax is used:
 --   data T = MkT { ... }