Split HsConDecl{H98,GADT}Detailswip/T18844

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, 4 insertions, 4 deletions

diff --git a/compiler/GHC/ThToHs.hs b/compiler/GHC/ThToHs.hs
index 2ff30da251..0130989940 100644
--- a/compiler/GHC/ThToHs.hs
+++ b/compiler/GHC/ThToHs.hs
@@ -622,7 +622,7 @@ cvtConstr (GadtC c strtys ty)
   = do  { c'      <- mapM cNameL c
         ; args    <- mapM cvt_arg strtys
         ; ty'     <- cvtType ty
-        ; returnL $ mk_gadt_decl c' (PrefixCon $ map hsLinear args) ty'}
+        ; returnL $ mk_gadt_decl c' (PrefixConGADT $ map hsLinear args) ty'}
 
 cvtConstr (RecGadtC [] _varstrtys _ty)
   = failWith (text "RecGadtC must have at least one constructor name")
@@ -631,9 +631,9 @@ cvtConstr (RecGadtC c varstrtys ty)
   = do  { c'       <- mapM cNameL c
         ; ty'      <- cvtType ty
         ; rec_flds <- mapM cvt_id_arg varstrtys
-        ; returnL $ mk_gadt_decl c' (RecCon $ noLoc rec_flds) ty' }
+        ; returnL $ mk_gadt_decl c' (RecConGADT $ noLoc rec_flds) ty' }
 
-mk_gadt_decl :: [Located RdrName] -> HsConDeclDetails GhcPs -> LHsType GhcPs
+mk_gadt_decl :: [Located RdrName] -> HsConDeclGADTDetails GhcPs -> LHsType GhcPs
              -> ConDecl GhcPs
 mk_gadt_decl names args res_ty
   = ConDeclGADT { con_g_ext  = noExtField
@@ -641,7 +641,7 @@ mk_gadt_decl names args res_ty
                 , con_forall = noLoc False
                 , con_qvars  = []
                 , con_mb_cxt = Nothing
-                , con_args   = args
+                , con_g_args = args
                 , con_res_ty = res_ty
                 , con_doc    = Nothing }