Split HsConDecl{H98,GADT}Details

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, 41 insertions, 3 deletions

diff --git a/docs/users_guide/9.2.1-notes.rst b/docs/users_guide/9.2.1-notes.rst
index b691fc0537..aa495444db 100644
--- a/docs/users_guide/9.2.1-notes.rst
+++ b/docs/users_guide/9.2.1-notes.rst
@@ -43,14 +43,52 @@ Compiler
 - ``Void#`` is now a type synonym for the unboxed tuple ``(# #)``.
   Code using ``Void#`` now has to enable :extension:`UnboxedTuples`.
 
+``ghc`` library
+~~~~~~~~~~~~~
+
+- The ``con_args`` field of ``ConDeclGADT`` has been renamed to ``con_g_args``.
+  This is because the type of ``con_g_args`` is now different from the type of
+  the ``con_args`` field in ``ConDeclH98``: ::
+
+    data ConDecl pass
+      = ConDeclGADT
+          { ...
+          , con_g_args :: HsConDeclGADTDetails pass -- ^ Arguments; never infix
+          , ...
+          }
+
+      | ConDeclH98
+          { ...
+          , con_args :: HsConDeclH98Details pass -- ^ Arguments; can be infix
+          , ...
+          }
+
+  Where: ::
+
+    -- Introduced in GHC 9.2; was called `HsConDeclDetails` in previous versions of GHC
+    type HsConDeclH98Details pass
+       = HsConDetails (HsScaled pass (LBangType pass)) (XRec pass [LConDeclField pass])
+
+    -- Introduced in GHC 9.2
+    data HsConDeclGADTDetails pass
+       = PrefixConGADT [HsScaled pass (LBangType pass)]
+       | RecConGADT (XRec pass [LConDeclField pass])
+
+  Unlike Haskell98-style constructors, GADT constructors cannot be declared
+  using infix syntax, which is why ``HsConDeclGADTDetails`` lacks an
+  ``InfixConGADT`` constructor.
+
+  As a result of all this, the ``con_args`` field is now partial, so using
+  ``con_args`` as a top-level field selector is discouraged.
+
 ``base`` library
 ~~~~~~~~~~~~~~~~
 
-- It's possible now to promote the ``Natural`` type: :: 
-    
+- It's possible now to promote the ``Natural`` type: ::
+
     data Coordinate = Mk2D Natural Natural
     type MyCoordinate = Mk2D 1 10
-    
+
   The separate kind ``Nat`` is removed and now it is just a type synonym for
   ``Natural``. As a consequence, one must enable ``TypeSynonymInstances``
   in order to define instances for ``Nat``.