Don't generate datacon wrappers for `type data` declarations

Data constructor wrappers only make sense for _value_-level data constructors,
but data constructors for `type data` declarations only exist at the _type_
level. This patch does the following:

* The criteria in `GHC.Types.Id.Make.mkDataConRep` for whether a data
  constructor receives a wrapper now consider whether or not its parent data
  type was declared with `type data`, omitting a wrapper if this is the case.

* Now that `type data` data constructors no longer receive wrappers, there is a
  spot of code in `refineDefaultAlt` that panics when it encounters a value
  headed by a `type data` type constructor. I've fixed this with a special case
  in `refineDefaultAlt` and expanded `Note [Refine DEFAULT case alternatives]`
  to explain why we do this.

Fixes #22948.

1 files changed, 35 insertions, 3 deletions

diff --git a/compiler/GHC/Core/Utils.hs b/compiler/GHC/Core/Utils.hs
index d36f0a14f2..89824889ef 100644
--- a/compiler/GHC/Core/Utils.hs
+++ b/compiler/GHC/Core/Utils.hs
@@ -728,9 +728,8 @@ refineDefaultAlt :: [Unique]          -- ^ Uniques for constructing new binders
 refineDefaultAlt us mult tycon tys imposs_deflt_cons all_alts
   | Alt DEFAULT _ rhs : rest_alts <- all_alts
   , isAlgTyCon tycon            -- It's a data type, tuple, or unboxed tuples.
-  , not (isNewTyCon tycon)      -- We can have a newtype, if we are just doing an eval:
-                                --      case x of { DEFAULT -> e }
-                                -- and we don't want to fill in a default for them!
+  , not (isNewTyCon tycon)      -- Exception 1 in Note [Refine DEFAULT case alternatives]
+  , not (isTypeDataTyCon tycon) -- Exception 2 in Note [Refine DEFAULT case alternatives]
   , Just all_cons <- tyConDataCons_maybe tycon
   , let imposs_data_cons = mkUniqSet [con | DataAlt con <- imposs_deflt_cons]
                              -- We now know it's a data type, so we can use
@@ -815,6 +814,39 @@ with a specific constructor is desirable.
    `imposs_deflt_cons` argument is populated with constructors which
    are matched elsewhere.
 
+There are two exceptions where we avoid refining a DEFAULT case:
+
+* Exception 1: Newtypes
+
+  We can have a newtype, if we are just doing an eval:
+
+    case x of { DEFAULT -> e }
+
+  And we don't want to fill in a default for them!
+
+* Exception 2: `type data` declarations
+
+  The data constructors for a `type data` declaration (see
+  Note [Type data declarations] in GHC.Rename.Module) do not exist at the
+  value level. Nevertheless, it is possible to strictly evaluate a value
+  whose type is a `type data` declaration. Test case
+  type-data/should_compile/T2294b.hs contains an example:
+
+    type data T a where
+      A :: T Int
+
+    f :: T a -> ()
+    f !x = ()
+
+  We want to generate the following Core for f:
+
+    f = \(@a) (x :: T a) ->
+         case x of
+           __DEFAULT -> ()
+
+  Namely, we do _not_ want to match on `A`, as it doesn't exist at the value
+  level!
+
 Note [Combine identical alternatives]
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 If several alternatives are identical, merge them into a single