Add the Unsatisfiable class

This commit implements GHC proposal #433, adding the Unsatisfiable
class to the GHC.TypeError module. This provides an alternative to
TypeError for which error reporting is more predictable: we report it
when we are reporting unsolved Wanted constraints.

Fixes #14983 #16249 #16906 #18310 #20835

1 files changed, 7 insertions, 4 deletions

diff --git a/compiler/GHC/Core/Type.hs b/compiler/GHC/Core/Type.hs
index 40fa1ea2df..86f483abca 100644
--- a/compiler/GHC/Core/Type.hs
+++ b/compiler/GHC/Core/Type.hs
@@ -76,6 +76,7 @@ module GHC.Core.Type (
 
         mkCastTy, mkCoercionTy, splitCastTy_maybe,
 
+        ErrorMsgType,
         userTypeError_maybe, pprUserTypeErrorTy,
 
         coAxNthLHS,
@@ -1229,9 +1230,12 @@ isLitTy ty
   | LitTy l <- coreFullView ty = Just l
   | otherwise                  = Nothing
 
+-- | A type of kind 'ErrorMessage' (from the 'GHC.TypeError' module).
+type ErrorMsgType = Type
+
 -- | Is this type a custom user error?
--- If so, give us the kind and the error message.
-userTypeError_maybe :: Type -> Maybe Type
+-- If so, give us the error message.
+userTypeError_maybe :: Type -> Maybe ErrorMsgType
 userTypeError_maybe t
   = do { (tc, _kind : msg : _) <- splitTyConApp_maybe t
           -- There may be more than 2 arguments, if the type error is
@@ -1241,7 +1245,7 @@ userTypeError_maybe t
        ; return msg }
 
 -- | Render a type corresponding to a user type error into a SDoc.
-pprUserTypeErrorTy :: Type -> SDoc
+pprUserTypeErrorTy :: ErrorMsgType -> SDoc
 pprUserTypeErrorTy ty =
   case splitTyConApp_maybe ty of
 
@@ -1267,7 +1271,6 @@ pprUserTypeErrorTy ty =
     -- An unevaluated type function
     _ -> ppr ty
 
-
 {- *********************************************************************
 *                                                                      *
                       FunTy