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, 15 insertions, 0 deletions

diff --git a/docs/users_guide/9.8.1-notes.rst b/docs/users_guide/9.8.1-notes.rst
index 0f0848765f..9f2d51d329 100644
--- a/docs/users_guide/9.8.1-notes.rst
+++ b/docs/users_guide/9.8.1-notes.rst
@@ -93,6 +93,21 @@ Compiler
   with each other, communicating via the system semaphore specified by
   the flag argument.
 
+- GHC Proposal `#433
+  <https://github.com/ghc-proposals/ghc-proposals/blob/master/proposals/0433-unsatisfiable.rst>`_
+  has been implemented. This adds the class ``Unsatisfiable :: ErrorMessage -> Constraint``
+  to the ``GHC.TypeError`` module. Constraints of the form ``Unsatisfiable msg``
+  provide a mechanism for custom type errors that reports the errors in a more
+  predictable behaviour than ``TypeError``, as these constraints are
+  handled purely during constraint solving.
+
+  For example: ::
+
+      instance Unsatisfiable (Text "There is no Eq instance for functions") => Eq (a -> b) where
+        (==) = unsatisfiable
+
+  This allows errors to be reported when users use the instance, even when
+  type errors are being deferred.
 
 GHCi
 ~~~~