Implement the -XUnliftedNewtypes extension.

GHC Proposal: 0013-unlifted-newtypes.rst
Discussion: https://github.com/ghc-proposals/ghc-proposals/pull/98
Issues: #15219, #1311, #13595, #15883
Implementation Details:
  Note [Implementation of UnliftedNewtypes]
  Note [Unifying data family kinds]
  Note [Compulsory newtype unfolding]

This patch introduces the -XUnliftedNewtypes extension. When this
extension is enabled, GHC drops the restriction that the field in
a newtype must be of kind (TYPE 'LiftedRep). This allows types
like Int# and ByteArray# to be used in a newtype. Additionally,
coerce is made levity-polymorphic so that it can be used with
newtypes over unlifted types.

The bulk of the changes are in TcTyClsDecls.hs. With -XUnliftedNewtypes,
getInitialKind is more liberal, introducing a unification variable to
return the kind (TYPE r0) rather than just returning (TYPE 'LiftedRep).
When kind-checking a data constructor with kcConDecl, we attempt to
unify the kind of a newtype with the kind of its field's type. When
typechecking a data declaration with tcTyClDecl, we again perform a
unification. See the implementation note for more on this.

Co-authored-by: Richard Eisenberg <rae@richarde.dev>

1 files changed, 1 insertions, 1 deletions

diff --git a/testsuite/tests/typecheck/should_fail/tcfail004.stderr b/testsuite/tests/typecheck/should_fail/tcfail004.stderr
index 7bf64d841a..9d6657e651 100644
--- a/testsuite/tests/typecheck/should_fail/tcfail004.stderr
+++ b/testsuite/tests/typecheck/should_fail/tcfail004.stderr
@@ -1,7 +1,7 @@
 
 tcfail004.hs:3:9: error:
     • Couldn't match expected type ‘(a, b)’
-                  with actual type ‘(Integer, Integer, Integer)’
+                  with actual type ‘(a0, b0, c0)’
     • In the expression: (1, 2, 3)
       In a pattern binding: (f, g) = (1, 2, 3)
     • Relevant bindings include