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, 5 insertions, 0 deletions

diff --git a/testsuite/tests/typecheck/should_fail/UnliftedNewtypesMultiFieldGadt.stderr b/testsuite/tests/typecheck/should_fail/UnliftedNewtypesMultiFieldGadt.stderr
new file mode 100644
index 0000000000..70493e0d96
--- /dev/null
+++ b/testsuite/tests/typecheck/should_fail/UnliftedNewtypesMultiFieldGadt.stderr
@@ -0,0 +1,5 @@
+UnliftedNewtypesMultiFieldGadt.hs:19:3:
+     The constructor of a newtype must have exactly one field
+        but ‘FooC’ has two
+     In the definition of data constructor ‘FooC’
+      In the newtype declaration for ‘Foo’