Guard CUSKs behind a language pragma

GHC Proposal #36 describes a transition plan away from CUSKs and to
top-level kind signatures:

1. Introduce a new extension, -XCUSKs, on by default, that detects CUSKs
   as they currently exist.
2. We turn off the -XCUSKs extension in a few releases and remove it
   sometime thereafter.

This patch implements phase 1 of this plan, introducing a new language
extension to control whether CUSKs are enabled. When top-level kind
signatures are implemented, we can transition to phase 2.

10 files changed, 84 insertions, 29 deletions

diff --git a/compiler/hsSyn/HsDecls.hs b/compiler/hsSyn/HsDecls.hs
index c194c2e21a..e328bf43c7 100644
--- a/compiler/hsSyn/HsDecls.hs
+++ b/compiler/hsSyn/HsDecls.hs
@@ -679,11 +679,15 @@ countTyClDecls decls
 
 -- | Does this declaration have a complete, user-supplied kind signature?
 -- See Note [CUSKs: complete user-supplied kind signatures]
-hsDeclHasCusk :: TyClDecl GhcRn -> Bool
-hsDeclHasCusk (FamDecl { tcdFam = fam_decl })
-  = famDeclHasCusk False fam_decl
+hsDeclHasCusk
+  :: Bool  -- True <=> the -XCUSKs extension is enabled
+  -> TyClDecl GhcRn
+  -> Bool
+hsDeclHasCusk _cusks_enabled@False _ = False
+hsDeclHasCusk cusks_enabled (FamDecl { tcdFam = fam_decl })
+  = famDeclHasCusk cusks_enabled False fam_decl
     -- False: this is not: an associated type of a class with no cusk
-hsDeclHasCusk (SynDecl { tcdTyVars = tyvars, tcdRhs = rhs })
+hsDeclHasCusk _cusks_enabled@True (SynDecl { tcdTyVars = tyvars, tcdRhs = rhs })
   -- NB: Keep this synchronized with 'getInitialKind'
   = hsTvbAllKinded tyvars && rhs_annotated rhs
   where
@@ -691,9 +695,9 @@ hsDeclHasCusk (SynDecl { tcdTyVars = tyvars, tcdRhs = rhs })
       HsParTy _ lty  -> rhs_annotated lty
       HsKindSig {}   -> True
       _              -> False
-hsDeclHasCusk (DataDecl { tcdDExt = DataDeclRn { tcdDataCusk = cusk }}) = cusk
-hsDeclHasCusk (ClassDecl { tcdTyVars = tyvars }) = hsTvbAllKinded tyvars
-hsDeclHasCusk (XTyClDecl _) = panic "hsDeclHasCusk"
+hsDeclHasCusk _cusks_enabled@True (DataDecl { tcdDExt = DataDeclRn { tcdDataCusk = cusk }}) = cusk
+hsDeclHasCusk _cusks_enabled@True (ClassDecl { tcdTyVars = tyvars }) = hsTvbAllKinded tyvars
+hsDeclHasCusk _ (XTyClDecl _) = panic "hsDeclHasCusk"
 
 -- Pretty-printing TyClDecl
 -- ~~~~~~~~~~~~~~~~~~~~~~~~
@@ -787,6 +791,10 @@ declaration before checking all of the others, supporting polymorphic recursion.
 See https://gitlab.haskell.org/ghc/ghc/wikis/ghc-kinds/kind-inference#proposed-new-strategy
 and #9200 for lots of discussion of how we got here.
 
+The detection of CUSKs is enabled by the -XCUSKs extension, switched on by default.
+Under -XNoCUSKs, all declarations are treated as if they have no CUSK.
+See https://github.com/ghc-proposals/ghc-proposals/blob/master/proposals/0036-kind-signatures.rst
+
 PRINCIPLE:
   a type declaration has a CUSK iff we could produce a separate kind signature
   for it, just like a type signature for a function,
@@ -1080,11 +1088,13 @@ data FamilyInfo pass
 
 -- | Does this family declaration have a complete, user-supplied kind signature?
 -- See Note [CUSKs: complete user-supplied kind signatures]
-famDeclHasCusk :: Bool -- ^ True <=> this is an associated type family,
+famDeclHasCusk :: Bool -- ^ True <=> the -XCUSKs extension is enabled
+               -> Bool -- ^ True <=> this is an associated type family,
                        --            and the parent class has /no/ CUSK
                -> FamilyDecl pass
                -> Bool
-famDeclHasCusk assoc_with_no_cusk
+famDeclHasCusk _cusks_enabled@False _ _ = False
+famDeclHasCusk _cusks_enabled@True assoc_with_no_cusk
                (FamilyDecl { fdInfo      = fam_info
                            , fdTyVars    = tyvars
                            , fdResultSig = L _ resultSig })
@@ -1095,7 +1105,7 @@ famDeclHasCusk assoc_with_no_cusk
             -- Un-associated open type/data families have CUSKs
             -- Associated type families have CUSKs iff the parent class does
 
-famDeclHasCusk _ (XFamilyDecl {}) = panic "famDeclHasCusk"
+famDeclHasCusk _ _ (XFamilyDecl {}) = panic "famDeclHasCusk"
 
 -- | Does this family declaration have user-supplied return kind signature?
 hasReturnKindSignature :: FamilyResultSig a -> Bool
diff --git a/compiler/main/DynFlags.hs b/compiler/main/DynFlags.hs
index d40a9aba36..e94798aede 100644
--- a/compiler/main/DynFlags.hs
+++ b/compiler/main/DynFlags.hs
@@ -2279,6 +2279,7 @@ languageExtensions (Just Haskell98)
     = [LangExt.ImplicitPrelude,
        -- See Note [When is StarIsType enabled]
        LangExt.StarIsType,
+       LangExt.CUSKs,
        LangExt.MonomorphismRestriction,
        LangExt.NPlusKPatterns,
        LangExt.DatatypeContexts,
@@ -2295,6 +2296,7 @@ languageExtensions (Just Haskell2010)
     = [LangExt.ImplicitPrelude,
        -- See Note [When is StarIsType enabled]
        LangExt.StarIsType,
+       LangExt.CUSKs,
        LangExt.MonomorphismRestriction,
        LangExt.DatatypeContexts,
        LangExt.TraditionalRecordSyntax,
@@ -4377,6 +4379,7 @@ xFlagsDeps = [
   flagSpec "BinaryLiterals"                   LangExt.BinaryLiterals,
   flagSpec "CApiFFI"                          LangExt.CApiFFI,
   flagSpec "CPP"                              LangExt.Cpp,
+  flagSpec "CUSKs"                            LangExt.CUSKs,
   flagSpec "ConstrainedClassMethods"          LangExt.ConstrainedClassMethods,
   flagSpec "ConstraintKinds"                  LangExt.ConstraintKinds,
   flagSpec "DataKinds"                        LangExt.DataKinds,
diff --git a/compiler/rename/RnSource.hs b/compiler/rename/RnSource.hs
index e7ff909c02..537f283183 100644
--- a/compiler/rename/RnSource.hs
+++ b/compiler/rename/RnSource.hs
@@ -1552,7 +1552,8 @@ rnTyClDecl (DataDecl { tcdLName = tycon, tcdTyVars = tyvars,
        ; bindHsQTyVars doc Nothing Nothing kvs tyvars $ \ tyvars' no_rhs_kvs ->
     do { (defn', fvs) <- rnDataDefn doc defn
           -- See Note [Complete user-supplied kind signatures] in HsDecls
-       ; let cusk = hsTvbAllKinded tyvars' && no_rhs_kvs
+       ; cusks_enabled <- xoptM LangExt.CUSKs
+       ; let cusk = cusks_enabled && hsTvbAllKinded tyvars' && no_rhs_kvs
              rn_info = DataDeclRn { tcdDataCusk = cusk
                                   , tcdFVs      = fvs }
        ; traceRn "rndata" (ppr tycon <+> ppr cusk <+> ppr no_rhs_kvs)
diff --git a/compiler/typecheck/TcTyClsDecls.hs b/compiler/typecheck/TcTyClsDecls.hs
index 8b5142158d..a825573dba 100644
--- a/compiler/typecheck/TcTyClsDecls.hs
+++ b/compiler/typecheck/TcTyClsDecls.hs
@@ -510,8 +510,9 @@ kcTyClGroup decls
           --    3. Generalise the inferred kinds
           -- See Note [Kind checking for type and class decls]
 
+        ; cusks_enabled <- xoptM LangExt.CUSKs
         ; let (cusk_decls, no_cusk_decls)
-                 = partition (hsDeclHasCusk . unLoc) decls
+                 = partition (hsDeclHasCusk cusks_enabled . unLoc) decls
 
         ; poly_cusk_tcs <- getInitialKinds True cusk_decls
 
@@ -1040,17 +1041,25 @@ getInitialKind cusk (FamDecl { tcdFam = decl })
 getInitialKind cusk (SynDecl { tcdLName = dL->L _ name
                              , tcdTyVars = ktvs
                              , tcdRhs = rhs })
-  = do  { tycon <- kcLHsQTyVars name TypeSynonymFlavour cusk ktvs $
-                   case kind_annotation rhs of
+  = do  { cusks_enabled <- xoptM LangExt.CUSKs
+        ; tycon <- kcLHsQTyVars name TypeSynonymFlavour cusk ktvs $
+                   case kind_annotation cusks_enabled rhs of
                      Just ksig -> tcLHsKindSig (TySynKindCtxt name) ksig
-                     Nothing   -> newMetaKindVar
+                     Nothing -> newMetaKindVar
         ; return [tycon] }
   where
     -- Keep this synchronized with 'hsDeclHasCusk'.
-    kind_annotation (dL->L _ ty) = case ty of
-        HsParTy _ lty     -> kind_annotation lty
-        HsKindSig _ _ k   -> Just k
-        _                 -> Nothing
+    kind_annotation
+      :: Bool           --  cusks_enabled?
+      -> LHsType GhcRn  --  rhs
+      -> Maybe (LHsKind GhcRn)
+    kind_annotation False = const Nothing
+    kind_annotation True = go
+      where
+        go (dL->L _ ty) = case ty of
+          HsParTy _ lty     -> go lty
+          HsKindSig _ _ k   -> Just k
+          _                 -> Nothing
 
 getInitialKind _ (DataDecl _ _ _ _ (XHsDataDefn _)) = panic "getInitialKind"
 getInitialKind _ (XTyClDecl _) = panic "getInitialKind"
@@ -1074,18 +1083,20 @@ getFamDeclInitialKind parent_cusk mb_parent_tycon
                      , fdTyVars    = ktvs
                      , fdResultSig = (dL->L _ resultSig)
                      , fdInfo      = info })
-  = kcLHsQTyVars name flav fam_cusk ktvs $
-    case resultSig of
-      KindSig _ ki                              -> tcLHsKindSig ctxt ki
-      TyVarSig _ (dL->L _ (KindedTyVar _ _ ki)) -> tcLHsKindSig ctxt ki
-      _ -- open type families have * return kind by default
-        | tcFlavourIsOpen flav              -> return liftedTypeKind
-               -- closed type families have their return kind inferred
-               -- by default
-        | otherwise                         -> newMetaKindVar
+  = do { cusks_enabled <- xoptM LangExt.CUSKs
+       ; kcLHsQTyVars name flav (fam_cusk cusks_enabled) ktvs $
+         case resultSig of
+           KindSig _ ki                              -> tcLHsKindSig ctxt ki
+           TyVarSig _ (dL->L _ (KindedTyVar _ _ ki)) -> tcLHsKindSig ctxt ki
+           _ -- open type families have * return kind by default
+             | tcFlavourIsOpen flav              -> return liftedTypeKind
+                    -- closed type families have their return kind inferred
+                    -- by default
+             | otherwise                         -> newMetaKindVar
+       }
   where
     assoc_with_no_cusk = isJust mb_parent_tycon && not parent_cusk
-    fam_cusk = famDeclHasCusk assoc_with_no_cusk decl
+    fam_cusk cusks_enabled = famDeclHasCusk cusks_enabled assoc_with_no_cusk decl
     flav = case info of
       DataFamily         -> DataFamilyFlavour mb_parent_tycon
       OpenTypeFamily     -> OpenTypeFamilyFlavour mb_parent_tycon
diff --git a/docs/users_guide/glasgow_exts.rst b/docs/users_guide/glasgow_exts.rst
index 5fef204831..bce2bf8370 100644
--- a/docs/users_guide/glasgow_exts.rst
+++ b/docs/users_guide/glasgow_exts.rst
@@ -9012,6 +9012,11 @@ do so.
 Complete user-supplied kind signatures and polymorphic recursion
 ----------------------------------------------------------------
 
+.. extension:: CUSKs
+    :shortdesc: Enable detection of complete user-supplied kind signatures.
+
+    :since: 8.10.1
+
 Just as in type inference, kind inference for recursive types can only
 use *monomorphic* recursion. Consider this (contrived) example: ::
 
@@ -9110,6 +9115,13 @@ example, consider ::
 According to the rules above ``X`` has a CUSK. Yet, the kind of ``k`` is undetermined.
 It is thus quantified over, giving ``X`` the kind ``forall k1 (k :: k1). Proxy k -> Type``.
 
+The detection of CUSKs is enabled by the :extension:`CUSKs` flag, which is
+switched on by default. When :extension:`CUSKs` is switched off, there is
+currently no way to enable polymorphic recursion in types. In the future, the
+notion of a CUSK will be replaced by top-level kind signatures
+(`GHC Proposal #36 <https://github.com/ghc-proposals/ghc-proposals/blob/master/proposals/0036-kind-signatures.rst>`__),
+then, after a transition period, this extension will be turned off by default, and eventually removed.
+
 Kind inference in closed type families
 --------------------------------------
 
diff --git a/libraries/ghc-boot-th/GHC/LanguageExtensions/Type.hs b/libraries/ghc-boot-th/GHC/LanguageExtensions/Type.hs
index 565187be59..ac47e165ff 100644
--- a/libraries/ghc-boot-th/GHC/LanguageExtensions/Type.hs
+++ b/libraries/ghc-boot-th/GHC/LanguageExtensions/Type.hs
@@ -140,4 +140,5 @@ data Extension
    | QuantifiedConstraints
    | StarIsType
    | ImportQualifiedPost
+   | CUSKs
    deriving (Eq, Enum, Show, Generic, Bounded)
diff --git a/testsuite/tests/driver/T4437.hs b/testsuite/tests/driver/T4437.hs
index 2f28c05ec2..b8ef646a9b 100644
--- a/testsuite/tests/driver/T4437.hs
+++ b/testsuite/tests/driver/T4437.hs
@@ -41,6 +41,7 @@ expectedGhcOnlyExtensions = ["RelaxedLayout",
                              "AlternativeLayoutRuleTransitional",
                              "EmptyDataDeriving",
                              "GeneralisedNewtypeDeriving",
+                             "CUSKs",
                              "ImportQualifiedPost"]
 
 expectedCabalOnlyExtensions :: [String]
diff --git a/testsuite/tests/typecheck/should_fail/all.T b/testsuite/tests/typecheck/should_fail/all.T
index c51398f00b..c4c5040b9b 100644
--- a/testsuite/tests/typecheck/should_fail/all.T
+++ b/testsuite/tests/typecheck/should_fail/all.T
@@ -241,6 +241,7 @@ test('tcfail217', normal, compile_fail, [''])
 test('tcfail218', normal, compile_fail, [''])
 test('tcfail223', normal, compile_fail, [''])
 test('tcfail224', normal, compile_fail, [''])
+test('tcfail225', normal, compile_fail, [''])
 
 test('SilentParametersOverlapping', normal, compile, [''])
 test('FailDueToGivenOverlapping', normal, compile_fail, [''])
diff --git a/testsuite/tests/typecheck/should_fail/tcfail225.hs b/testsuite/tests/typecheck/should_fail/tcfail225.hs
new file mode 100644
index 0000000000..c01f49fdd1
--- /dev/null
+++ b/testsuite/tests/typecheck/should_fail/tcfail225.hs
@@ -0,0 +1,9 @@
+{-# LANGUAGE PolyKinds, GADTs #-}
+{-# LANGUAGE NoCUSKs #-}
+
+module TcFail225 where
+
+import Data.Kind (Type)
+
+data T (m :: k -> Type) :: k -> Type where
+  MkT :: m a -> T Maybe (m a) -> T m a
diff --git a/testsuite/tests/typecheck/should_fail/tcfail225.stderr b/testsuite/tests/typecheck/should_fail/tcfail225.stderr
new file mode 100644
index 0000000000..5a3ba3681f
--- /dev/null
+++ b/testsuite/tests/typecheck/should_fail/tcfail225.stderr
@@ -0,0 +1,6 @@
+
+tcfail225.hs:9:19: error:
+    • Expected kind ‘k -> *’, but ‘Maybe’ has kind ‘* -> *’
+    • In the first argument of ‘T’, namely ‘Maybe’
+      In the type ‘T Maybe (m a)’
+      In the definition of data constructor ‘MkT’