Fix #8958.

We now do role inference on stupid datatype contexts, allowing a
lightweight role annotation syntax.

compiler/typecheck/TcTyClsDecls.lhs

@@ -780,7 +780,8 @@ tcDataDefn rec_info tc_name tvs kind
   = do { extra_tvs <- tcDataKindSig kind
        ; let final_tvs  = tvs ++ extra_tvs
              roles      = rti_roles rec_info tc_name
-       ; stupid_theta <- tcHsContext ctxt
+       ; stupid_tc_theta <- tcHsContext ctxt
+       ; stupid_theta    <- zonkTcTypeToTypes emptyZonkEnv stupid_tc_theta
        ; kind_signatures <- xoptM Opt_KindSignatures
        ; is_boot         <- tcIsHsBoot  -- Are we compiling an hs-boot file?
 

compiler/typecheck/TcTyDecls.lhs

@@ -709,6 +709,8 @@ irTyCon tc
        ; unless (all (== Nominal) old_roles) $  -- also catches data families,
                                                 -- which don't want or need role inference
     do { whenIsJust (tyConClass_maybe tc) (irClass tc_name)
+       ; addRoleInferenceInfo tc_name (tyConTyVars tc) $
+         mapM_ (irType emptyVarSet) (tyConStupidTheta tc)  -- See #8958
        ; mapM_ (irDataCon tc_name) (visibleDataCons $ algTyConRhs tc) }}
 
   | Just (SynonymTyCon ty) <- synTyConRhs_maybe tc
@@ -778,7 +780,7 @@ lookupRoles tc
            Just roles -> return roles
            Nothing    -> return $ tyConRoles tc }
 
--- tries to update a role; won't even update a role "downwards"
+-- tries to update a role; won't ever update a role "downwards"
 updateRole :: Role -> TyVar -> RoleM ()
 updateRole role tv
   = do { var_ns <- getVarNs

testsuite/tests/ghci/scripts/ghci031.stdout

-data Eq a => D a = C a 	-- Defined at ghci031.hs:7:1
+type role D nominal
+data Eq a => D a = C a
+  	-- Defined at ghci031.hs:7:1

testsuite/tests/rename/should_fail/rnfail055.stderr

@@ -32,9 +32,10 @@ RnFail055.hs-boot:12:1:
 RnFail055.hs-boot:14:1:
     Type constructor ‘T2’ has conflicting definitions in the module
     and its hs-boot file
-    Main module: type role T2 representational phantom
+    Main module: type role T2 representational nominal
                  data Eq b => T2 a b = T2 a
-    Boot file:   data Eq a => T2 a b = T2 a
+    Boot file:   type role T2 nominal representational
+                 data Eq a => T2 a b = T2 a
 
 RnFail055.hs-boot:16:11:
     T3 is exported by the hs-boot file, but not exported by the module

testsuite/tests/roles/should_compile/T8958.hs

+{-# LANGUAGE RoleAnnotations, DatatypeContexts, IncoherentInstances,
+             FlexibleInstances #-}
+
+module T8958 where
+
+class Nominal a
+instance Nominal a
+
+class Representational a
+instance Representational a
+type role Representational representational
+
+newtype (Nominal k, Representational v) => Map k v = MkMap [(k,v)]

testsuite/tests/roles/should_compile/T8958.stderr

+
+T8958.hs:1:31: Warning:
+    -XDatatypeContexts is deprecated: It was widely considered a misfeature, and has been removed from the Haskell language.
+TYPE SIGNATURES
+TYPE CONSTRUCTORS
+  Map :: * -> * -> *
+  newtype (Nominal k, Representational v) => Map k v
+    No C type associated
+    Roles: [nominal, representational]
+    RecFlag NonRecursive, Promotable
+    = MkMap :: [(k, v)] -> Map k v Stricts: _
+    FamilyInstance: none
+  Nominal :: * -> Constraint
+  class Nominal a
+    Roles: [nominal]
+    RecFlag NonRecursive
+  Representational :: * -> Constraint
+  class Representational a
+    Roles: [representational]
+    RecFlag NonRecursive
+COERCION AXIOMS
+  axiom T8958.NTCo:Map :: Map k v = [(k, v)]
+INSTANCES
+  instance [incoherent] Representational a
+    -- Defined at T8958.hs:10:10
+  instance [incoherent] Nominal a -- Defined at T8958.hs:7:10
+Dependent modules: []
+Dependent packages: [base, ghc-prim, integer-gmp]
+
+==================== Typechecker ====================
+AbsBinds [a] []
+  {Exports: [T8958.$fRepresentationala <= $dRepresentational_aJ6
+               <>]
+   Exported types: T8958.$fRepresentationala [InlPrag=[ALWAYS] CONLIKE]
+                     :: forall a. Representational a
+                   [LclIdX[DFunId],
+                    Str=DmdType,
+                    Unf=DFun: \ (@ a) -> T8958.D:Representational TYPE a]
+   Binds: $dRepresentational_aJ6 = T8958.D:Representational}
+AbsBinds [a] []
+  {Exports: [T8958.$fNominala <= $dNominal_aJ7
+               <>]
+   Exported types: T8958.$fNominala [InlPrag=[ALWAYS] CONLIKE]
+                     :: forall a. Nominal a
+                   [LclIdX[DFunId],
+                    Str=DmdType,
+                    Unf=DFun: \ (@ a) -> T8958.D:Nominal TYPE a]
+   Binds: $dNominal_aJ7 = T8958.D:Nominal}
+

testsuite/tests/roles/should_compile/all.T

@@ -5,3 +5,4 @@ test('Roles4', only_ways('normal'), compile, ['-ddump-tc'])
 test('Roles13', only_ways('normal'), compile, ['-ddump-simpl -dsuppress-uniques'])
 test('Roles14', only_ways('normal'), compile, ['-ddump-tc'])
 test('RolesIArray', only_ways('normal'), compile, [''])
+test('T8958', only_ways('normal'), compile, ['-ddump-tc'])