diff options
Diffstat (limited to 'compiler/GHC/Tc/TyCl.hs')
| -rw-r--r-- | compiler/GHC/Tc/TyCl.hs | 43 |
1 files changed, 40 insertions, 3 deletions
diff --git a/compiler/GHC/Tc/TyCl.hs b/compiler/GHC/Tc/TyCl.hs index a2d507475a..6abbbfde26 100644 --- a/compiler/GHC/Tc/TyCl.hs +++ b/compiler/GHC/Tc/TyCl.hs @@ -4976,9 +4976,13 @@ checkValidRoleAnnots role_annots tc | isVisibleTyConBinder tvb = Just (role, binderVar tvb) | otherwise = Nothing - check_roles - = whenIsJust role_annot_decl_maybe $ - \decl@(L loc (RoleAnnotDecl _ _ the_role_annots)) -> + check_roles = case role_annot_decl_maybe of + Nothing -> + setSrcSpan (getSrcSpan name) $ + -- See Note [Missing role annotations warning] + warnIf (not (isClassTyCon tc) && not (null vis_roles)) $ + TcRnMissingRoleAnnotation name vis_roles + Just (decl@(L loc (RoleAnnotDecl _ _ the_role_annots))) -> addRoleAnnotCtxt name $ setSrcSpanA loc $ do { role_annots_ok <- xoptM LangExt.RoleAnnotations @@ -5001,6 +5005,39 @@ checkValidRoleAnnots role_annots tc check_no_roles = whenIsJust role_annot_decl_maybe illegalRoleAnnotDecl +-- Note [Missing role annotations warning] +-- +-- We warn about missing role annotations for tycons +-- 1. not type-classes: +-- type classes are nominal by default, which is most conservative +-- choice. E.g. we cannot have a type-class with an (accidentally) +-- phantom or representational type variable, as we can with +-- data types. +-- 2. with visible roles +-- +-- We don't make any exceptions for other data types. +-- In particular we explicitly warn about omitted (default and common) +-- representational roles. That is the point of the warning. +-- For example the default representational role for `Map`s key type parameter +-- would be wrong, and this warning is there to warn about it, +-- asking users to be explicit. +-- +-- If the default roles have been nominal, i.e. as conservative as possible, +-- the warning would still be valuable, as most types can be `representational` +-- (c.f. type-classes, which usually cannot). +-- +-- We don't warn about types with invisible roles only, because users cannot +-- specify them: +-- +-- type Foo :: forall {k}. Type +-- data Foo = Foo Int +-- type role Foo phantom +-- +-- is incorrect, GHC complains: +-- Wrong number of roles listed in role annotation; +-- Expected 0, got 1: +-- + checkRoleAnnot :: TyVar -> LocatedAn NoEpAnns (Maybe Role) -> Role -> TcM () checkRoleAnnot _ (L _ Nothing) _ = return () checkRoleAnnot tv (L _ (Just r1)) r2 |