Use tcSplitForAllInvisTyVars (not tcSplitForAllTyVars) in more placeswip/T18939

The use of `tcSplitForAllTyVars` in `tcDataFamInstHeader` was the immediate
cause of #18939, and replacing it with a new `tcSplitForAllInvisTyVars`
function (which behaves like `tcSplitForAllTyVars` but only splits invisible
type variables) fixes the issue. However, this led me to realize that _most_
uses of `tcSplitForAllTyVars` in GHC really ought to be
`tcSplitForAllInvisTyVars` instead. While I was in town, I opted to replace
most uses of `tcSplitForAllTys` with `tcSplitForAllTysInvis` to reduce the
likelihood of such bugs in the future.

I say "most uses" above since there is one notable place where we _do_ want
to use `tcSplitForAllTyVars`: in `GHC.Tc.Validity.forAllTyErr`, which produces
the "`Illegal polymorphic type`" error message if you try to use a higher-rank
`forall` without having `RankNTypes` enabled. Here, we really do want to split
all `forall`s, not just invisible ones, or we run the risk of giving an
inaccurate error message in the newly added `T18939_Fail` test case.

I debated at some length whether I wanted to name the new function
`tcSplitForAllInvisTyVars` or `tcSplitForAllTyVarsInvisible`, but in the end,
I decided that I liked the former better. For consistency's sake, I opted to
rename the existing `splitPiTysInvisible` and `splitPiTysInvisibleN` functions
to `splitInvisPiTys` and `splitPiTysInvisN`, respectively, so that they use the
same naming convention. As a consequence, this ended up requiring a `haddock`
submodule bump.

Fixes #18939.

6 files changed, 22 insertions, 15 deletions

diff --git a/compiler/GHC/Tc/Gen/HsType.hs b/compiler/GHC/Tc/Gen/HsType.hs
index 5e5ffd41ad..6e42b9e21e 100644
--- a/compiler/GHC/Tc/Gen/HsType.hs
+++ b/compiler/GHC/Tc/Gen/HsType.hs
@@ -2568,13 +2568,13 @@ kcCheckDeclHeader_sig kisig name flav
     split_invis :: Kind -> Maybe Kind -> ([TyCoBinder], Kind)
     split_invis sig_ki Nothing =
       -- instantiate all invisible binders
-      splitPiTysInvisible sig_ki
+      splitInvisPiTys sig_ki
     split_invis sig_ki (Just res_ki) =
       -- subtraction a la checkExpectedKind
       let n_res_invis_bndrs = invisibleTyBndrCount res_ki
           n_sig_invis_bndrs = invisibleTyBndrCount sig_ki
           n_inst = n_sig_invis_bndrs - n_res_invis_bndrs
-      in splitPiTysInvisibleN n_inst sig_ki
+      in splitInvisPiTysN n_inst sig_ki
 
 -- A quantifier from a kind signature zipped with a user-written binder for it.
 data ZippedBinder =
diff --git a/compiler/GHC/Tc/TyCl/Instance.hs b/compiler/GHC/Tc/TyCl/Instance.hs
index 5ff7308f80..e3e1b7aa16 100644
--- a/compiler/GHC/Tc/TyCl/Instance.hs
+++ b/compiler/GHC/Tc/TyCl/Instance.hs
@@ -950,7 +950,7 @@ tcDataFamInstHeader mb_clsinfo fam_tc outer_bndrs fixity
     tc_kind_sig (Just hs_kind)
       = do { sig_kind <- tcLHsKindSig data_ctxt hs_kind
            ; lvl <- getTcLevel
-           ; let (tvs, inner_kind) = tcSplitForAllTyVars sig_kind
+           ; let (tvs, inner_kind) = tcSplitForAllInvisTyVars sig_kind
            ; (subst, _tvs') <- tcInstSkolTyVarsAt lvl False emptyTCvSubst tvs
              -- Perhaps surprisingly, we don't need the skolemised tvs themselves
            ; return (substTy subst inner_kind) }
diff --git a/compiler/GHC/Tc/Utils/Backpack.hs b/compiler/GHC/Tc/Utils/Backpack.hs
index de114c3817..6214434fce 100644
--- a/compiler/GHC/Tc/Utils/Backpack.hs
+++ b/compiler/GHC/Tc/Utils/Backpack.hs
@@ -223,8 +223,8 @@ check_inst sig_inst = do
         skol_info = InstSkol
         -- Based off of tcSplitDFunTy
         (tvs, theta, pred) =
-           case tcSplitForAllTyVars ty of { (tvs, rho)    ->
-           case splitFunTys rho        of { (theta, pred) ->
+           case tcSplitForAllInvisTyVars ty of { (tvs, rho)    ->
+           case splitFunTys rho             of { (theta, pred) ->
            (tvs, theta, pred) }}
         origin = InstProvidedOrigin (tcg_semantic_mod tcg_env) sig_inst
     (skol_subst, tvs_skols) <- tcInstSkolTyVars tvs -- Skolemize
diff --git a/compiler/GHC/Tc/Utils/Instantiate.hs b/compiler/GHC/Tc/Utils/Instantiate.hs
index 9bb0675f6c..27d01a5c4d 100644
--- a/compiler/GHC/Tc/Utils/Instantiate.hs
+++ b/compiler/GHC/Tc/Utils/Instantiate.hs
@@ -464,7 +464,7 @@ tcInstType inst_tyvars id
              subst'  = extendTCvInScopeSet subst (tyCoVarsOfType rho)
        ; return (tv_prs, substTheta subst' theta, substTy subst' tau) }
   where
-    (tyvars, rho) = tcSplitForAllTyVars (idType id)
+    (tyvars, rho) = tcSplitForAllInvisTyVars (idType id)
     (theta, tau)  = tcSplitPhiTy rho
 
 tcInstTypeBndrs :: Id -> TcM ([(Name, InvisTVBinder)], TcThetaType, TcType)
diff --git a/compiler/GHC/Tc/Utils/TcType.hs b/compiler/GHC/Tc/Utils/TcType.hs
index f355a016ae..c408ffb54c 100644
--- a/compiler/GHC/Tc/Utils/TcType.hs
+++ b/compiler/GHC/Tc/Utils/TcType.hs
@@ -60,7 +60,7 @@ module GHC.Tc.Utils.TcType (
   -- These are important because they do not look through newtypes
   getTyVar,
   tcSplitForAllTyVarBinder_maybe,
-  tcSplitForAllTyVars, tcSplitSomeForAllTyVars,
+  tcSplitForAllTyVars, tcSplitForAllInvisTyVars, tcSplitSomeForAllTyVars,
   tcSplitForAllReqTVBinders, tcSplitForAllInvisTVBinders,
   tcSplitPiTys, tcSplitPiTy_maybe, tcSplitForAllTyVarBinders,
   tcSplitPhiTy, tcSplitPredFunTy_maybe,
@@ -1223,12 +1223,17 @@ tcSplitForAllTyVarBinder_maybe (ForAllTy tv ty) = ASSERT( isTyVarBinder tv ) Jus
 tcSplitForAllTyVarBinder_maybe _                = Nothing
 
 -- | Like 'tcSplitPiTys', but splits off only named binders,
--- returning just the tycovars.
+-- returning just the tyvars.
 tcSplitForAllTyVars :: Type -> ([TyVar], Type)
 tcSplitForAllTyVars ty
   = ASSERT( all isTyVar (fst sty) ) sty
   where sty = splitForAllTyCoVars ty
 
+-- | Like 'tcSplitForAllTyVars', but only splits 'ForAllTy's with 'Invisible'
+-- type variable binders.
+tcSplitForAllInvisTyVars :: Type -> ([TyVar], Type)
+tcSplitForAllInvisTyVars ty = tcSplitSomeForAllTyVars isInvisibleArgFlag ty
+
 -- | Like 'tcSplitForAllTyVars', but only splits a 'ForAllTy' if @argf_pred argf@
 -- is 'True', where @argf@ is the visibility of the @ForAllTy@'s binder and
 -- @argf_pred@ is a predicate over visibilities provided as an argument to this
@@ -1284,9 +1289,11 @@ tcSplitPhiTy ty
           Just (pred, ty) -> split ty (pred:ts)
           Nothing         -> (reverse ts, ty)
 
--- | Split a sigma type into its parts.
+-- | Split a sigma type into its parts. This only splits /invisible/ type
+-- variable binders, as these are the only forms of binder that the typechecker
+-- will implicitly instantiate.
 tcSplitSigmaTy :: Type -> ([TyVar], ThetaType, Type)
-tcSplitSigmaTy ty = case tcSplitForAllTyVars ty of
+tcSplitSigmaTy ty = case tcSplitForAllInvisTyVars ty of
                         (tvs, rho) -> case tcSplitPhiTy rho of
                                         (theta, tau) -> (tvs, theta, tau)
 
@@ -1469,9 +1476,9 @@ tcSplitDFunTy :: Type -> ([TyVar], [Type], Class, [Type])
 -- the latter specifically stops at PredTy arguments,
 -- and we don't want to do that here
 tcSplitDFunTy ty
-  = case tcSplitForAllTyVars ty of { (tvs, rho)    ->
-    case splitFunTys rho        of { (theta, tau)  ->
-    case tcSplitDFunHead tau    of { (clas, tys)   ->
+  = case tcSplitForAllInvisTyVars ty of { (tvs, rho)    ->
+    case splitFunTys rho             of { (theta, tau)  ->
+    case tcSplitDFunHead tau         of { (clas, tys)   ->
     (tvs, map scaledThing theta, clas, tys) }}}
 
 tcSplitDFunHead :: Type -> (Class, [Type])
@@ -1489,7 +1496,7 @@ tcSplitMethodTy :: Type -> ([TyVar], PredType, Type)
 -- tcSplitMethodTy just peels off the outer forall and
 -- that first predicate
 tcSplitMethodTy ty
-  | (sel_tyvars,sel_rho) <- tcSplitForAllTyVars ty
+  | (sel_tyvars,sel_rho) <- tcSplitForAllInvisTyVars ty
   , Just (first_pred, local_meth_ty) <- tcSplitPredFunTy_maybe sel_rho
   = (sel_tyvars, first_pred, local_meth_ty)
   | otherwise
diff --git a/compiler/GHC/Tc/Validity.hs b/compiler/GHC/Tc/Validity.hs
index 87be216d9b..6f290cb7ab 100644
--- a/compiler/GHC/Tc/Validity.hs
+++ b/compiler/GHC/Tc/Validity.hs
@@ -924,7 +924,7 @@ forAllTyErr env rank ty
      , vcat [ hang herald 2 (ppr_tidy env ty)
             , suggestion ] )
   where
-    (tvs, _theta, _tau) = tcSplitSigmaTy ty
+    (tvs, _rho) = tcSplitForAllTyVars ty
     herald | null tvs  = text "Illegal qualified type:"
            | otherwise = text "Illegal polymorphic type:"
     suggestion = case rank of