Don't zap to Any; error insteadwip/t16775

This changes GHC's treatment of so-called Naughty Quantification
Candidates to issue errors, instead of zapping to Any.

Close #16775.

No new test cases, because existing ones cover this well.

33 files changed, 298 insertions, 124 deletions

diff --git a/compiler/basicTypes/OccName.hs b/compiler/basicTypes/OccName.hs
index 7584fa4380..ac2ad47100 100644
--- a/compiler/basicTypes/OccName.hs
+++ b/compiler/basicTypes/OccName.hs
@@ -94,7 +94,7 @@ module OccName (
 
         -- * Tidying up
         TidyOccEnv, emptyTidyOccEnv, initTidyOccEnv,
-        tidyOccName, avoidClashesOccEnv,
+        tidyOccName, avoidClashesOccEnv, delTidyOccEnvList,
 
         -- FsEnv
         FastStringEnv, emptyFsEnv, lookupFsEnv, extendFsEnv, mkFsEnv
@@ -818,7 +818,7 @@ Every id contributes a type variable to the type signature, and all of them are
 
     (id,id,id) :: (a2 -> a2, a1 -> a1, a -> a)
 
-which is a bit unfortunate, as it unfairly renames only one of them. What we
+which is a bit unfortunate, as it unfairly renames only two of them. What we
 would like to see is
 
     (id,id,id) :: (a3 -> a3, a2 -> a2, a1 -> a1)
@@ -846,6 +846,9 @@ initTidyOccEnv = foldl' add emptyUFM
   where
     add env (OccName _ fs) = addToUFM env fs 1
 
+delTidyOccEnvList :: TidyOccEnv -> [FastString] -> TidyOccEnv
+delTidyOccEnvList = delListFromUFM
+
 -- see Note [Tidying multiple names at once]
 avoidClashesOccEnv :: TidyOccEnv -> [OccName] -> TidyOccEnv
 avoidClashesOccEnv env occs = go env emptyUFM occs
diff --git a/compiler/basicTypes/VarEnv.hs b/compiler/basicTypes/VarEnv.hs
index 4c23b1f141..dfdc933b67 100644
--- a/compiler/basicTypes/VarEnv.hs
+++ b/compiler/basicTypes/VarEnv.hs
@@ -71,13 +71,14 @@ module VarEnv (
 
         -- * TidyEnv and its operation
         TidyEnv,
-        emptyTidyEnv, mkEmptyTidyEnv
+        emptyTidyEnv, mkEmptyTidyEnv, delTidyEnvList
     ) where
 
 import GhcPrelude
 import qualified Data.IntMap.Strict as IntMap -- TODO: Move this to UniqFM
 
 import OccName
+import Name
 import Var
 import VarSet
 import UniqSet
@@ -424,6 +425,12 @@ emptyTidyEnv = (emptyTidyOccEnv, emptyVarEnv)
 mkEmptyTidyEnv :: TidyOccEnv -> TidyEnv
 mkEmptyTidyEnv occ_env = (occ_env, emptyVarEnv)
 
+delTidyEnvList :: TidyEnv -> [Var] -> TidyEnv
+delTidyEnvList (occ_env, var_env) vs = (occ_env', var_env')
+  where
+    occ_env' = occ_env `delTidyOccEnvList` map (occNameFS . getOccName) vs
+    var_env' = var_env `delVarEnvList` vs
+
 {-
 ************************************************************************
 *                                                                      *
diff --git a/compiler/typecheck/TcHsType.hs b/compiler/typecheck/TcHsType.hs
index 17c027318b..61980788cd 100644
--- a/compiler/typecheck/TcHsType.hs
+++ b/compiler/typecheck/TcHsType.hs
@@ -1770,7 +1770,7 @@ the surrounding context, we must obey the following dictum:
   Every metavariable in a type must either be
     (A) generalized, or
     (B) promoted, or        See Note [Promotion in signatures]
-    (C) zapped to Any       See Note [Naughty quantification candidates] in TcMType
+    (C) a cause to error    See Note [Naughty quantification candidates] in TcMType
 
 The kindGeneralize functions do not require pre-zonking; they zonk as they
 go.
diff --git a/compiler/typecheck/TcMType.hs b/compiler/typecheck/TcMType.hs
index 0ac553c0ea..c86e3162c0 100644
--- a/compiler/typecheck/TcMType.hs
+++ b/compiler/typecheck/TcMType.hs
@@ -1137,8 +1137,7 @@ So alpha is entirely unconstrained.
 
 What then should we do with alpha?  During generalization, every
 metavariable is either (A) promoted, (B) generalized, or (C) zapped
-(according again to Note [Recipe for checking a signature] in
-TcHsType).
+(according to Note [Recipe for checking a signature] in TcHsType).
 
  * We can't generalise it.
  * We can't promote it, because its kind prevents that
@@ -1146,11 +1145,14 @@ TcHsType).
    go into the typing environment (as the type of some let-bound
    variable, say), and then chaos erupts when we try to instantiate.
 
-So, we zap it, eagerly, to Any. We don't have to do this eager zapping
-in terms (say, in `length []`) because terms are never re-examined before
-the final zonk (which zaps any lingering metavariables to Any).
+Previously, we zapped it to Any. This worked, but it had the unfortunate
+effect of causing Any sometimes to appear in error messages. If this
+kind of signature happens, the user probably has made a mistake -- no
+one really wants Any in their types. So we now error. This must be
+a hard error (failure in the monad) to avoid other messages from mentioning
+Any.
 
-We do this eager zapping in candidateQTyVars, which always precedes
+We do this eager erroring in candidateQTyVars, which always precedes
 generalisation, because at that moment we have a clear picture of what
 skolems are in scope within the type itself (e.g. that 'forall arg').
 
@@ -1235,13 +1237,14 @@ partitionCandidates dvs@(DV { dv_kvs = kvs, dv_tvs = tvs }) pred
 -- See Note [Dependent type variables]
 candidateQTyVarsOfType :: TcType       -- not necessarily zonked
                        -> TcM CandidatesQTvs
-candidateQTyVarsOfType ty = collect_cand_qtvs False emptyVarSet mempty ty
+candidateQTyVarsOfType ty = collect_cand_qtvs ty False emptyVarSet mempty ty
 
 -- | Like 'candidateQTyVarsOfType', but over a list of types
 -- The variables to quantify must have a TcLevel strictly greater than
 -- the ambient level. (See Wrinkle in Note [Naughty quantification candidates])
 candidateQTyVarsOfTypes :: [Type] -> TcM CandidatesQTvs
-candidateQTyVarsOfTypes tys = foldlM (collect_cand_qtvs False emptyVarSet) mempty tys
+candidateQTyVarsOfTypes tys = foldlM (\acc ty -> collect_cand_qtvs ty False emptyVarSet acc ty)
+                                     mempty tys
 
 -- | Like 'candidateQTyVarsOfType', but consider every free variable
 -- to be dependent. This is appropriate when generalizing a *kind*,
@@ -1249,11 +1252,12 @@ candidateQTyVarsOfTypes tys = foldlM (collect_cand_qtvs False emptyVarSet) mempt
 -- to Type.)
 candidateQTyVarsOfKind :: TcKind       -- Not necessarily zonked
                        -> TcM CandidatesQTvs
-candidateQTyVarsOfKind ty = collect_cand_qtvs True emptyVarSet mempty ty
+candidateQTyVarsOfKind ty = collect_cand_qtvs ty True emptyVarSet mempty ty
 
 candidateQTyVarsOfKinds :: [TcKind]    -- Not necessarily zonked
                        -> TcM CandidatesQTvs
-candidateQTyVarsOfKinds tys = foldM (collect_cand_qtvs True emptyVarSet) mempty tys
+candidateQTyVarsOfKinds tys = foldM (\acc ty -> collect_cand_qtvs ty True emptyVarSet acc ty)
+                                    mempty tys
 
 delCandidates :: CandidatesQTvs -> [Var] -> CandidatesQTvs
 delCandidates (DV { dv_kvs = kvs, dv_tvs = tvs, dv_cvs = cvs }) vars
@@ -1262,7 +1266,8 @@ delCandidates (DV { dv_kvs = kvs, dv_tvs = tvs, dv_cvs = cvs }) vars
        , dv_cvs = cvs `delVarSetList`  vars }
 
 collect_cand_qtvs
-  :: Bool            -- True <=> consider every fv in Type to be dependent
+  :: TcType          -- original type that we started recurring into; for errors
+  -> Bool            -- True <=> consider every fv in Type to be dependent
   -> VarSet          -- Bound variables (locals only)
   -> CandidatesQTvs  -- Accumulating parameter
   -> Type            -- Not necessarily zonked
@@ -1272,14 +1277,14 @@ collect_cand_qtvs
 --   * Looks through meta-tyvars as it goes;
 --     no need to zonk in advance
 --
---   * Needs to be monadic anyway, because it does the zap-naughty
---     stuff; see Note [Naughty quantification candidates]
+--   * Needs to be monadic anyway, because it handles naughty
+--     quantification; see Note [Naughty quantification candidates]
 --
 --   * Returns fully-zonked CandidateQTvs, including their kinds
 --     so that subsequent dependency analysis (to build a well
 --     scoped telescope) works correctly
 
-collect_cand_qtvs is_dep bound dvs ty
+collect_cand_qtvs orig_ty is_dep bound dvs ty
   = go dvs ty
   where
     is_bound tv = tv `elemVarSet` bound
@@ -1292,8 +1297,8 @@ collect_cand_qtvs is_dep bound dvs ty
     go dv (FunTy _ arg res) = foldlM go dv [arg, res]
     go dv (LitTy {})        = return dv
     go dv (CastTy ty co)    = do dv1 <- go dv ty
-                                 collect_cand_qtvs_co bound dv1 co
-    go dv (CoercionTy co)   = collect_cand_qtvs_co bound dv co
+                                 collect_cand_qtvs_co orig_ty bound dv1 co
+    go dv (CoercionTy co)   = collect_cand_qtvs_co orig_ty bound dv co
 
     go dv (TyVarTy tv)
       | is_bound tv      = return dv
@@ -1303,8 +1308,8 @@ collect_cand_qtvs is_dep bound dvs ty
                                   Nothing     -> go_tv dv tv }
 
     go dv (ForAllTy (Bndr tv _) ty)
-      = do { dv1 <- collect_cand_qtvs True bound dv (tyVarKind tv)
-           ; collect_cand_qtvs is_dep (bound `extendVarSet` tv) dv1 ty }
+      = do { dv1 <- collect_cand_qtvs orig_ty True bound dv (tyVarKind tv)
+           ; collect_cand_qtvs orig_ty is_dep (bound `extendVarSet` tv) dv1 ty }
 
     -----------------
     go_tv dv@(DV { dv_kvs = kvs, dv_tvs = tvs }) tv
@@ -1322,25 +1327,23 @@ collect_cand_qtvs is_dep bound dvs ty
                  -- (#15795) and to make the naughty check
                  -- (which comes next) works correctly
 
+           ; let tv_kind_vars = tyCoVarsOfType tv_kind
            ; cur_lvl <- getTcLevel
            ; if |  tcTyVarLevel tv <= cur_lvl
                 -> return dv   -- this variable is from an outer context; skip
                                -- See Note [Use level numbers ofor quantification]
 
-                |  intersectsVarSet bound (tyCoVarsOfType tv_kind)
+                |  intersectsVarSet bound tv_kind_vars
                    -- the tyvar must not be from an outer context, but we have
                    -- already checked for this.
                    -- See Note [Naughty quantification candidates]
-                -> do { traceTc "Zapping naughty quantifier" $
+                -> do { traceTc "Naughty quantifier" $
                           vcat [ ppr tv <+> dcolon <+> ppr tv_kind
                                , text "bound:" <+> pprTyVars (nonDetEltsUniqSet bound)
-                               , text "fvs:" <+> pprTyVars (nonDetEltsUniqSet $
-                                                            tyCoVarsOfType tv_kind) ]
+                               , text "fvs:" <+> pprTyVars (nonDetEltsUniqSet tv_kind_vars) ]
 
-                      ; writeMetaTyVar tv (anyTypeOfKind tv_kind)
-
-                      -- See Note [Recurring into kinds for candidateQTyVars]
-                      ; collect_cand_qtvs True bound dv tv_kind }
+                      ; let escapees = intersectVarSet bound tv_kind_vars
+                      ; naughtyQuantification orig_ty tv escapees }
 
                 |  otherwise
                 -> do { let tv' = tv `setTyVarKind` tv_kind
@@ -1349,15 +1352,16 @@ collect_cand_qtvs is_dep bound dvs ty
                                 -- See Note [Order of accumulation]
 
                         -- See Note [Recurring into kinds for candidateQTyVars]
-                      ; collect_cand_qtvs True bound dv' tv_kind } }
+                      ; collect_cand_qtvs orig_ty True bound dv' tv_kind } }
 
-collect_cand_qtvs_co :: VarSet -- bound variables
+collect_cand_qtvs_co :: TcType -- original type at top of recursion; for errors
+                     -> VarSet -- bound variables
                      -> CandidatesQTvs -> Coercion
                      -> TcM CandidatesQTvs
-collect_cand_qtvs_co bound = go_co
+collect_cand_qtvs_co orig_ty bound = go_co
   where
-    go_co dv (Refl ty)             = collect_cand_qtvs True bound dv ty
-    go_co dv (GRefl _ ty mco)      = do dv1 <- collect_cand_qtvs True bound dv ty
+    go_co dv (Refl ty)             = collect_cand_qtvs orig_ty True bound dv ty
+    go_co dv (GRefl _ ty mco)      = do dv1 <- collect_cand_qtvs orig_ty True bound dv ty
                                         go_mco dv1 mco
     go_co dv (TyConAppCo _ _ cos)  = foldlM go_co dv cos
     go_co dv (AppCo co1 co2)       = foldlM go_co dv [co1, co2]
@@ -1365,8 +1369,8 @@ collect_cand_qtvs_co bound = go_co
     go_co dv (AxiomInstCo _ _ cos) = foldlM go_co dv cos
     go_co dv (AxiomRuleCo _ cos)   = foldlM go_co dv cos
     go_co dv (UnivCo prov _ t1 t2) = do dv1 <- go_prov dv prov
-                                        dv2 <- collect_cand_qtvs True bound dv1 t1
-                                        collect_cand_qtvs True bound dv2 t2
+                                        dv2 <- collect_cand_qtvs orig_ty True bound dv1 t1
+                                        collect_cand_qtvs orig_ty True bound dv2 t2
     go_co dv (SymCo co)            = go_co dv co
     go_co dv (TransCo co1 co2)     = foldlM go_co dv [co1, co2]
     go_co dv (NthCo _ _ co)        = go_co dv co
@@ -1385,7 +1389,7 @@ collect_cand_qtvs_co bound = go_co
 
     go_co dv (ForAllCo tcv kind_co co)
       = do { dv1 <- go_co dv kind_co
-           ; collect_cand_qtvs_co (bound `extendVarSet` tcv) dv1 co }
+           ; collect_cand_qtvs_co orig_ty (bound `extendVarSet` tcv) dv1 co }
 
     go_mco dv MRefl    = return dv
     go_mco dv (MCo co) = go_co dv co
@@ -1401,7 +1405,7 @@ collect_cand_qtvs_co bound = go_co
       | cv `elemVarSet` cvs = return dv
 
         -- See Note [Recurring into kinds for candidateQTyVars]
-      | otherwise           = collect_cand_qtvs True bound
+      | otherwise           = collect_cand_qtvs orig_ty True bound
                                     (dv { dv_cvs = cvs `extendVarSet` cv })
                                     (idType cv)
 
@@ -2305,8 +2309,8 @@ tidySigSkol env cx ty tv_prs
 %************************************************************************
 %*                                                                      *
              Levity polymorphism checks
-*                                                                      *
-************************************************************************
+*                                                                       *
+*************************************************************************
 
 See Note [Levity polymorphism checking] in DsMonad
 
@@ -2351,3 +2355,48 @@ formatLevPolyErr ty
   where
     (tidy_env, tidy_ty) = tidyOpenType emptyTidyEnv ty
     tidy_ki             = tidyType tidy_env (tcTypeKind ty)
+
+{-
+%************************************************************************
+%*                                                                      *
+             Error messages
+*                                                                       *
+*************************************************************************
+
+-}
+
+-- See Note [Naughty quantification candidates]
+naughtyQuantification :: TcType   -- original type user wanted to quantify
+                      -> TcTyVar  -- naughty var
+                      -> TyVarSet -- skolems that would escape
+                      -> TcM a
+naughtyQuantification orig_ty tv escapees
+  = do { orig_ty1 <- zonkTcType orig_ty  -- in case it's not zonked
+
+       ; escapees' <- mapM zonkTcTyVarToTyVar $
+                      nonDetEltsUniqSet escapees
+                     -- we'll just be printing, so no harmful non-determinism
+
+       ; let fvs  = tyCoVarsOfTypeWellScoped orig_ty1
+             env0 = tidyFreeTyCoVars emptyTidyEnv fvs
+             env  = env0 `delTidyEnvList` escapees'
+                    -- this avoids gratuitous renaming of the escaped
+                    -- variables; very confusing to users!
+
+             orig_ty'   = tidyType env orig_ty1
+             ppr_tidied = pprTyVars . map (tidyTyCoVarOcc env)
+             doc = pprWithExplicitKindsWhen True $
+                   vcat [ sep [ text "Cannot generalise type; skolem" <> plural escapees'
+                              , quotes $ ppr_tidied escapees'
+                              , text "would escape" <+> itsOrTheir escapees' <+> text "scope"
+                              ]
+                        , sep [ text "if I tried to quantify"
+                              , ppr_tidied [tv]
+                              , text "in this type:"
+                              ]
+                        , nest 2 (pprTidiedType orig_ty')
+                        , text "(Indeed, I sometimes struggle even printing this correctly,"
+                        , text " due to its ill-scoped nature.)"
+                        ]
+
+       ; failWithTcM (env, doc) }
diff --git a/compiler/typecheck/TcPatSyn.hs b/compiler/typecheck/TcPatSyn.hs
index 1a6d3ea1e4..6984d06907 100644
--- a/compiler/typecheck/TcPatSyn.hs
+++ b/compiler/typecheck/TcPatSyn.hs
@@ -237,7 +237,7 @@ dependentArgErr (arg, bad_cos)
 ~~-~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 Consider
   data AST a = Sym [a]
-  class Prj s where { prj :: [a] -> Maybe (s a)
+  class Prj s where { prj :: [a] -> Maybe (s a) }
   pattern P x <= Sym (prj -> Just x)
 
 Here we get a matcher with this type
@@ -261,7 +261,7 @@ mentions the existentials.  We can conveniently do that by making the
    forall ex_tvs. arg_ty
 
 After that, Note [Naughty quantification candidates] in TcMType takes
-over, and zonks any such naughty variables to Any.
+over and errors.
 
 Note [Remove redundant provided dicts]
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
diff --git a/compiler/types/TyCoPpr.hs b/compiler/types/TyCoPpr.hs
index fbbdfbd7a0..f7a768210b 100644
--- a/compiler/types/TyCoPpr.hs
+++ b/compiler/types/TyCoPpr.hs
@@ -5,7 +5,7 @@ module TyCoPpr
         PprPrec(..), topPrec, sigPrec, opPrec, funPrec, appPrec, maybeParen,
 
         -- * Pretty-printing types
-        pprType, pprParendType, pprPrecType, pprPrecTypeX,
+        pprType, pprParendType, pprTidiedType, pprPrecType, pprPrecTypeX,
         pprTypeApp, pprTCvBndr, pprTCvBndrs,
         pprSigmaType,
         pprTheta, pprParendTheta, pprForAll, pprUserForAll,
@@ -81,10 +81,13 @@ See Note [Precedence in types] in BasicTypes.
 -- See Note [Pretty printing via Iface syntax] in PprTyThing
 --------------------------------------------------------
 
-pprType, pprParendType :: Type -> SDoc
+pprType, pprParendType, pprTidiedType :: Type -> SDoc
 pprType       = pprPrecType topPrec
 pprParendType = pprPrecType appPrec
 
+-- already pre-tidied
+pprTidiedType = pprIfaceType . toIfaceTypeX emptyVarSet
+
 pprPrecType :: PprPrec -> Type -> SDoc
 pprPrecType = pprPrecTypeX emptyTidyEnv
 
diff --git a/compiler/types/TyCoTidy.hs b/compiler/types/TyCoTidy.hs
index f4639ca303..b6f87c2230 100644
--- a/compiler/types/TyCoTidy.hs
+++ b/compiler/types/TyCoTidy.hs
@@ -95,8 +95,8 @@ tidyTyCoVarBinders tidy_env tvbs
 tidyFreeTyCoVars :: TidyEnv -> [TyCoVar] -> TidyEnv
 -- ^ Add the free 'TyVar's to the env in tidy form,
 -- so that we can tidy the type they are free in
-tidyFreeTyCoVars (full_occ_env, var_env) tyvars
-  = fst (tidyOpenTyCoVars (full_occ_env, var_env) tyvars)
+tidyFreeTyCoVars tidy_env tyvars
+  = fst (tidyOpenTyCoVars tidy_env tyvars)
 
 ---------------
 tidyOpenTyCoVars :: TidyEnv -> [TyCoVar] -> (TidyEnv, [TyCoVar])
@@ -232,5 +232,3 @@ tidyCo env@(_, subst) co
 
 tidyCos :: TidyEnv -> [Coercion] -> [Coercion]
 tidyCos env = map (tidyCo env)
-
-
diff --git a/compiler/utils/Outputable.hs b/compiler/utils/Outputable.hs
index d81c754866..02805c6c7c 100644
--- a/compiler/utils/Outputable.hs
+++ b/compiler/utils/Outputable.hs
@@ -34,7 +34,7 @@ module Outputable (
         sep, cat,
         fsep, fcat,
         hang, hangNotEmpty, punctuate, ppWhen, ppUnless,
-        speakNth, speakN, speakNOf, plural, isOrAre, doOrDoes,
+        speakNth, speakN, speakNOf, plural, isOrAre, doOrDoes, itsOrTheir,
         unicodeSyntax,
 
         coloured, keyword,
@@ -1160,6 +1160,15 @@ doOrDoes :: [a] -> SDoc
 doOrDoes [_] = text "does"
 doOrDoes _   = text "do"
 
+-- | Determines the form of possessive appropriate for the length of a list:
+--
+-- > itsOrTheir [x]   = text "its"
+-- > itsOrTheir [x,y] = text "their"
+-- > itsOrTheir []    = text "their"  -- probably avoid this
+itsOrTheir :: [a] -> SDoc
+itsOrTheir [_] = text "its"
+itsOrTheir _   = text "their"
+
 {-
 ************************************************************************
 *                                                                      *
diff --git a/docs/users_guide/8.12.1-notes.rst b/docs/users_guide/8.12.1-notes.rst
index 69890b2c10..95a0096c69 100644
--- a/docs/users_guide/8.12.1-notes.rst
+++ b/docs/users_guide/8.12.1-notes.rst
@@ -18,6 +18,12 @@ Full details
 Language
 ~~~~~~~~
 
+* In obscure scenarios, GHC now rejects programs it previously accepted, but
+  with unhelpful types. For example, if (with ``-XPartialTypeSignatures``) you
+  were to write ``x :: forall (f :: forall a (b :: a -> Type). b _). f _``, GHC previously
+  would have accepted ``x``, but its type would have involved the mysterious ``Any``
+  internal type family. Now, GHC rejects, explaining the situation.
+
 Compiler
 ~~~~~~~~
 
diff --git a/testsuite/tests/dependent/should_compile/all.T b/testsuite/tests/dependent/should_compile/all.T
index 823244ebca..6cc9f12bca 100644
--- a/testsuite/tests/dependent/should_compile/all.T
+++ b/testsuite/tests/dependent/should_compile/all.T
@@ -54,13 +54,9 @@ test('T15419', normal, compile, [''])
 test('T14066h', normal, compile, [''])
 test('T15666', normal, compile, [''])
 test('T15725', normal, compile, [''])
-test('T14880', normal, compile, [''])
-test('T14880-2', normal, compile, [''])
 test('T15743', normal, compile, ['-ddump-types -fprint-explicit-foralls'])
 test('InferDependency', normal, compile, [''])
 test('T15743e', normal, compile, ['-ddump-types -fprint-explicit-foralls'])
-test('T15076', normal, compile, [''])
-test('T15076b', normal, compile, [''])
 test('T15076c', normal, compile, [''])
 test('T15829', normal, compile, [''])
 test('T14729', normal, compile, ['-ddump-types -fprint-typechecker-elaboration -fprint-explicit-coercions'])
diff --git a/testsuite/tests/dependent/should_compile/T14880-2.hs b/testsuite/tests/dependent/should_fail/T14880-2.hs
index e7057a3f00..e7057a3f00 100644
--- a/testsuite/tests/dependent/should_compile/T14880-2.hs
+++ b/testsuite/tests/dependent/should_fail/T14880-2.hs
diff --git a/testsuite/tests/dependent/should_fail/T14880-2.stderr b/testsuite/tests/dependent/should_fail/T14880-2.stderr
new file mode 100644
index 0000000000..2082ca6c34
--- /dev/null
+++ b/testsuite/tests/dependent/should_fail/T14880-2.stderr
@@ -0,0 +1,8 @@
+
+T14880-2.hs:12:9: error:
+    • Cannot generalise type; skolem ‘arg’ would escape its scope
+      if I tried to quantify (a0 :: arg) in this type:
+        forall arg. Proxy @{Proxy @{arg} a0 -> *} (Foo arg @a0) -> ()
+      (Indeed, I sometimes struggle even printing this correctly,
+       due to its ill-scoped nature.)
+    • In the type signature: quux :: forall arg. Proxy (Foo arg) -> ()
diff --git a/testsuite/tests/dependent/should_compile/T14880.hs b/testsuite/tests/dependent/should_fail/T14880.hs
index 91cfb20a4a..91cfb20a4a 100644
--- a/testsuite/tests/dependent/should_compile/T14880.hs
+++ b/testsuite/tests/dependent/should_fail/T14880.hs
diff --git a/testsuite/tests/dependent/should_fail/T14880.stderr b/testsuite/tests/dependent/should_fail/T14880.stderr
new file mode 100644
index 0000000000..a5aa1df8d2
--- /dev/null
+++ b/testsuite/tests/dependent/should_fail/T14880.stderr
@@ -0,0 +1,12 @@
+
+T14880.hs:12:5: error:
+    • Cannot generalise type; skolem ‘arg’ would escape its scope
+      if I tried to quantify (a0 :: arg) in this type:
+        forall x arg.
+        ((Foo arg @a0 :: (Proxy @{arg} a0 -> *))
+         ~ (Foo arg @a0 :: (Proxy @{arg} a0 -> *))) =>
+        Bar x
+      (Indeed, I sometimes struggle even printing this correctly,
+       due to its ill-scoped nature.)
+    • In the definition of data constructor ‘MkBar’
+      In the data declaration for ‘Bar’
diff --git a/testsuite/tests/dependent/should_compile/T15076.hs b/testsuite/tests/dependent/should_fail/T15076.hs
index 0890cf9eab..0890cf9eab 100644
--- a/testsuite/tests/dependent/should_compile/T15076.hs
+++ b/testsuite/tests/dependent/should_fail/T15076.hs
diff --git a/testsuite/tests/dependent/should_fail/T15076.stderr b/testsuite/tests/dependent/should_fail/T15076.stderr
new file mode 100644
index 0000000000..814d459c3c
--- /dev/null
+++ b/testsuite/tests/dependent/should_fail/T15076.stderr
@@ -0,0 +1,11 @@
+
+T15076.hs:10:8: error:
+    • Cannot generalise type; skolem ‘a’ would escape its scope
+      if I tried to quantify (x0 :: a) in this type:
+        forall a (f :: forall (x :: a). Proxy @{a} x -> *).
+        Proxy @{Proxy @{a} x0 -> *} (f @x0) -> ()
+      (Indeed, I sometimes struggle even printing this correctly,
+       due to its ill-scoped nature.)
+    • In the type signature:
+        foo :: forall (a :: Type) (f :: forall (x :: a). Proxy x -> Type).
+               Proxy f -> ()
diff --git a/testsuite/tests/dependent/should_compile/T15076b.hs b/testsuite/tests/dependent/should_fail/T15076b.hs
index 15fce826c0..15fce826c0 100644
--- a/testsuite/tests/dependent/should_compile/T15076b.hs
+++ b/testsuite/tests/dependent/should_fail/T15076b.hs
diff --git a/testsuite/tests/dependent/should_fail/T15076b.stderr b/testsuite/tests/dependent/should_fail/T15076b.stderr
new file mode 100644
index 0000000000..3ee27a82b3
--- /dev/null
+++ b/testsuite/tests/dependent/should_fail/T15076b.stderr
@@ -0,0 +1,11 @@
+
+T15076b.hs:8:8: error:
+    • Cannot generalise type; skolem ‘a’ would escape its scope
+      if I tried to quantify (x0 :: a) in this type:
+        forall a (f :: forall (x :: a). Proxy @{a} x -> *).
+        Proxy @{Proxy @{a} x0 -> *} (f @x0) -> ()
+      (Indeed, I sometimes struggle even printing this correctly,
+       due to its ill-scoped nature.)
+    • In the type signature:
+        foo :: forall (a :: Type) (f :: forall (x :: a). Proxy x -> Type).
+               Proxy f -> ()
diff --git a/testsuite/tests/dependent/should_fail/T15825.stderr b/testsuite/tests/dependent/should_fail/T15825.stderr
index 97582ba952..5d989303a6 100644
--- a/testsuite/tests/dependent/should_fail/T15825.stderr
+++ b/testsuite/tests/dependent/should_fail/T15825.stderr
@@ -1,6 +1,8 @@
 
-T15825.hs:14:31: error:
-    • Illegal type synonym family application ‘GHC.Types.Any
-                                                 @k’ in instance:
-        X (a @(GHC.Types.Any @k))
+T15825.hs:14:10: error:
+    • Cannot generalise type; skolem ‘k’ would escape its scope
+      if I tried to quantify (x0 :: k) in this type:
+        forall k (a :: C k). X (a @x0)
+      (Indeed, I sometimes struggle even printing this correctly,
+       due to its ill-scoped nature.)
     • In the instance declaration for ‘X (a :: *)’
diff --git a/testsuite/tests/dependent/should_fail/all.T b/testsuite/tests/dependent/should_fail/all.T
index 8ff5a88961..dde686af7a 100644
--- a/testsuite/tests/dependent/should_fail/all.T
+++ b/testsuite/tests/dependent/should_fail/all.T
@@ -60,3 +60,7 @@ test('T16418', normal, compile_fail, [''])
 test('T17541', normal, compile_fail, [''])
 test('T17541b', normal, compile_fail, [''])
 test('T17131', normal, compile_fail, [''])
+test('T14880', normal, compile_fail, [''])
+test('T14880-2', normal, compile_fail, [''])
+test('T15076', normal, compile_fail, [''])
+test('T15076b', normal, compile_fail, [''])
diff --git a/testsuite/tests/partial-sigs/should_fail/T14040a.stderr b/testsuite/tests/partial-sigs/should_fail/T14040a.stderr
index d03d0dc32b..1d122cf590 100644
--- a/testsuite/tests/partial-sigs/should_fail/T14040a.stderr
+++ b/testsuite/tests/partial-sigs/should_fail/T14040a.stderr
@@ -1,34 +1,25 @@
 
-T14040a.hs:34:8: error:
-    • Cannot apply expression of type ‘Sing wl
-                                       -> (forall y. p _0 'WeirdNil)
-                                       -> (forall z (x :: z) (xs :: WeirdList (WeirdList z)).
-                                           Sing x
-                                           -> Sing xs
-                                           -> p GHC.Types.Any xs
-                                           -> p GHC.Types.Any ('WeirdCons x xs))
-                                       -> p _1 wl’
-      to a visible type argument ‘(WeirdList z)’
-    • In the sixth argument of ‘pWeirdCons’, namely
-        ‘(elimWeirdList @(WeirdList z) @xs @p xs pWeirdNil pWeirdCons)’
-      In the expression:
-        pWeirdCons
-          @z
-          @x
-          @xs
-          x
-          xs
-          (elimWeirdList @(WeirdList z) @xs @p xs pWeirdNil pWeirdCons)
-      In an equation for ‘elimWeirdList’:
-          elimWeirdList
-            (SWeirdCons (x :: Sing (x :: z))
-                        (xs :: Sing (xs :: WeirdList (WeirdList z))))
-            pWeirdNil
-            pWeirdCons
-            = pWeirdCons
-                @z
-                @x
-                @xs
-                x
-                xs
-                (elimWeirdList @(WeirdList z) @xs @p xs pWeirdNil pWeirdCons)
+T14040a.hs:21:18: error:
+    • Cannot generalise type; skolem ‘z’ would escape its scope
+      if I tried to quantify (_1 :: WeirdList z) in this type:
+        forall a1 (wl :: WeirdList a1)
+               (p :: forall x. x -> WeirdList x -> *).
+        Sing @(WeirdList a1) wl
+        -> (forall y. p @x0 _0 ('WeirdNil @x0))
+        -> (forall z (x :: z) (xs :: WeirdList (WeirdList z)).
+            Sing @z x
+            -> Sing @(WeirdList (WeirdList z)) xs
+            -> p @(WeirdList z) _1 xs
+            -> p @z _2 ('WeirdCons @z x xs))
+        -> p @a1 _3 wl
+      (Indeed, I sometimes struggle even printing this correctly,
+       due to its ill-scoped nature.)
+    • In the type signature:
+        elimWeirdList :: forall (a :: Type)
+                                (wl :: WeirdList a)
+                                (p :: forall (x :: Type). x -> WeirdList x -> Type).
+                         Sing wl
+                         -> (forall (y :: Type). p _ WeirdNil)
+                            -> (forall (z :: Type) (x :: z) (xs :: WeirdList (WeirdList z)).
+                                Sing x -> Sing xs -> p _ xs -> p _ (WeirdCons x xs))
+                               -> p _ wl
diff --git a/testsuite/tests/patsyn/should_compile/all.T b/testsuite/tests/patsyn/should_compile/all.T
index 2ac343f635..6ef1928768 100644
--- a/testsuite/tests/patsyn/should_compile/all.T
+++ b/testsuite/tests/patsyn/should_compile/all.T
@@ -75,7 +75,6 @@ test('T14058', [extra_files(['T14058.hs', 'T14058a.hs'])],
                multimod_compile, ['T14058', '-v0'])
 test('T14326', normal, compile, [''])
 test('T14394', normal, ghci_script, ['T14394.script'])
-test('T14552', normal, compile, [''])
 test('T14498', normal, compile, [''])
 test('T16682', [extra_files(['T16682.hs', 'T16682a.hs'])],
                multimod_compile, ['T16682', '-v0 -fwarn-incomplete-patterns -fno-code'])
diff --git a/testsuite/tests/patsyn/should_compile/T14552.hs b/testsuite/tests/patsyn/should_fail/T14552.hs
index a4a7493530..a4a7493530 100644
--- a/testsuite/tests/patsyn/should_compile/T14552.hs
+++ b/testsuite/tests/patsyn/should_fail/T14552.hs
diff --git a/testsuite/tests/patsyn/should_fail/T14552.stderr b/testsuite/tests/patsyn/should_fail/T14552.stderr
new file mode 100644
index 0000000000..b9b6b8448b
--- /dev/null
+++ b/testsuite/tests/patsyn/should_fail/T14552.stderr
@@ -0,0 +1,8 @@
+
+T14552.hs:22:9: error:
+    • Cannot generalise type; skolem ‘k’ would escape its scope
+      if I tried to quantify (aa0 :: k) in this type:
+        forall k (w :: k --> *). Exp a0 (F @k @(*) w aa0)
+      (Indeed, I sometimes struggle even printing this correctly,
+       due to its ill-scoped nature.)
+    • In the declaration for pattern synonym ‘FOO’
diff --git a/testsuite/tests/patsyn/should_fail/all.T b/testsuite/tests/patsyn/should_fail/all.T
index 27ebc8bdd4..02cc2cec2c 100644
--- a/testsuite/tests/patsyn/should_fail/all.T
+++ b/testsuite/tests/patsyn/should_fail/all.T
@@ -46,3 +46,4 @@ test('T15685', normal, compile_fail, [''])
 test('T15692', normal, compile, [''])   # It has -fdefer-type-errors inside
 test('T15694', normal, compile_fail, [''])
 test('T16900', normal, compile_fail, ['-fdiagnostics-show-caret'])
+test('T14552', normal, compile_fail, [''])
diff --git a/testsuite/tests/polykinds/T15795.stderr b/testsuite/tests/polykinds/T15795.stderr
new file mode 100644
index 0000000000..65bfdddecc
--- /dev/null
+++ b/testsuite/tests/polykinds/T15795.stderr
@@ -0,0 +1,9 @@
+
+T15795.hs:12:3: error:
+    • Cannot generalise type; skolem ‘k’ would escape its scope
+      if I tried to quantify (a0 :: k) in this type:
+        forall k (b :: k). T @k @a0 b
+      (Indeed, I sometimes struggle even printing this correctly,
+       due to its ill-scoped nature.)
+    • In the definition of data constructor ‘MkT’
+      In the data declaration for ‘T’
diff --git a/testsuite/tests/polykinds/T15795a.stderr b/testsuite/tests/polykinds/T15795a.stderr
new file mode 100644
index 0000000000..f4757137ce
--- /dev/null
+++ b/testsuite/tests/polykinds/T15795a.stderr
@@ -0,0 +1,9 @@
+
+T15795a.hs:9:3: error:
+    • Cannot generalise type; skolem ‘u’ would escape its scope
+      if I tried to quantify (cat10 :: u) in this type:
+        forall u (a :: u). F @u @cat10 a
+      (Indeed, I sometimes struggle even printing this correctly,
+       due to its ill-scoped nature.)
+    • In the definition of data constructor ‘Prod’
+      In the data declaration for ‘F’
diff --git a/testsuite/tests/polykinds/all.T b/testsuite/tests/polykinds/all.T
index 74ab266308..4312691755 100644
--- a/testsuite/tests/polykinds/all.T
+++ b/testsuite/tests/polykinds/all.T
@@ -204,8 +204,8 @@ test('T15817', normal, compile, [''])
 test('T15874', normal, compile, [''])
 test('T14887a', normal, compile, [''])
 test('T14847', normal, compile, [''])
-test('T15795', normal, compile, [''])
-test('T15795a', normal, compile, [''])
+test('T15795', normal, compile_fail, [''])
+test('T15795a', normal, compile_fail, [''])
 test('T16247', normal, compile_fail, [''])
 test('T16247a', normal, compile_fail, [''])
 test('KindVarOrder', normal, ghci_script, ['KindVarOrder.script'])
diff --git a/testsuite/tests/typecheck/should_compile/all.T b/testsuite/tests/typecheck/should_compile/all.T
index c51ff0b36b..80b11ca851 100644
--- a/testsuite/tests/typecheck/should_compile/all.T
+++ b/testsuite/tests/typecheck/should_compile/all.T
@@ -685,7 +685,6 @@ test('UnlifNewUnify', normal, compile, [''])
 test('UnliftedNewtypesLPFamily', normal, compile, [''])
 test('UnliftedNewtypesDifficultUnification', normal, compile, [''])
 test('T16832', normal, ghci_script, ['T16832.script'])
-test('T16946', normal, compile, [''])
 test('T16995', normal, compile, [''])
 test('T17007', normal, compile, [''])
 test('T17067', normal, compile, [''])
diff --git a/testsuite/tests/typecheck/should_fail/T14350.stderr b/testsuite/tests/typecheck/should_fail/T14350.stderr
index eb80d3ec80..955554ab8d 100644
--- a/testsuite/tests/typecheck/should_fail/T14350.stderr
+++ b/testsuite/tests/typecheck/should_fail/T14350.stderr
@@ -1,30 +1,52 @@
 
-T14350.hs:59:15: error:
-    • Couldn't match expected type ‘Proxy a1
-                                    -> Apply (Apply c 'Proxy) (Apply g 'Proxy)’
-                  with actual type ‘Sing (f @@ t0)’
-    • The function ‘applySing’ is applied to three arguments,
-      but its type ‘Sing f -> Sing t0 -> Sing (f @@ t0)’ has only two
-      In the expression: applySing f Proxy Proxy
-      In an equation for ‘dcomp’: dcomp f g x = applySing f Proxy Proxy
-    • Relevant bindings include
-        x :: Sing x (bound at T14350.hs:59:11)
-        g :: Sing g (bound at T14350.hs:59:9)
-        f :: Sing f (bound at T14350.hs:59:7)
-        dcomp :: Sing f
-                 -> Sing g -> Sing x -> (c @@ 'Proxy) @@ (g @@ 'Proxy)
-          (bound at T14350.hs:59:1)
-
-T14350.hs:59:27: error:
-    • Couldn't match expected type ‘Sing t0’
-                  with actual type ‘Proxy a0’
-    • In the second argument of ‘applySing’, namely ‘Proxy’
-      In the expression: applySing f Proxy Proxy
-      In an equation for ‘dcomp’: dcomp f g x = applySing f Proxy Proxy
-    • Relevant bindings include
-        x :: Sing x (bound at T14350.hs:59:11)
-        g :: Sing g (bound at T14350.hs:59:9)
-        f :: Sing f (bound at T14350.hs:59:7)
-        dcomp :: Sing f
-                 -> Sing g -> Sing x -> (c @@ 'Proxy) @@ (g @@ 'Proxy)
-          (bound at T14350.hs:59:1)
+T14350.hs:49:10: error:
+    • Cannot generalise type; skolem ‘a’ would escape its scope
+      if I tried to quantify (x0 :: a) in this type:
+        forall a (b1 :: a ~> *)
+               (c :: forall (x :: a). Proxy @{a} x ~> ((@@) @{*} @{a} b1 x ~> *))
+               (f :: forall (x :: a) (y :: (@@) @{*} @{a} b1 x).
+                     Proxy @{a} x
+                     ~> (Proxy @{Apply @a @(*) b1 x} y
+                         ~> (@@)
+                              @{*}
+                              @{Apply @a @(*) b1 x}
+                              ((@@)
+                                 @{Apply @a @(*) b1 x ~> *} @{Proxy @{a} x} (c @x) ('Proxy @{a} @x))
+                              y))
+               (g :: forall (x :: a). Proxy @{a} x ~> (@@) @{*} @{a} b1 x)
+               (x :: a).
+        Sing
+          @(Proxy @{a} x0
+            ~> (Proxy @{Apply @a @(*) b1 x0} y0
+                ~> Apply
+                     @(Apply @a @(*) b1 x0)
+                     @(*)
+                     (Apply
+                        @(Proxy @{a} x0)
+                        @(Apply @a @(*) b1 x0 ~> *)
+                        (c @x0)
+                        ('Proxy @{a} @x0))
+                     y0))
+          (f @x0 @y0)
+        -> Sing @(Proxy @{a} x1 ~> Apply @a @(*) b1 x1) (g @x1)
+        -> Sing @a x
+        -> (@@)
+             @{*}
+             @{Apply @a @(*) b1 x}
+             ((@@)
+                @{Apply @a @(*) b1 x ~> *} @{Proxy @{a} x} (c @x) ('Proxy @{a} @x))
+             ((@@)
+                @{Apply @a @(*) b1 x} @{Proxy @{a} x} (g @x) ('Proxy @{a} @x))
+      (Indeed, I sometimes struggle even printing this correctly,
+       due to its ill-scoped nature.)
+    • In the type signature:
+        dcomp :: forall (a :: Type)
+                        (b :: a ~> Type)
+                        (c :: forall (x :: a). Proxy x ~> b @@ x ~> Type)
+                        (f :: forall (x :: a) (y :: b @@ x).
+                              Proxy x ~> Proxy y ~> c @@ ('Proxy :: Proxy x) @@ y)
+                        (g :: forall (x :: a). Proxy x ~> b @@ x)
+                        (x :: a).
+                 Sing f
+                 -> Sing g
+                    -> Sing x -> c @@ ('Proxy :: Proxy x) @@ (g @@ ('Proxy :: Proxy x))
diff --git a/testsuite/tests/typecheck/should_compile/T16946.hs b/testsuite/tests/typecheck/should_fail/T16946.hs
index e824f7cec8..e824f7cec8 100644
--- a/testsuite/tests/typecheck/should_compile/T16946.hs
+++ b/testsuite/tests/typecheck/should_fail/T16946.hs
diff --git a/testsuite/tests/typecheck/should_fail/T16946.stderr b/testsuite/tests/typecheck/should_fail/T16946.stderr
new file mode 100644
index 0000000000..a923fe778e
--- /dev/null
+++ b/testsuite/tests/typecheck/should_fail/T16946.stderr
@@ -0,0 +1,16 @@
+
+T16946.hs:11:9: error:
+    • Cannot generalise type; skolem ‘k’ would escape its scope
+      if I tried to quantify (y0 :: k) in this type:
+        forall k (c :: k -> k -> *)
+               (m :: forall (x :: k) (y :: k). c x y -> * -> *) a.
+        CatMonad @k c m =>
+        a -> m @y0 @y0 (Id @{k} @y0 c) a
+      (Indeed, I sometimes struggle even printing this correctly,
+       due to its ill-scoped nature.)
+    • In the type signature:
+        boom :: forall k
+                       (c :: k -> k -> Type)
+                       (m :: forall (x :: k) (y :: k). c x y -> Type -> Type)
+                       a.
+                CatMonad c m => a -> m (Id c) a
diff --git a/testsuite/tests/typecheck/should_fail/all.T b/testsuite/tests/typecheck/should_fail/all.T
index e46f694dd3..e11c239742 100644
--- a/testsuite/tests/typecheck/should_fail/all.T
+++ b/testsuite/tests/typecheck/should_fail/all.T
@@ -549,3 +549,4 @@ test('T17213', [extra_files(['T17213a.hs'])], multimod_compile_fail, ['T17213',
 test('T17355', normal, compile_fail, [''])
 test('T17360', normal, compile_fail, [''])
 test('T17563', normal, compile_fail, [''])
+test('T16946', normal, compile_fail, [''])