Add structured error messages for GHC.Tc.Utils.TcMType

Tracking ticket: #20119

MR: !10138

This converts uses of `mkTcRnUnknownMessage` to newly added constructors
of `TcRnMessage`.

26 files changed, 152 insertions, 82 deletions

diff --git a/compiler/GHC/Tc/Errors/Ppr.hs b/compiler/GHC/Tc/Errors/Ppr.hs
index 3de952f2d8..432163d6f1 100644
--- a/compiler/GHC/Tc/Errors/Ppr.hs
+++ b/compiler/GHC/Tc/Errors/Ppr.hs
@@ -40,7 +40,7 @@ import GHC.Core.FamInstEnv ( FamInst(..), famInstAxiom, pprFamInst )
 import GHC.Core.InstEnv
 import GHC.Core.TyCo.Rep (Type(..))
 import GHC.Core.TyCo.Ppr (pprWithExplicitKindsWhen,
-                          pprSourceTyCon, pprTyVars, pprWithTYPE)
+                          pprSourceTyCon, pprTyVars, pprWithTYPE, pprTyVar, pprTidiedType)
 import GHC.Core.PatSyn ( patSynName, pprPatSynType )
 import GHC.Core.Predicate
 import GHC.Core.Type
@@ -1453,6 +1453,34 @@ instance Diagnostic TcRnMessage where
     TcRnTyThingUsedWrong sort thing name
       -> mkSimpleDecorated $
          pprTyThingUsedWrong sort thing name
+    TcRnCannotDefaultKindVar var knd ->
+      mkSimpleDecorated $
+      (vcat [ text "Cannot default kind variable" <+> quotes (ppr var)
+            , text "of kind:" <+> ppr knd
+            , text "Perhaps enable PolyKinds or add a kind signature" ])
+    TcRnUninferrableTyvar tidied_tvs context ->
+      mkSimpleDecorated $
+      pprWithExplicitKindsWhen True $
+      vcat [ text "Uninferrable type variable"
+              <> plural tidied_tvs
+              <+> pprWithCommas pprTyVar tidied_tvs
+              <+> text "in"
+            , pprUninferrableTyvarCtx context ]
+    TcRnSkolemEscape escapees tv orig_ty ->
+      mkSimpleDecorated $
+      pprWithExplicitKindsWhen True $
+      vcat [ sep [ text "Cannot generalise type; skolem" <> plural escapees
+                , quotes $ pprTyVars escapees
+                , text "would escape" <+> itsOrTheir escapees <+> text "scope"
+                ]
+          , sep [ text "if I tried to quantify"
+                , pprTyVar 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.)"
+          ]
 
   diagnosticReason = \case
     TcRnUnknownMessage m
@@ -1931,6 +1959,12 @@ instance Diagnostic TcRnMessage where
       -> ErrorWithoutFlag
     TcRnTyThingUsedWrong{}
       -> ErrorWithoutFlag
+    TcRnCannotDefaultKindVar{}
+      -> ErrorWithoutFlag
+    TcRnUninferrableTyvar{}
+      -> ErrorWithoutFlag
+    TcRnSkolemEscape{}
+      -> ErrorWithoutFlag
 
   diagnosticHints = \case
     TcRnUnknownMessage m
@@ -2427,6 +2461,12 @@ instance Diagnostic TcRnMessage where
       -> noHints
     TcRnTyThingUsedWrong{}
       -> noHints
+    TcRnCannotDefaultKindVar{}
+      -> noHints
+    TcRnUninferrableTyvar{}
+      -> noHints
+    TcRnSkolemEscape{}
+      -> noHints
 
   diagnosticCode = constructorCode
 
@@ -4505,3 +4545,19 @@ pprStageCheckReason = \case
     text "instance for" <+> quotes (ppr t)
   StageCheckSplice t ->
     quotes (ppr t)
+
+pprUninferrableTyvarCtx :: UninferrableTyvarCtx -> SDoc
+pprUninferrableTyvarCtx = \case
+  UninfTyCtx_ClassContext theta ->
+    sep [ text "the class context:", pprTheta theta ]
+  UninfTyCtx_DataContext theta ->
+    sep [ text "the datatype context:", pprTheta theta ]
+  UninfTyCtx_ProvidedContext theta ->
+    sep [ text "the provided context:" , pprTheta theta ]
+  UninfTyCtx_TyfamRhs rhs_ty ->
+    sep [ text "the type family equation right-hand side:" , ppr rhs_ty ]
+  UninfTyCtx_TysynRhs rhs_ty ->
+    sep [ text "the type synonym right-hand side:" , ppr rhs_ty ]
+  UninfTyCtx_Sig exp_kind full_hs_ty ->
+    hang (text "the kind" <+> ppr exp_kind) 2
+         (text "of the type signature:" <+> ppr full_hs_ty)
diff --git a/compiler/GHC/Tc/Errors/Types.hs b/compiler/GHC/Tc/Errors/Types.hs
index aa43f6f581..d84aca8146 100644
--- a/compiler/GHC/Tc/Errors/Types.hs
+++ b/compiler/GHC/Tc/Errors/Types.hs
@@ -94,6 +94,7 @@ module GHC.Tc.Errors.Types (
   , HsigShapeMismatchReason(..)
   , WrongThingSort(..)
   , StageCheckReason(..)
+  , UninferrableTyvarCtx(..)
   ) where
 
 import GHC.Prelude
@@ -3257,6 +3258,41 @@ data TcRnMessage where
     -> !Name -- ^ Name of the thing used wrongly.
     -> TcRnMessage
 
+  {-| TcRnCannotDefaultKindVar is an error that occurs when attempting to use
+    unconstrained kind variables whose type isn't @Type@, without -XPolyKinds.
+
+    Test cases:
+      T11334b
+  -}
+  TcRnCannotDefaultKindVar
+    :: !TyVar -- ^ The unconstrained variable.
+    -> !Kind -- ^ Kind of the variable.
+    -> TcRnMessage
+
+  {-| TcRnUninferrableTyvar is an error that occurs when metavariables
+    in a type could not be defaulted.
+
+    Test cases:
+      T17301, T17562, T17567, T17567StupidTheta, T15474, T21479
+  -}
+  TcRnUninferrableTyvar
+    :: ![TyCoVar] -- ^ The variables that could not be defaulted.
+    -> !UninferrableTyvarCtx -- ^ Description of the surrounding context.
+    -> TcRnMessage
+
+  {-| TcRnSkolemEscape is an error that occurs when type variables from an
+    outer scope is used in a context where they should be locally scoped.
+
+    Test cases:
+      T15076, T15076b, T14880-2, T15825, T14880, T15807, T16946, T14350,
+      T14040A, T15795, T15795a, T14552
+  -}
+  TcRnSkolemEscape
+    :: ![TcTyVar] -- ^ The variables that would escape.
+    -> !TcTyVar -- ^ The variable that is being quantified.
+    -> !Type -- ^ The type in which they occur.
+    -> TcRnMessage
+
   deriving Generic
 
 -- | Things forbidden in @type data@ declarations.
@@ -4538,3 +4574,11 @@ data WrongThingSort
 data StageCheckReason
   = StageCheckInstance !InstanceWhat !PredType
   | StageCheckSplice !Name
+
+data UninferrableTyvarCtx
+  = UninfTyCtx_ClassContext [TcType]
+  | UninfTyCtx_DataContext [TcType]
+  | UninfTyCtx_ProvidedContext [TcType]
+  | UninfTyCtx_TyfamRhs TcType
+  | UninfTyCtx_TysynRhs TcType
+  | UninfTyCtx_Sig TcType (LHsSigType GhcRn)
diff --git a/compiler/GHC/Tc/Gen/HsType.hs b/compiler/GHC/Tc/Gen/HsType.hs
index c002d8cc3e..0a0ec7230a 100644
--- a/compiler/GHC/Tc/Gen/HsType.hs
+++ b/compiler/GHC/Tc/Gen/HsType.hs
@@ -473,7 +473,7 @@ tc_lhs_sig_type skol_info full_hs_ty@(L loc (HsSig { sig_bndrs = hs_outer_bndrs
        -- Default any unconstrained variables free in the kind
        -- See Note [Escaping kind in type signatures]
        ; exp_kind_dvs <- candidateQTyVarsOfType exp_kind
-       ; doNotQuantifyTyVars exp_kind_dvs (mk_doc exp_kind)
+       ; doNotQuantifyTyVars exp_kind_dvs (err_ctx exp_kind)
 
        ; traceTc "tc_lhs_sig_type" (ppr hs_outer_bndrs $$ ppr outer_bndrs)
        ; outer_bndrs <- scopedSortOuter outer_bndrs
@@ -488,10 +488,9 @@ tc_lhs_sig_type skol_info full_hs_ty@(L loc (HsSig { sig_bndrs = hs_outer_bndrs
 
        ; return (implic, mkInfForAllTys kvs ty1) }
   where
-    mk_doc exp_kind tidy_env
+    err_ctx exp_kind tidy_env
       = do { (tidy_env2, exp_kind) <- zonkTidyTcType tidy_env exp_kind
-           ; return (tidy_env2, hang (text "The kind" <+> ppr exp_kind)
-                                   2 (text "of type signature:" <+> ppr full_hs_ty)) }
+           ; return (tidy_env2, UninfTyCtx_Sig exp_kind full_hs_ty) }
 
 
 
diff --git a/compiler/GHC/Tc/TyCl.hs b/compiler/GHC/Tc/TyCl.hs
index f3e02c0fd0..afb2047d63 100644
--- a/compiler/GHC/Tc/TyCl.hs
+++ b/compiler/GHC/Tc/TyCl.hs
@@ -35,7 +35,8 @@ import GHC.Driver.Config.HsToCore
 import GHC.Hs
 
 import GHC.Tc.Errors.Types ( TcRnMessage(..), FixedRuntimeRepProvenance(..)
-                           , mkTcRnUnknownMessage, IllegalNewtypeReason (..) )
+                           , mkTcRnUnknownMessage, IllegalNewtypeReason (..)
+                           , UninferrableTyvarCtx (..) )
 import GHC.Tc.TyCl.Build
 import GHC.Tc.Solver( pushLevelAndSolveEqualities, pushLevelAndSolveEqualitiesX
                     , reportUnsolvedEqualities )
@@ -2455,11 +2456,9 @@ tcClassDecl1 roles_info class_name hs_ctxt meths fundeps sigs ats at_defs
        --   class (forall a. a b ~ a c) => C b c
        -- The kind of `a` is unconstrained.
        ; dvs <- candidateQTyVarsOfTypes ctxt
-       ; let mk_doc tidy_env = do { (tidy_env2, ctxt) <- zonkTidyTcTypes tidy_env ctxt
-                                  ; return ( tidy_env2
-                                           , sep [ text "the class context:"
-                                                 , pprTheta ctxt ] ) }
-       ; doNotQuantifyTyVars dvs mk_doc
+       ; let err_ctx tidy_env = do { (tidy_env2, ctxt) <- zonkTidyTcTypes tidy_env ctxt
+                                  ; return (tidy_env2, UninfTyCtx_ClassContext ctxt) }
+       ; doNotQuantifyTyVars dvs err_ctx
 
        -- The pushLevelAndSolveEqualities will report errors for any
        -- unsolved equalities, so these zonks should not encounter
@@ -2873,11 +2872,9 @@ tcTySynRhs roles_info tc_name hs_ty
          --   type T = forall a. Proxy a
          -- The kind of `a` is unconstrained.
        ; dvs <- candidateQTyVarsOfType rhs_ty
-       ; let mk_doc tidy_env = do { (tidy_env2, rhs_ty) <- zonkTidyTcType tidy_env rhs_ty
-                                  ; return ( tidy_env2
-                                           , sep [ text "the type synonym right-hand side:"
-                                                 , ppr rhs_ty ] ) }
-       ; doNotQuantifyTyVars dvs mk_doc
+       ; let err_ctx tidy_env = do { (tidy_env2, rhs_ty) <- zonkTidyTcType tidy_env rhs_ty
+                                  ; return (tidy_env2, UninfTyCtx_TysynRhs rhs_ty) }
+       ; doNotQuantifyTyVars dvs err_ctx
 
        ; ze          <- mkEmptyZonkEnv NoFlexi
        ; (ze, bndrs) <- zonkTyVarBindersX ze tc_bndrs
@@ -2918,12 +2915,10 @@ tcDataDefn err_ctxt roles_info tc_name
        --   data (forall a. a b ~ a c) => T b c
        -- The kind of 'a' is unconstrained.
        ; dvs <- candidateQTyVarsOfTypes stupid_tc_theta
-       ; let mk_doc tidy_env
+       ; let err_ctx tidy_env
                = do { (tidy_env2, theta) <- zonkTidyTcTypes tidy_env stupid_tc_theta
-                    ; return ( tidy_env2
-                             , sep [ text "the datatype context:"
-                                   , pprTheta theta ] ) }
-       ; doNotQuantifyTyVars dvs mk_doc
+                    ; return (tidy_env2, UninfTyCtx_DataContext theta) }
+       ; doNotQuantifyTyVars dvs err_ctx
 
              -- Check that we don't use kind signatures without the extension
        ; kind_signatures <- xoptM LangExt.KindSignatures
@@ -3178,12 +3173,10 @@ tcTyFamInstEqnGuts fam_tc mb_clsinfo outer_hs_bndrs hs_pats hs_rhs_ty
        -- See Note [Error on unconstrained meta-variables] in GHC.Tc.Utils.TcMType
        -- Example: typecheck/should_fail/T17301
        ; dvs_rhs <- candidateQTyVarsOfType rhs_ty
-       ; let mk_doc tidy_env
+       ; let err_ctx tidy_env
                = do { (tidy_env2, rhs_ty) <- zonkTidyTcType tidy_env rhs_ty
-                    ; return ( tidy_env2
-                             , sep [ text "type family equation right-hand side:"
-                                   , ppr rhs_ty ] ) }
-       ; doNotQuantifyTyVars dvs_rhs mk_doc
+                    ; return (tidy_env2, UninfTyCtx_TyfamRhs rhs_ty) }
+       ; doNotQuantifyTyVars dvs_rhs err_ctx
 
        ; ze              <- mkEmptyZonkEnv NoFlexi
        ; (ze, final_tvs) <- zonkTyBndrsX      ze final_tvs
diff --git a/compiler/GHC/Tc/TyCl/PatSyn.hs b/compiler/GHC/Tc/TyCl/PatSyn.hs
index 8741770977..82fa7db1f7 100644
--- a/compiler/GHC/Tc/TyCl/PatSyn.hs
+++ b/compiler/GHC/Tc/TyCl/PatSyn.hs
@@ -194,12 +194,10 @@ tcInferPatSynDecl (PSB { psb_id = lname@(L _ name), psb_args = details
        -- Report un-quantifiable type variables:
        -- see Note [Unquantified tyvars in a pattern synonym]
        ; dvs <- candidateQTyVarsOfTypes prov_theta
-       ; let mk_doc tidy_env
+       ; let err_ctx tidy_env
                = do { (tidy_env2, theta) <- zonkTidyTcTypes tidy_env prov_theta
-                    ; return ( tidy_env2
-                             , sep [ text "the provided context:"
-                                   , pprTheta theta ] ) }
-       ; doNotQuantifyTyVars dvs mk_doc
+                    ; return ( tidy_env2, UninfTyCtx_ProvidedContext theta ) }
+       ; doNotQuantifyTyVars dvs err_ctx
 
        ; traceTc "tcInferPatSynDecl }" $ (ppr name $$ ppr ex_tvs)
        ; rec_fields <- lookupConstructorFields name
diff --git a/compiler/GHC/Tc/Utils/TcMType.hs b/compiler/GHC/Tc/Utils/TcMType.hs
index e3ca947cdd..e14dae75cf 100644
--- a/compiler/GHC/Tc/Utils/TcMType.hs
+++ b/compiler/GHC/Tc/Utils/TcMType.hs
@@ -131,7 +131,6 @@ import GHC.Types.Name
 import GHC.Types.Var.Set
 
 import GHC.Builtin.Types
-import GHC.Types.Error
 import GHC.Types.Var.Env
 import GHC.Types.Unique.Set
 import GHC.Types.Basic ( TypeOrKind(..)
@@ -1853,10 +1852,7 @@ defaultTyVar def_strat tv
            ; writeMetaTyVar kv liftedTypeKind
            ; return True }
       | otherwise
-      = do { addErr $ mkTcRnUnknownMessage $ mkPlainError noHints $
-               (vcat [ text "Cannot default kind variable" <+> quotes (ppr kv')
-                     , text "of kind:" <+> ppr (tyVarKind kv')
-                     , text "Perhaps enable PolyKinds or add a kind signature" ])
+      = do { addErr $ TcRnCannotDefaultKindVar kv' (tyVarKind kv')
            -- We failed to default it, so return False to say so.
            -- Hence, it'll get skolemised.  That might seem odd, but we must either
            -- promote, skolemise, or zap-to-Any, to satisfy GHC.Tc.Gen.HsType
@@ -2053,7 +2049,7 @@ C. Examine the class declaration at the top of this Note again.
 -}
 
 doNotQuantifyTyVars :: CandidatesQTvs
-                    -> (TidyEnv -> TcM (TidyEnv, SDoc))
+                    -> (TidyEnv -> TcM (TidyEnv, UninferrableTyvarCtx))
                             -- ^ like "the class context (D a b, E foogle)"
                     -> TcM ()
 -- See Note [Error on unconstrained meta-variables]
@@ -2072,14 +2068,7 @@ doNotQuantifyTyVars dvs where_found
        ; unless (null leftover_metas) $
          do { let (tidy_env1, tidied_tvs) = tidyOpenTyCoVars emptyTidyEnv leftover_metas
             ; (tidy_env2, where_doc) <- where_found tidy_env1
-            ; let msg = mkTcRnUnknownMessage            $
-                        mkPlainError noHints          $
-                        pprWithExplicitKindsWhen True $
-                    vcat [ text "Uninferrable type variable"
-                           <> plural tidied_tvs
-                           <+> pprWithCommas pprTyVar tidied_tvs
-                           <+> text "in"
-                         , where_doc ]
+            ; let msg = TcRnUninferrableTyvar tidied_tvs where_doc
             ; failWithTcM (tidy_env2, msg) }
        ; traceTc "doNotQuantifyTyVars success" empty }
 
@@ -2741,21 +2730,8 @@ naughtyQuantification orig_ty tv escapees
                     -- variables; very confusing to users!
 
              orig_ty'   = tidyType env orig_ty1
-             ppr_tidied = pprTyVars . map (tidyTyCoVarOcc env)
-             msg = mkTcRnUnknownMessage $ mkPlainError noHints $
-                   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.)"
-                        ]
+             tidied = map (tidyTyCoVarOcc env) escapees'
+             msg = TcRnSkolemEscape tidied (tidyTyCoVarOcc env tv) orig_ty'
 
        ; failWithTcM (env, msg) }
 
diff --git a/compiler/GHC/Types/Error/Codes.hs b/compiler/GHC/Types/Error/Codes.hs
index 1f9fb29905..a5c751ed3d 100644
--- a/compiler/GHC/Types/Error/Codes.hs
+++ b/compiler/GHC/Types/Error/Codes.hs
@@ -539,6 +539,9 @@ type family GhcDiagnosticCode c = n | n -> c where
   GhcDiagnosticCode "TcRnBadlyStaged"                               = 28914
   GhcDiagnosticCode "TcRnStageRestriction"                          = 18157
   GhcDiagnosticCode "TcRnTyThingUsedWrong"                          = 10969
+  GhcDiagnosticCode "TcRnCannotDefaultKindVar"                      = 79924
+  GhcDiagnosticCode "TcRnUninferrableTyvar"                         = 16220
+  GhcDiagnosticCode "TcRnSkolemEscape"                              = 71451
 
   -- IllegalNewtypeReason
   GhcDiagnosticCode "DoesNotHaveSingleField"                        = 23517
diff --git a/testsuite/tests/dependent/should_fail/T11334b.stderr b/testsuite/tests/dependent/should_fail/T11334b.stderr
index 57045d6df1..c9eda334bb 100644
--- a/testsuite/tests/dependent/should_fail/T11334b.stderr
+++ b/testsuite/tests/dependent/should_fail/T11334b.stderr
@@ -1,5 +1,5 @@
 
-T11334b.hs:8:14: error:
+T11334b.hs:8:14: error: [GHC-79924]
     • Cannot default kind variable ‘a0’
       of kind: k10
       Perhaps enable PolyKinds or add a kind signature
@@ -7,7 +7,7 @@ T11334b.hs:8:14: error:
       In the expression: Proxy :: Proxy 'Compose
       In an equation for ‘p’: p = Proxy :: Proxy 'Compose
 
-T11334b.hs:8:14: error:
+T11334b.hs:8:14: error: [GHC-79924]
     • Cannot default kind variable ‘g0’
       of kind: k10 -> k0
       Perhaps enable PolyKinds or add a kind signature
@@ -15,7 +15,7 @@ T11334b.hs:8:14: error:
       In the expression: Proxy :: Proxy 'Compose
       In an equation for ‘p’: p = Proxy :: Proxy 'Compose
 
-T11334b.hs:8:14: error:
+T11334b.hs:8:14: error: [GHC-79924]
     • Cannot default kind variable ‘f0’
       of kind: k0 -> *
       Perhaps enable PolyKinds or add a kind signature
diff --git a/testsuite/tests/dependent/should_fail/T14880-2.stderr b/testsuite/tests/dependent/should_fail/T14880-2.stderr
index 56d8ff68ba..2e4df34c79 100644
--- a/testsuite/tests/dependent/should_fail/T14880-2.stderr
+++ b/testsuite/tests/dependent/should_fail/T14880-2.stderr
@@ -1,5 +1,5 @@
 
-T14880-2.hs:13:9: error:
+T14880-2.hs:13:9: error: [GHC-71451]
     • 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) -> ()
diff --git a/testsuite/tests/dependent/should_fail/T14880.stderr b/testsuite/tests/dependent/should_fail/T14880.stderr
index 90e54427c0..85f8bdb80d 100644
--- a/testsuite/tests/dependent/should_fail/T14880.stderr
+++ b/testsuite/tests/dependent/should_fail/T14880.stderr
@@ -1,5 +1,5 @@
 
-T14880.hs:13:5: error:
+T14880.hs:13:5: error: [GHC-71451]
     • Cannot generalise type; skolem ‘arg’ would escape its scope
       if I tried to quantify (a0 :: arg) in this type:
         forall x arg.
diff --git a/testsuite/tests/dependent/should_fail/T15076.stderr b/testsuite/tests/dependent/should_fail/T15076.stderr
index 71d3c7c156..827b2bcdcc 100644
--- a/testsuite/tests/dependent/should_fail/T15076.stderr
+++ b/testsuite/tests/dependent/should_fail/T15076.stderr
@@ -1,5 +1,5 @@
 
-T15076.hs:11:8: error:
+T15076.hs:11:8: error: [GHC-71451]
     • 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 -> *).
diff --git a/testsuite/tests/dependent/should_fail/T15076b.stderr b/testsuite/tests/dependent/should_fail/T15076b.stderr
index 8da932a044..600253fb8d 100644
--- a/testsuite/tests/dependent/should_fail/T15076b.stderr
+++ b/testsuite/tests/dependent/should_fail/T15076b.stderr
@@ -1,4 +1,4 @@
-T15076b.hs:9:8: error:
+T15076b.hs:9:8: error: [GHC-71451]
     • 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 -> *).
diff --git a/testsuite/tests/dependent/should_fail/T15825.stderr b/testsuite/tests/dependent/should_fail/T15825.stderr
index 5d989303a6..5a4822f1f7 100644
--- a/testsuite/tests/dependent/should_fail/T15825.stderr
+++ b/testsuite/tests/dependent/should_fail/T15825.stderr
@@ -1,5 +1,5 @@
 
-T15825.hs:14:10: error:
+T15825.hs:14:10: error: [GHC-71451]
     • 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)
diff --git a/testsuite/tests/patsyn/should_fail/T14552.stderr b/testsuite/tests/patsyn/should_fail/T14552.stderr
index 92c1adb57b..2c1bdb8c49 100644
--- a/testsuite/tests/patsyn/should_fail/T14552.stderr
+++ b/testsuite/tests/patsyn/should_fail/T14552.stderr
@@ -1,5 +1,5 @@
 
-T14552.hs:22:1: error:
+T14552.hs:22:1: error: [GHC-71451]
     • Cannot generalise type; skolem ‘k’ would escape its scope
       if I tried to quantify (t0 :: k) in this type:
         forall k (w :: k --> *). Exp a0 (F @k @(*) w t0)
diff --git a/testsuite/tests/patsyn/should_fail/T21479.stderr b/testsuite/tests/patsyn/should_fail/T21479.stderr
index 07601492a8..dc6677580d 100644
--- a/testsuite/tests/patsyn/should_fail/T21479.stderr
+++ b/testsuite/tests/patsyn/should_fail/T21479.stderr
@@ -1,5 +1,5 @@
 
-T21479.hs:13:1: error:
+T21479.hs:13:1: error: [GHC-16220]
     • Uninferrable type variable a0 in
       the provided context: (a0 :: *) ~ (Int :: *)
     • In the declaration for pattern synonym ‘T1’
diff --git a/testsuite/tests/polykinds/T15795.stderr b/testsuite/tests/polykinds/T15795.stderr
index 65bfdddecc..20fb997e4c 100644
--- a/testsuite/tests/polykinds/T15795.stderr
+++ b/testsuite/tests/polykinds/T15795.stderr
@@ -1,5 +1,5 @@
 
-T15795.hs:12:3: error:
+T15795.hs:12:3: error: [GHC-71451]
     • 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
diff --git a/testsuite/tests/polykinds/T15795a.stderr b/testsuite/tests/polykinds/T15795a.stderr
index f4757137ce..00fa09ac82 100644
--- a/testsuite/tests/polykinds/T15795a.stderr
+++ b/testsuite/tests/polykinds/T15795a.stderr
@@ -1,5 +1,5 @@
 
-T15795a.hs:9:3: error:
+T15795a.hs:9:3: error: [GHC-71451]
     • 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
diff --git a/testsuite/tests/typecheck/no_skolem_info/T14040A.stderr b/testsuite/tests/typecheck/no_skolem_info/T14040A.stderr
index 554a4e0a32..176b198b99 100644
--- a/testsuite/tests/typecheck/no_skolem_info/T14040A.stderr
+++ b/testsuite/tests/typecheck/no_skolem_info/T14040A.stderr
@@ -1,5 +1,5 @@
 
-T14040A.hs:13:8: error:
+T14040A.hs:13:8: error: [GHC-71451]
     • 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 -> *).
diff --git a/testsuite/tests/typecheck/should_fail/T14350.stderr b/testsuite/tests/typecheck/should_fail/T14350.stderr
index 955554ab8d..4adc54c497 100644
--- a/testsuite/tests/typecheck/should_fail/T14350.stderr
+++ b/testsuite/tests/typecheck/should_fail/T14350.stderr
@@ -1,5 +1,5 @@
 
-T14350.hs:49:10: error:
+T14350.hs:49:10: error: [GHC-71451]
     • Cannot generalise type; skolem ‘a’ would escape its scope
       if I tried to quantify (x0 :: a) in this type:
         forall a (b1 :: a ~> *)
diff --git a/testsuite/tests/typecheck/should_fail/T15474.stderr b/testsuite/tests/typecheck/should_fail/T15474.stderr
index 98f64213d9..23ae019c77 100644
--- a/testsuite/tests/typecheck/should_fail/T15474.stderr
+++ b/testsuite/tests/typecheck/should_fail/T15474.stderr
@@ -1,5 +1,5 @@
 
-T15474.hs:10:1: error:
+T15474.hs:10:1: error: [GHC-16220]
     • Uninferrable type variable k0 in
       the type synonym right-hand side: forall (t :: k0). Proxy @{k0} t
     • In the type declaration for ‘Forall’
diff --git a/testsuite/tests/typecheck/should_fail/T15807.stderr b/testsuite/tests/typecheck/should_fail/T15807.stderr
index 8589ec1268..5ef0fe2ef8 100644
--- a/testsuite/tests/typecheck/should_fail/T15807.stderr
+++ b/testsuite/tests/typecheck/should_fail/T15807.stderr
@@ -1,5 +1,5 @@
 
-T15807.hs:12:3: error:
+T15807.hs:12:3: error: [GHC-71451]
     • Cannot generalise type; skolem ‘f’ would escape its scope
       if I tried to quantify (f0 :: f -> *) in this type:
         forall f (a :: f). f a %1 -> App @f @f0 a
diff --git a/testsuite/tests/typecheck/should_fail/T16946.stderr b/testsuite/tests/typecheck/should_fail/T16946.stderr
index a923fe778e..c26e4fb339 100644
--- a/testsuite/tests/typecheck/should_fail/T16946.stderr
+++ b/testsuite/tests/typecheck/should_fail/T16946.stderr
@@ -1,5 +1,5 @@
 
-T16946.hs:11:9: error:
+T16946.hs:11:9: error: [GHC-71451]
     • Cannot generalise type; skolem ‘k’ would escape its scope
       if I tried to quantify (y0 :: k) in this type:
         forall k (c :: k -> k -> *)
diff --git a/testsuite/tests/typecheck/should_fail/T17301.stderr b/testsuite/tests/typecheck/should_fail/T17301.stderr
index 4c1c693bc1..ed6cf9c59e 100644
--- a/testsuite/tests/typecheck/should_fail/T17301.stderr
+++ b/testsuite/tests/typecheck/should_fail/T17301.stderr
@@ -1,5 +1,6 @@
 
-T17301.hs:22:3: error:
+T17301.hs:22:3: error: [GHC-16220]
     • Uninferrable type variable (a0 :: A) in
-      type family equation right-hand side: MkATySing @(B a0) (SB @a0)
+      the type family equation right-hand side:
+      MkATySing @(B a0) (SB @a0)
     • In the type family declaration for ‘Forget’
diff --git a/testsuite/tests/typecheck/should_fail/T17562.stderr b/testsuite/tests/typecheck/should_fail/T17562.stderr
index 22086bdd0d..24e4bc17bd 100644
--- a/testsuite/tests/typecheck/should_fail/T17562.stderr
+++ b/testsuite/tests/typecheck/should_fail/T17562.stderr
@@ -1,5 +1,5 @@
 
-T17562.hs:7:1: error:
+T17562.hs:7:1: error: [GHC-16220]
     • Uninferrable type variable k0 in
       the class context: forall (a :: k -> k0). (a b :: k0) ~ (a c :: k0)
     • In the class declaration for ‘C’
diff --git a/testsuite/tests/typecheck/should_fail/T17567.stderr b/testsuite/tests/typecheck/should_fail/T17567.stderr
index 75b4de2960..841a91eac0 100644
--- a/testsuite/tests/typecheck/should_fail/T17567.stderr
+++ b/testsuite/tests/typecheck/should_fail/T17567.stderr
@@ -1,5 +1,5 @@
 
-T17567.hs:7:1: error:
+T17567.hs:7:1: error: [GHC-16220]
     • Uninferrable type variable k0 in
       the type synonym right-hand side: forall (a :: k0). Proxy @{k0} a
     • In the type declaration for ‘T’
diff --git a/testsuite/tests/typecheck/should_fail/T17567StupidTheta.stderr b/testsuite/tests/typecheck/should_fail/T17567StupidTheta.stderr
index 425ac8728b..376f9733c0 100644
--- a/testsuite/tests/typecheck/should_fail/T17567StupidTheta.stderr
+++ b/testsuite/tests/typecheck/should_fail/T17567StupidTheta.stderr
@@ -2,7 +2,7 @@
 T17567StupidTheta.hs:1:37: warning: [-Wdeprecated-flags (in -Wdefault)]
     -XDatatypeContexts is deprecated: It was widely considered a misfeature, and has been removed from the Haskell language.
 
-T17567StupidTheta.hs:6:1: error:
+T17567StupidTheta.hs:6:1: error: [GHC-16220]
     • Uninferrable type variable k0 in
       the datatype context:
       forall (a :: k -> k0). (a b :: k0) ~ (a c :: k0)