Improve overlap error for polykinded constraints

There were two problems around `mkDictErr`:

  1. An outdated call to `flattenTys` meant that we missed out on some
     instances. As we no longer flatten type-family applications,
     the logic is obsolete and can be removed.

  2. We reported "out of scope" errors in a poly-kinded situation
     because `BoxedRep` and `Lifted` were considered out of scope.
     We fix this by using `pretendNameIsInScope`.

fixes #20465

31 files changed, 141 insertions, 102 deletions

diff --git a/compiler/GHC/Builtin/Names.hs b/compiler/GHC/Builtin/Names.hs
index b7f54060b9..364d7a913d 100644
--- a/compiler/GHC/Builtin/Names.hs
+++ b/compiler/GHC/Builtin/Names.hs
@@ -2794,17 +2794,41 @@ interactiveClassKeys = map getUnique interactiveClassNames
 *                                                                      *
 ************************************************************************
 
-GHCi's :info command will usually filter out instances mentioning types whose
-names are not in scope. GHCi makes an exception for some commonly used names,
-such as Data.Kind.Type, which may not actually be in scope but should be
-treated as though they were in scope. The list in the definition of
-pretendNameIsInScope below contains these commonly used names.
+Note [pretendNameIsInScope]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~
+In general, we filter out instances that mention types whose names are
+not in scope. However, in the situations listed below, we make an exception
+for some commonly used names, such as Data.Kind.Type, which may not actually
+be in scope but should be treated as though they were in scope.
+This includes built-in names, as well as a few extra names such as
+'Type', 'TYPE', 'BoxedRep', etc.
 
+Situations in which we apply this special logic:
+
+  - GHCi's :info command, see GHC.Runtime.Eval.getInfo.
+    This fixes #1581.
+
+  - When reporting instance overlap errors. Not doing so could mean
+    that we would omit instances for typeclasses like
+
+      type Cls :: k -> Constraint
+      class Cls a
+
+    because BoxedRep/Lifted were not in scope.
+    See GHC.Tc.Errors.pprPotentials.
+    This fixes one of the issues reported in #20465.
 -}
 
+-- | Should this name be considered in-scope, even though it technically isn't?
+--
+-- This ensures that we don't filter out information because, e.g.,
+-- Data.Kind.Type isn't imported.
+--
+-- See Note [pretendNameIsInScope].
 pretendNameIsInScope :: Name -> Bool
 pretendNameIsInScope n
-  = any (n `hasKey`)
+  = isBuiltInSyntax n
+  || any (n `hasKey`)
     [ liftedTypeKindTyConKey, unliftedTypeKindTyConKey
     , liftedDataConKey, unliftedDataConKey
     , tYPETyConKey
diff --git a/compiler/GHC/Runtime/Eval.hs b/compiler/GHC/Runtime/Eval.hs
index e28b2daeba..bceb9a4159 100644
--- a/compiler/GHC/Runtime/Eval.hs
+++ b/compiler/GHC/Runtime/Eval.hs
@@ -81,7 +81,6 @@ import GHC.Tc.Types.Constraint
 import GHC.Tc.Types.Origin
 
 import GHC.Builtin.Names ( toDynName, pretendNameIsInScope )
-import GHC.Builtin.Types ( isCTupleTyConName )
 
 import GHC.Data.Maybe
 import GHC.Data.FastString
@@ -873,8 +872,7 @@ getInfo allInfo name
           ok n | n == name              = True
                        -- The one we looked for in the first place!
                | pretendNameIsInScope n = True
-               | isBuiltInSyntax n      = True
-               | isCTupleTyConName n    = True
+                   -- See Note [pretendNameIsInScope] in GHC.Builtin.Names
                | isExternalName n       = isJust (lookupGRE_Name rdr_env n)
                | otherwise              = True
 
diff --git a/compiler/GHC/Tc/Errors.hs b/compiler/GHC/Tc/Errors.hs
index 51ab0fca2a..e420bd1c23 100644
--- a/compiler/GHC/Tc/Errors.hs
+++ b/compiler/GHC/Tc/Errors.hs
@@ -50,7 +50,7 @@ import GHC.Types.Error
 import GHC.Rename.Unbound ( unknownNameSuggestions, WhatLooking(..) )
 import GHC.Unit.Module
 import GHC.Hs.Binds ( PatSynBind(..) )
-import GHC.Builtin.Names ( typeableClassName )
+import GHC.Builtin.Names ( typeableClassName, pretendNameIsInScope )
 import qualified GHC.LanguageExtensions as LangExt
 
 import GHC.Core.Predicate
@@ -58,7 +58,7 @@ import GHC.Core.Type
 import GHC.Core.Coercion
 import GHC.Core.TyCo.Rep
 import GHC.Core.TyCo.Ppr  ( pprTyVars, pprWithExplicitKindsWhen, pprSourceTyCon, pprWithTYPE )
-import GHC.Core.Unify     ( tcMatchTys, flattenTys )
+import GHC.Core.Unify     ( tcMatchTys )
 import GHC.Core.InstEnv
 import GHC.Core.TyCon
 import GHC.Core.Class
@@ -2420,8 +2420,7 @@ mkDictErr ctxt cts
       && (null unifiers || all (not . isAmbiguousTyVar) (tyCoVarsOfCtList ct))
 
     lookup_cls_inst inst_envs ct
-                -- Note [Flattening in error message generation]
-      = (ct, lookupInstEnv True inst_envs clas (flattenTys emptyInScopeSet tys))
+      = (ct, lookupInstEnv True inst_envs clas tys)
       where
         (clas, tys) = getClassPredTys (ctPred ct)
 
@@ -2862,8 +2861,8 @@ pprPotentials (PrintPotentialInstances show_potentials) sty herald insts
                              orphNamesOfTypes (is_tys cls_inst)
 
     name_in_scope name
-      | isBuiltInSyntax name
-      = True -- E.g. (->)
+      | pretendNameIsInScope name
+      = True -- E.g. (->); see Note [pretendNameIsInScope] in GHC.Builtin.Names
       | Just mod <- nameModule_maybe name
       = qual_in_scope (qualName sty mod (nameOccName name))
       | otherwise
@@ -2897,7 +2896,7 @@ we want to give it a bit of structure.  Here's the plan
   These are the ones most likely to be useful to the programmer.
 
 * Show at most n_show in-scope instances,
-  and summarise the rest ("plus 3 others")
+  and summarise the rest ("plus N others")
 
 * Summarise the not-in-scope instances ("plus 4 not in scope")
 
@@ -2906,18 +2905,6 @@ we want to give it a bit of structure.  Here's the plan
 -}
 
 {-
-Note [Flattening in error message generation]
-~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
-Consider (C (Maybe (F x))), where F is a type function, and we have
-instances
-                C (Maybe Int) and C (Maybe a)
-Since (F x) might turn into Int, this is an overlap situation, and
-indeed the main solver will have refrained
-from solving.  But by the time we get to error message generation, we've
-un-flattened the constraint.  So we must *re*-flatten it before looking
-up in the instance environment, lest we only report one matching
-instance when in fact there are two.
-
 Note [Kind arguments in error messages]
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 It can be terribly confusing to get an error message like (#9171)
diff --git a/testsuite/driver/testlib.py b/testsuite/driver/testlib.py
index fba69533e0..7a1fafbbd7 100644
--- a/testsuite/driver/testlib.py
+++ b/testsuite/driver/testlib.py
@@ -2232,9 +2232,11 @@ def normalise_errmsg(s: str) -> str:
     # Error messages sometimes contain ghc-bignum implementation package
     s = re.sub('ghc-bignum-[0-9.]+', 'ghc-bignum-<VERSION>', s)
 
-    # Error messages sometimes contain this blurb which can vary
+    # Error messages sometimes contain these blurbs which can vary
     # spuriously depending upon build configuration (e.g. based on bignum
     # backend)
+    s = re.sub('...plus ([a-z]+|[0-9]+) others',
+                 '...plus N others', s)
     s = re.sub('...plus ([a-z]+|[0-9]+) instances involving out-of-scope types',
                  '...plus N instances involving out-of-scope types', s)
 
diff --git a/testsuite/tests/ado/T13242a.stderr b/testsuite/tests/ado/T13242a.stderr
index f5dce341ac..4ada4ecaed 100644
--- a/testsuite/tests/ado/T13242a.stderr
+++ b/testsuite/tests/ado/T13242a.stderr
@@ -29,8 +29,8 @@ T13242a.hs:13:13: error:
         instance Eq Ordering -- Defined in ‘GHC.Classes’
         instance Eq Integer -- Defined in ‘GHC.Num.Integer’
         instance Eq () -- Defined in ‘GHC.Classes’
-        ...plus 22 others
-        ...plus five instances involving out-of-scope types
+        ...plus N others
+        ...plus N instances involving out-of-scope types
         (use -fprint-potential-instances to see them all)
     • In a stmt of a 'do' block: return (x == x)
       In the expression:
diff --git a/testsuite/tests/annotations/should_fail/annfail10.stderr b/testsuite/tests/annotations/should_fail/annfail10.stderr
index a6e767f9e5..bd28fb4b17 100644
--- a/testsuite/tests/annotations/should_fail/annfail10.stderr
+++ b/testsuite/tests/annotations/should_fail/annfail10.stderr
@@ -10,8 +10,8 @@ annfail10.hs:9:1: error:
         instance Data.Data.Data Ordering -- Defined in ‘Data.Data’
         instance Data.Data.Data a => Data.Data.Data (Maybe a)
           -- Defined in ‘Data.Data’
-        ...plus 16 others
-        ...plus 50 instances involving out-of-scope types
+        ...plus N others
+        ...plus N instances involving out-of-scope types
         (use -fprint-potential-instances to see them all)
     • In the annotation: {-# ANN f 1 #-}
 
@@ -23,7 +23,7 @@ annfail10.hs:9:11: error:
         instance Num Integer -- Defined in ‘GHC.Num’
         instance Num Double -- Defined in ‘GHC.Float’
         instance Num Float -- Defined in ‘GHC.Float’
-        ...plus two others
-        ...plus 19 instances involving out-of-scope types
+        ...plus N others
+        ...plus N instances involving out-of-scope types
         (use -fprint-potential-instances to see them all)
     • In the annotation: {-# ANN f 1 #-}
diff --git a/testsuite/tests/ghci.debugger/scripts/break006.stderr b/testsuite/tests/ghci.debugger/scripts/break006.stderr
index 1b97299d92..198bc0df49 100644
--- a/testsuite/tests/ghci.debugger/scripts/break006.stderr
+++ b/testsuite/tests/ghci.debugger/scripts/break006.stderr
@@ -8,8 +8,8 @@
         instance Show Ordering -- Defined in ‘GHC.Show’
         instance Show a => Show (Maybe a) -- Defined in ‘GHC.Show’
         instance Show Integer -- Defined in ‘GHC.Show’
-        ...plus 23 others
-        ...plus 12 instances involving out-of-scope types
+        ...plus N others
+        ...plus N instances involving out-of-scope types
         (use -fprint-potential-instances to see them all)
     • In a stmt of an interactive GHCi command: print it
 
@@ -22,7 +22,7 @@
         instance Show Ordering -- Defined in ‘GHC.Show’
         instance Show a => Show (Maybe a) -- Defined in ‘GHC.Show’
         instance Show Integer -- Defined in ‘GHC.Show’
-        ...plus 23 others
-        ...plus 12 instances involving out-of-scope types
+        ...plus N others
+        ...plus N instances involving out-of-scope types
         (use -fprint-potential-instances to see them all)
     • In a stmt of an interactive GHCi command: print it
diff --git a/testsuite/tests/ghci.debugger/scripts/print019.stderr b/testsuite/tests/ghci.debugger/scripts/print019.stderr
index bac95541f4..09b1bfbd8c 100644
--- a/testsuite/tests/ghci.debugger/scripts/print019.stderr
+++ b/testsuite/tests/ghci.debugger/scripts/print019.stderr
@@ -8,7 +8,7 @@
         instance Show Ordering -- Defined in ‘GHC.Show’
         instance Show TyCon -- Defined in ‘GHC.Show’
         instance Show a => Show (List1 a) -- Defined at Test.hs:11:12
-        ...plus 30 others
-        ...plus 13 instances involving out-of-scope types
+        ...plus N others
+        ...plus N instances involving out-of-scope types
         (use -fprint-potential-instances to see them all)
     • In a stmt of an interactive GHCi command: print it
diff --git a/testsuite/tests/ghci/scripts/T10963.stderr b/testsuite/tests/ghci/scripts/T10963.stderr
index aa081391c4..7082d6e6cf 100644
--- a/testsuite/tests/ghci/scripts/T10963.stderr
+++ b/testsuite/tests/ghci/scripts/T10963.stderr
@@ -7,7 +7,7 @@
         instance Num Integer -- Defined in ‘GHC.Num’
         instance Num Double -- Defined in ‘GHC.Float’
         instance Num Float -- Defined in ‘GHC.Float’
-        ...plus two others
-        ...plus 8 instances involving out-of-scope types
+        ...plus N others
+        ...plus N instances involving out-of-scope types
         (use -fprint-potential-instances to see them all)
     • In the expression: foo
diff --git a/testsuite/tests/ghci/scripts/T15325.stderr b/testsuite/tests/ghci/scripts/T15325.stderr
index c767528e2c..99efb5585e 100644
--- a/testsuite/tests/ghci/scripts/T15325.stderr
+++ b/testsuite/tests/ghci/scripts/T15325.stderr
@@ -12,7 +12,7 @@ T15325.hs:11:9: warning: [-Wdeferred-type-errors (in -Wdefault)]
         instance Num Integer -- Defined in ‘GHC.Num’
         instance Num Double -- Defined in ‘GHC.Float’
         instance Num Float -- Defined in ‘GHC.Float’
-        ...plus two others
+        ...plus N others
         ...plus one instance involving out-of-scope types
         (use -fprint-potential-instances to see them all)
     • In the first argument of ‘f’, namely ‘0’
diff --git a/testsuite/tests/indexed-types/should_fail/T12522a.stderr b/testsuite/tests/indexed-types/should_fail/T12522a.stderr
index a91c31c764..7fc4787a5c 100644
--- a/testsuite/tests/indexed-types/should_fail/T12522a.stderr
+++ b/testsuite/tests/indexed-types/should_fail/T12522a.stderr
@@ -10,8 +10,8 @@ T12522a.hs:23:26: error:
         instance Show Ordering -- Defined in ‘GHC.Show’
         instance Show a => Show (Maybe a) -- Defined in ‘GHC.Show’
         instance Show Integer -- Defined in ‘GHC.Show’
-        ...plus 23 others
-        ...plus 12 instances involving out-of-scope types
+        ...plus N others
+        ...plus N instances involving out-of-scope types
         (use -fprint-potential-instances to see them all)
     • In the first argument of ‘(++)’, namely ‘show n’
       In the second argument of ‘($)’, namely ‘show n ++ s’
diff --git a/testsuite/tests/indexed-types/should_fail/T20465.hs b/testsuite/tests/indexed-types/should_fail/T20465.hs
new file mode 100644
index 0000000000..8ec790b619
--- /dev/null
+++ b/testsuite/tests/indexed-types/should_fail/T20465.hs
@@ -0,0 +1,17 @@
+{-# LANGUAGE FlexibleContexts #-}
+{-# LANGUAGE FlexibleInstances #-}
+{-# LANGUAGE PolyKinds #-}
+{-# LANGUAGE TypeFamilies #-}
+
+module T20465 where
+
+import Data.Kind
+import Data.Proxy
+
+class Cls (a :: (Type -> Constraint) -> Type)
+instance Cls a
+instance Cls Proxy
+foo :: Cls Proxy => Int
+foo = 42
+bar :: Int
+bar = foo
diff --git a/testsuite/tests/indexed-types/should_fail/T20465.stderr b/testsuite/tests/indexed-types/should_fail/T20465.stderr
new file mode 100644
index 0000000000..90e90c9f57
--- /dev/null
+++ b/testsuite/tests/indexed-types/should_fail/T20465.stderr
@@ -0,0 +1,8 @@
+
+T20465.hs:17:7: error:
+    • Overlapping instances for Cls Proxy arising from a use of ‘foo’
+      Matching instances:
+        instance Cls a -- Defined at T20465.hs:12:10
+        instance Cls Proxy -- Defined at T20465.hs:13:10
+    • In the expression: foo
+      In an equation for ‘bar’: bar = foo
diff --git a/testsuite/tests/indexed-types/should_fail/all.T b/testsuite/tests/indexed-types/should_fail/all.T
index 9d2c68f095..c97c8c56e4 100644
--- a/testsuite/tests/indexed-types/should_fail/all.T
+++ b/testsuite/tests/indexed-types/should_fail/all.T
@@ -164,3 +164,4 @@ test('T13571', normal, compile_fail, [''])
 test('T13571a', normal, compile_fail, [''])
 test('T18648', normal, compile_fail, [''])
 test('ExpandTFs', normal, compile_fail, [''])
+test('T20465', normal, compile_fail, [''])
diff --git a/testsuite/tests/overloadedlists/should_fail/overloadedlistsfail01.stderr b/testsuite/tests/overloadedlists/should_fail/overloadedlistsfail01.stderr
index 6e759d7f2a..5d391c4d9e 100644
--- a/testsuite/tests/overloadedlists/should_fail/overloadedlistsfail01.stderr
+++ b/testsuite/tests/overloadedlists/should_fail/overloadedlistsfail01.stderr
@@ -7,8 +7,8 @@ overloadedlistsfail01.hs:5:8: error:
         instance Show Ordering -- Defined in ‘GHC.Show’
         instance Show a => Show (Maybe a) -- Defined in ‘GHC.Show’
         instance Show Integer -- Defined in ‘GHC.Show’
-        ...plus 23 others
-        ...plus 14 instances involving out-of-scope types
+        ...plus N others
+        ...plus N instances involving out-of-scope types
         (use -fprint-potential-instances to see them all)
     • In the expression: print [1]
       In an equation for ‘main’: main = print [1]
@@ -19,7 +19,7 @@ overloadedlistsfail01.hs:5:14: error:
       Probable fix: use a type annotation to specify what ‘a0’ should be.
       These potential instances exist:
         instance GHC.Exts.IsList [a] -- Defined in ‘GHC.Exts’
-        ...plus four instances involving out-of-scope types
+        ...plus N instances involving out-of-scope types
         (use -fprint-potential-instances to see them all)
     • In the first argument of ‘print’, namely ‘[1]’
       In the expression: print [1]
@@ -34,7 +34,7 @@ overloadedlistsfail01.hs:5:15: error:
         instance Num Integer -- Defined in ‘GHC.Num’
         instance Num Double -- Defined in ‘GHC.Float’
         instance Num Float -- Defined in ‘GHC.Float’
-        ...plus two others
+        ...plus N others
         ...plus one instance involving out-of-scope types
         (use -fprint-potential-instances to see them all)
     • In the expression: 1
diff --git a/testsuite/tests/parser/should_fail/RecordDotSyntaxFail11.stderr b/testsuite/tests/parser/should_fail/RecordDotSyntaxFail11.stderr
index 2378585a6a..4c26d77b10 100644
--- a/testsuite/tests/parser/should_fail/RecordDotSyntaxFail11.stderr
+++ b/testsuite/tests/parser/should_fail/RecordDotSyntaxFail11.stderr
@@ -1,24 +1,25 @@
-RecordDotSyntaxFail11.hs:8:3:
-     Ambiguous type variable ‘a0’ arising from a use of ‘print’
+
+RecordDotSyntaxFail11.hs:8:3: error:
+    • Ambiguous type variable ‘a0’ arising from a use of ‘print’
       prevents the constraint ‘(Show a0)’ from being solved.
       Probable fix: use a type annotation to specify what ‘a0’ should be.
       These potential instances exist:
         instance Show Ordering -- Defined in ‘GHC.Show’
         instance Show a => Show (Maybe a) -- Defined in ‘GHC.Show’
         instance Show Integer -- Defined in ‘GHC.Show’
-        ...plus 23 others
+        ...plus N others
         ...plus N instances involving out-of-scope types
         (use -fprint-potential-instances to see them all)
-     In the first argument of ‘($)’, namely ‘print’
+    • In the first argument of ‘($)’, namely ‘print’
       In a stmt of a 'do' block: print $ (.foo.bar.baz) a
       In the expression:
         do let a = ...
            print $ (.foo.bar.baz) a
 
-RecordDotSyntaxFail11.hs:8:11:
-     No instance for (GHC.Records.HasField "baz" Int a0)
+RecordDotSyntaxFail11.hs:8:11: error:
+    • No instance for (GHC.Records.HasField "baz" Int a0)
         arising from a use of ‘GHC.Records.getField’
-     In the second argument of ‘($)’, namely ‘(.foo.bar.baz) a’
+    • In the second argument of ‘($)’, namely ‘(.foo.bar.baz) a’
       In a stmt of a 'do' block: print $ (.foo.bar.baz) a
       In the expression:
         do let a = ...
diff --git a/testsuite/tests/parser/should_fail/RecordDotSyntaxFail8.stderr b/testsuite/tests/parser/should_fail/RecordDotSyntaxFail8.stderr
index 8bf921b79f..e66a9dfb28 100644
--- a/testsuite/tests/parser/should_fail/RecordDotSyntaxFail8.stderr
+++ b/testsuite/tests/parser/should_fail/RecordDotSyntaxFail8.stderr
@@ -1,24 +1,25 @@
-RecordDotSyntaxFail8.hs:37:3:
-     Ambiguous type variable ‘a0’ arising from a use of ‘print’
+
+RecordDotSyntaxFail8.hs:37:3: error:
+    • Ambiguous type variable ‘a0’ arising from a use of ‘print’
       prevents the constraint ‘(Show a0)’ from being solved.
       Probable fix: use a type annotation to specify what ‘a0’ should be.
       These potential instances exist:
         instance Show Ordering -- Defined in ‘GHC.Show’
         instance Show Bar -- Defined at RecordDotSyntaxFail8.hs:22:41
         instance Show Baz -- Defined at RecordDotSyntaxFail8.hs:27:42
-        ...plus 27 others
+        ...plus N others
         ...plus N instances involving out-of-scope types
         (use -fprint-potential-instances to see them all)
-     In the first argument of ‘($)’, namely ‘print’
+    • In the first argument of ‘($)’, namely ‘print’
       In a stmt of a 'do' block: print $ ....baz.quux
       In the expression:
         do let a = ...
            print $ ....quux
 
-RecordDotSyntaxFail8.hs:37:11:
-     No instance for (HasField "quux" Quux a0)
+RecordDotSyntaxFail8.hs:37:11: error:
+    • No instance for (HasField "quux" Quux a0)
         arising from selecting the field ‘quux’
-     In the second argument of ‘($)’, namely ‘....baz.quux’
+    • In the second argument of ‘($)’, namely ‘....baz.quux’
       In a stmt of a 'do' block: print $ ....baz.quux
       In the expression:
         do let a = ...
diff --git a/testsuite/tests/partial-sigs/should_fail/T10999.stderr b/testsuite/tests/partial-sigs/should_fail/T10999.stderr
index 71bab83508..22df588742 100644
--- a/testsuite/tests/partial-sigs/should_fail/T10999.stderr
+++ b/testsuite/tests/partial-sigs/should_fail/T10999.stderr
@@ -25,8 +25,8 @@ T10999.hs:8:28: error:
         instance Ord a => Ord (Set.Set a) -- Defined in ‘Data.Set.Internal’
         instance Ord Ordering -- Defined in ‘GHC.Classes’
         instance Ord Integer -- Defined in ‘GHC.Num.Integer’
-        ...plus 23 others
-        ...plus two instances involving out-of-scope types
+        ...plus N others
+        ...plus N instances involving out-of-scope types
         (use -fprint-potential-instances to see them all)
     • In the second argument of ‘($)’, namely ‘f ()’
       In the second argument of ‘($)’, namely ‘Set.toList $ f ()’
diff --git a/testsuite/tests/polykinds/T13393.stderr b/testsuite/tests/polykinds/T13393.stderr
index a06aecff70..759f3408b0 100644
--- a/testsuite/tests/polykinds/T13393.stderr
+++ b/testsuite/tests/polykinds/T13393.stderr
@@ -7,8 +7,8 @@ T13393.hs:61:3: error:
         instance Traversable (Either a) -- Defined in ‘Data.Traversable’
         instance Traversable Identity -- Defined in ‘Data.Traversable’
         instance Traversable Maybe -- Defined in ‘Data.Traversable’
-        ...plus three others
-        ...plus 28 instances involving out-of-scope types
+        ...plus N others
+        ...plus N instances involving out-of-scope types
         (use -fprint-potential-instances to see them all)
     • In a stmt of a 'do' block:
         mapM putBackLeftOverInputAndReturnOutput undefined
diff --git a/testsuite/tests/typecheck/should_compile/T14273.stderr b/testsuite/tests/typecheck/should_compile/T14273.stderr
index daff685704..7aa78d56a9 100644
--- a/testsuite/tests/typecheck/should_compile/T14273.stderr
+++ b/testsuite/tests/typecheck/should_compile/T14273.stderr
@@ -11,8 +11,8 @@ T14273.hs:7:27: warning: [-Wdeferred-type-errors (in -Wdefault)]
           -- Defined in ‘Data.Either’
         instance Show Ordering -- Defined in ‘GHC.Show’
         instance Show a => Show (Maybe a) -- Defined in ‘GHC.Show’
-        ...plus 24 others
-        ...plus 70 instances involving out-of-scope types
+        ...plus N others
+        ...plus N instances involving out-of-scope types
         (use -fprint-potential-instances to see them all)
     • In the first argument of ‘Just’, namely ‘(show _a)’
       In the expression: Just (show _a)
@@ -65,8 +65,8 @@ T14273.hs:13:10: warning: [-Wdeferred-type-errors (in -Wdefault)]
           -- Defined in ‘Data.Either’
         instance Show Ordering -- Defined in ‘GHC.Show’
         instance Show a => Show (Maybe a) -- Defined in ‘GHC.Show’
-        ...plus 24 others
-        ...plus 70 instances involving out-of-scope types
+        ...plus N others
+        ...plus N instances involving out-of-scope types
         (use -fprint-potential-instances to see them all)
     • In the expression: show (_h ++ [])
       In an equation for ‘foo’: foo xs = show (_h ++ [])
diff --git a/testsuite/tests/typecheck/should_compile/holes2.stderr b/testsuite/tests/typecheck/should_compile/holes2.stderr
index 1e30e87882..a88ea524e0 100644
--- a/testsuite/tests/typecheck/should_compile/holes2.stderr
+++ b/testsuite/tests/typecheck/should_compile/holes2.stderr
@@ -8,8 +8,8 @@ holes2.hs:3:5: warning: [-Wdeferred-type-errors (in -Wdefault)]
           -- Defined in ‘Data.Either’
         instance Show Ordering -- Defined in ‘GHC.Show’
         instance Show a => Show (Maybe a) -- Defined in ‘GHC.Show’
-        ...plus 24 others
-        ...plus 70 instances involving out-of-scope types
+        ...plus N others
+        ...plus N instances involving out-of-scope types
         (use -fprint-potential-instances to see them all)
     • In the expression: show _
       In an equation for ‘f’: f = show _
diff --git a/testsuite/tests/typecheck/should_fail/T10971b.stderr b/testsuite/tests/typecheck/should_fail/T10971b.stderr
index 0947ab1b6f..1a6104d501 100644
--- a/testsuite/tests/typecheck/should_fail/T10971b.stderr
+++ b/testsuite/tests/typecheck/should_fail/T10971b.stderr
@@ -10,8 +10,8 @@ T10971b.hs:4:11: error:
         instance Foldable (Either a) -- Defined in ‘Data.Foldable’
         instance Foldable Maybe -- Defined in ‘Data.Foldable’
         instance Foldable ((,) a) -- Defined in ‘Data.Foldable’
-        ...plus two others
-        ...plus 29 instances involving out-of-scope types
+        ...plus N others
+        ...plus N instances involving out-of-scope types
         (use -fprint-potential-instances to see them all)
     • In the expression: length x
       In the expression: \ x -> length x
@@ -28,8 +28,8 @@ T10971b.hs:5:13: error:
         instance Traversable (Either a) -- Defined in ‘Data.Traversable’
         instance Traversable Maybe -- Defined in ‘Data.Traversable’
         instance Traversable ((,) a) -- Defined in ‘Data.Traversable’
-        ...plus two others
-        ...plus 29 instances involving out-of-scope types
+        ...plus N others
+        ...plus N instances involving out-of-scope types
         (use -fprint-potential-instances to see them all)
     • In the expression: fmapDefault f x
       In the expression: \ f x -> fmapDefault f x
@@ -46,8 +46,8 @@ T10971b.hs:6:14: error:
         instance Traversable (Either a) -- Defined in ‘Data.Traversable’
         instance Traversable Maybe -- Defined in ‘Data.Traversable’
         instance Traversable ((,) a) -- Defined in ‘Data.Traversable’
-        ...plus two others
-        ...plus 29 instances involving out-of-scope types
+        ...plus N others
+        ...plus N instances involving out-of-scope types
         (use -fprint-potential-instances to see them all)
     • In the expression: fmapDefault f x
       In the expression: (fmapDefault f x, length x)
@@ -64,8 +64,8 @@ T10971b.hs:6:31: error:
         instance Foldable (Either a) -- Defined in ‘Data.Foldable’
         instance Foldable Maybe -- Defined in ‘Data.Foldable’
         instance Foldable ((,) a) -- Defined in ‘Data.Foldable’
-        ...plus two others
-        ...plus 29 instances involving out-of-scope types
+        ...plus N others
+        ...plus N instances involving out-of-scope types
         (use -fprint-potential-instances to see them all)
     • In the expression: length x
       In the expression: (fmapDefault f x, length x)
diff --git a/testsuite/tests/typecheck/should_fail/T12921.stderr b/testsuite/tests/typecheck/should_fail/T12921.stderr
index 478d2f03c8..b3de93e2f4 100644
--- a/testsuite/tests/typecheck/should_fail/T12921.stderr
+++ b/testsuite/tests/typecheck/should_fail/T12921.stderr
@@ -10,8 +10,8 @@ T12921.hs:4:1: error:
         instance Data.Data.Data Ordering -- Defined in ‘Data.Data’
         instance Data.Data.Data a => Data.Data.Data (Maybe a)
           -- Defined in ‘Data.Data’
-        ...plus 16 others
-        ...plus 50 instances involving out-of-scope types
+        ...plus N others
+        ...plus N instances involving out-of-scope types
         (use -fprint-potential-instances to see them all)
     • In the annotation:
         {-# ANN module "HLint: ignore Reduce duplication" #-}
@@ -24,7 +24,7 @@ T12921.hs:4:16: error:
       These potential instances exist:
         instance (a ~ Char) => Data.String.IsString [a]
           -- Defined in ‘Data.String’
-        ...plus two instances involving out-of-scope types
+        ...plus N instances involving out-of-scope types
         (use -fprint-potential-instances to see them all)
     • In the annotation:
         {-# ANN module "HLint: ignore Reduce duplication" #-}
diff --git a/testsuite/tests/typecheck/should_fail/T13292.stderr b/testsuite/tests/typecheck/should_fail/T13292.stderr
index a3a7ba3bae..2cc7bb41c3 100644
--- a/testsuite/tests/typecheck/should_fail/T13292.stderr
+++ b/testsuite/tests/typecheck/should_fail/T13292.stderr
@@ -8,9 +8,9 @@ T13292a.hs:4:12: warning: [-Wdeferred-type-errors (in -Wdefault)]
       These potential instances exist:
         instance Monad IO -- Defined in ‘GHC.Base’
         instance Monad Maybe -- Defined in ‘GHC.Base’
-        instance Monoid a => Monad ((,) a) -- Defined in ‘GHC.Base’
-        ...plus four others
-        ...plus two instances involving out-of-scope types
+        instance Monad ((->) r) -- Defined in ‘GHC.Base’
+        ...plus N others
+        ...plus one instance involving out-of-scope types
         (use -fprint-potential-instances to see them all)
     • In the expression: return ()
       In an equation for ‘someFunc’: someFunc = return ()
diff --git a/testsuite/tests/typecheck/should_fail/T14884.stderr b/testsuite/tests/typecheck/should_fail/T14884.stderr
index 2c5abc33f0..e28ad780bf 100644
--- a/testsuite/tests/typecheck/should_fail/T14884.stderr
+++ b/testsuite/tests/typecheck/should_fail/T14884.stderr
@@ -40,8 +40,8 @@ T14884.hs:4:7: error:
           -- Defined in ‘Data.Either’
         instance Show Ordering -- Defined in ‘GHC.Show’
         instance Show a => Show (Maybe a) -- Defined in ‘GHC.Show’
-        ...plus 24 others
-        ...plus 67 instances involving out-of-scope types
+        ...plus N others
+        ...plus N instances involving out-of-scope types
         (use -fprint-potential-instances to see them all)
     • In the first argument of ‘_’, namely ‘print’
       In the expression: _ print "abc"
diff --git a/testsuite/tests/typecheck/should_fail/T5095.stderr b/testsuite/tests/typecheck/should_fail/T5095.stderr
index e30898f74f..3641d18f34 100644
--- a/testsuite/tests/typecheck/should_fail/T5095.stderr
+++ b/testsuite/tests/typecheck/should_fail/T5095.stderr
@@ -5,8 +5,8 @@ T5095.hs:9:11: error:
         instance [overlappable] Show a => Eq a -- Defined at T5095.hs:5:31
         instance Eq Ordering -- Defined in ‘GHC.Classes’
         instance Eq a => Eq (Maybe a) -- Defined in ‘GHC.Maybe’
-        ...plus 24 others
-        ...plus six instances involving out-of-scope types
+        ...plus N others
+        ...plus N instances involving out-of-scope types
         (use -fprint-potential-instances to see them all)
       (The choice depends on the instantiation of ‘a’
        To pick the first instance above, use IncoherentInstances
diff --git a/testsuite/tests/typecheck/should_fail/TyAppPat_PatternBindingExistential.stderr b/testsuite/tests/typecheck/should_fail/TyAppPat_PatternBindingExistential.stderr
index a05806e7be..e316f78fc6 100644
--- a/testsuite/tests/typecheck/should_fail/TyAppPat_PatternBindingExistential.stderr
+++ b/testsuite/tests/typecheck/should_fail/TyAppPat_PatternBindingExistential.stderr
@@ -24,8 +24,8 @@ TyAppPat_PatternBindingExistential.hs:13:3: error:
         instance Show Ordering -- Defined in ‘GHC.Show’
         instance Show a => Show (Maybe a) -- Defined in ‘GHC.Show’
         instance Show Integer -- Defined in ‘GHC.Show’
-        ...plus 23 others
-        ...plus 12 instances involving out-of-scope types
+        ...plus N others
+        ...plus N instances involving out-of-scope types
         (use -fprint-potential-instances to see them all)
     • In a stmt of a 'do' block: print (x :: a)
       In the expression: do print (x :: a)
diff --git a/testsuite/tests/typecheck/should_fail/tcfail008.stderr b/testsuite/tests/typecheck/should_fail/tcfail008.stderr
index 1e7bc19585..974eccb485 100644
--- a/testsuite/tests/typecheck/should_fail/tcfail008.stderr
+++ b/testsuite/tests/typecheck/should_fail/tcfail008.stderr
@@ -8,7 +8,7 @@ tcfail008.hs:3:5: error:
         instance Num Integer -- Defined in ‘GHC.Num’
         instance Num Double -- Defined in ‘GHC.Float’
         instance Num Float -- Defined in ‘GHC.Float’
-        ...plus two others
+        ...plus N others
         ...plus one instance involving out-of-scope types
         (use -fprint-potential-instances to see them all)
     • In the first argument of ‘(:)’, namely ‘1’
diff --git a/testsuite/tests/typecheck/should_fail/tcfail072.stderr b/testsuite/tests/typecheck/should_fail/tcfail072.stderr
index b91f96bf37..c916c92df1 100644
--- a/testsuite/tests/typecheck/should_fail/tcfail072.stderr
+++ b/testsuite/tests/typecheck/should_fail/tcfail072.stderr
@@ -10,8 +10,8 @@ tcfail072.hs:23:13: error:
         instance Ord Ordering -- Defined in ‘GHC.Classes’
         instance Ord Integer -- Defined in ‘GHC.Num.Integer’
         instance Ord () -- Defined in ‘GHC.Classes’
-        ...plus 22 others
-        ...plus two instances involving out-of-scope types
+        ...plus N others
+        ...plus N instances involving out-of-scope types
         (use -fprint-potential-instances to see them all)
     • In the expression: g A
       In an equation for ‘g’: g (B _ _) = g A
diff --git a/testsuite/tests/typecheck/should_fail/tcfail133.stderr b/testsuite/tests/typecheck/should_fail/tcfail133.stderr
index f5e2309a81..004c06a288 100644
--- a/testsuite/tests/typecheck/should_fail/tcfail133.stderr
+++ b/testsuite/tests/typecheck/should_fail/tcfail133.stderr
@@ -11,8 +11,8 @@ tcfail133.hs:68:7: error:
         instance (Number a, Digit b, Show a, Show b) => Show (a :@ b)
           -- Defined at tcfail133.hs:11:54
         instance Show One -- Defined at tcfail133.hs:9:28
-        ...plus 26 others
-        ...plus 12 instances involving out-of-scope types
+        ...plus N others
+        ...plus N instances involving out-of-scope types
         (use -fprint-potential-instances to see them all)
     • In the first argument of ‘($)’, namely ‘show’
       In the expression: show $ add (One :@ Zero) (One :@ One)
diff --git a/testsuite/tests/typecheck/should_fail/tcfail181.stderr b/testsuite/tests/typecheck/should_fail/tcfail181.stderr
index c21214a689..f878123949 100644
--- a/testsuite/tests/typecheck/should_fail/tcfail181.stderr
+++ b/testsuite/tests/typecheck/should_fail/tcfail181.stderr
@@ -9,9 +9,9 @@ tcfail181.hs:17:9: error:
       These potential instances exist:
         instance Monad IO -- Defined in ‘GHC.Base’
         instance Monad Maybe -- Defined in ‘GHC.Base’
-        instance Monoid a => Monad ((,) a) -- Defined in ‘GHC.Base’
-        ...plus four others
-        ...plus two instances involving out-of-scope types
+        instance Monad ((->) r) -- Defined in ‘GHC.Base’
+        ...plus N others
+        ...plus one instance involving out-of-scope types
         (use -fprint-potential-instances to see them all)
     • In the expression: foo
       In the expression: foo {bar = return True}