Fix type check error message grammar (fixes #20122)

Remove trailing spaces

10 files changed, 100 insertions, 55 deletions

diff --git a/compiler/GHC/Tc/Errors.hs b/compiler/GHC/Tc/Errors.hs
index 766b88408d..a504fd44ea 100644
--- a/compiler/GHC/Tc/Errors.hs
+++ b/compiler/GHC/Tc/Errors.hs
@@ -2522,8 +2522,8 @@ mk_dict_err ctxt@(CEC {cec_encl = implics}) (ct, (matches, unifiers, unsafe_over
           = vcat [ ppWhen lead_with_ambig $
                      text "Probable fix: use a type annotation to specify what"
                      <+> pprQuotedList ambig_tvs <+> text "should be."
-                 , text "These potential instance" <> plural unifiers
-                   <+> text "exist:"]
+                 , thisOrThese unifiers <+> text "potential instance" <> plural unifiers
+                   <+> text "exist" <> singular unifiers <> text ":"]
 
         mb_patsyn_prov :: Maybe SDoc
         mb_patsyn_prov
diff --git a/compiler/GHC/Utils/Outputable.hs b/compiler/GHC/Utils/Outputable.hs
index 7d0436f2f2..a6d20a2467 100644
--- a/compiler/GHC/Utils/Outputable.hs
+++ b/compiler/GHC/Utils/Outputable.hs
@@ -44,7 +44,7 @@ module GHC.Utils.Outputable (
         fsep, fcat,
         hang, hangNotEmpty, punctuate, ppWhen, ppUnless,
         ppWhenOption, ppUnlessOption,
-        speakNth, speakN, speakNOf, plural, isOrAre, doOrDoes, itsOrTheir,
+        speakNth, speakN, speakNOf, plural, singular, isOrAre, doOrDoes, itsOrTheir, thisOrThese,
         unicodeSyntax,
 
         coloured, keyword,
@@ -1425,6 +1425,15 @@ plural :: [a] -> SDoc
 plural [_] = empty  -- a bit frightening, but there you are
 plural _   = char 's'
 
+-- | Determines the singular verb suffix appropriate for the length of a list:
+--
+-- > singular [] = empty
+-- > singular["Hello"] = char 's'
+-- > singular ["Hello", "World"] = empty
+singular :: [a] -> SDoc
+singular [_] = char 's'
+singular _   = empty
+
 -- | Determines the form of to be appropriate for the length of a list:
 --
 -- > isOrAre [] = text "are"
@@ -1451,3 +1460,13 @@ doOrDoes _   = text "do"
 itsOrTheir :: [a] -> SDoc
 itsOrTheir [_] = text "its"
 itsOrTheir _   = text "their"
+
+
+-- | Determines the form of subject appropriate for the length of a list:
+--
+-- > thisOrThese [x]   = text "This"
+-- > thisOrThese [x,y] = text "These"
+-- > thisOrThese []    = text "These"  -- probably avoid this
+thisOrThese :: [a] -> SDoc
+thisOrThese [_] = text "This"
+thisOrThese _   = text "These"
\ No newline at end of file
diff --git a/testsuite/tests/typecheck/should_fail/T20122.hs b/testsuite/tests/typecheck/should_fail/T20122.hs
new file mode 100644
index 0000000000..c9e424dbc2
--- /dev/null
+++ b/testsuite/tests/typecheck/should_fail/T20122.hs
@@ -0,0 +1,11 @@
+module T20122 where
+
+class Testable a where
+    test :: a -> Bool
+
+data A = A
+
+instance Testable A where
+    test _ = True
+
+foo = test
diff --git a/testsuite/tests/typecheck/should_fail/T20122.stderr b/testsuite/tests/typecheck/should_fail/T20122.stderr
new file mode 100644
index 0000000000..f3e987ee23
--- /dev/null
+++ b/testsuite/tests/typecheck/should_fail/T20122.stderr
@@ -0,0 +1,12 @@
+
+T20122.hs:11:7: error:
+    • Ambiguous type variable ‘a0’ arising from a use of ‘test’
+      prevents the constraint ‘(Testable a0)’ from being solved.
+      Relevant bindings include
+        foo :: a0 -> Bool (bound at T20122.hs:11:1)
+      Probable fix: use a type annotation to specify what ‘a0’ should be.
+      This potential instance exists:
+        instance Testable A
+          -- Defined at T20122.hs:8:10
+    • In the expression: test
+      In an equation for ‘foo’: foo = test
diff --git a/testsuite/tests/typecheck/should_fail/T4921.stderr b/testsuite/tests/typecheck/should_fail/T4921.stderr
index 8eff919aea..c4510cbb08 100644
--- a/testsuite/tests/typecheck/should_fail/T4921.stderr
+++ b/testsuite/tests/typecheck/should_fail/T4921.stderr
@@ -3,8 +3,9 @@ T4921.hs:10:9: error:
     • Ambiguous type variables ‘a0’, ‘b1’ arising from a use of ‘f’
       prevents the constraint ‘(C a0 b1)’ from being solved.
       Relevant bindings include x :: a0 (bound at T4921.hs:10:1)
-      Probable fix: use a type annotation to specify what ‘a0’, ‘b1’ should be.
-      These potential instance exist:
+      Probable fix: use a type annotation to specify what ‘a0’,
+                                                          ‘b1’ should be.
+      This potential instance exists:
         instance C Int Char -- Defined at T4921.hs:7:10
     • In the first argument of ‘fst’, namely ‘f’
       In the expression: fst f
@@ -14,7 +15,7 @@ T4921.hs:12:9: error:
     • Ambiguous type variable ‘b0’ arising from a use of ‘f’
       prevents the constraint ‘(C Int b0)’ from being solved.
       Probable fix: use a type annotation to specify what ‘b0’ should be.
-      These potential instance exist:
+      This potential instance exists:
         instance C Int Char -- Defined at T4921.hs:7:10
     • In the first argument of ‘fst’, namely ‘f’
       In the expression: fst f :: Int
diff --git a/testsuite/tests/typecheck/should_fail/T5858.stderr b/testsuite/tests/typecheck/should_fail/T5858.stderr
index 829c027135..d5474da8a4 100644
--- a/testsuite/tests/typecheck/should_fail/T5858.stderr
+++ b/testsuite/tests/typecheck/should_fail/T5858.stderr
@@ -3,8 +3,9 @@ T5858.hs:11:7: error:
     • Ambiguous type variables ‘a0’, ‘a1’ arising from a use of ‘infer’
       prevents the constraint ‘(InferOverloaded
                                   ([a0], [a1]))’ from being solved.
-      Probable fix: use a type annotation to specify what ‘a0’, ‘a1’ should be.
-      These potential instance exist:
+      Probable fix: use a type annotation to specify what ‘a0’,
+                                                          ‘a1’ should be.
+      This potential instance exists:
         instance (t1 ~ String) => InferOverloaded (t1, t1)
           -- Defined at T5858.hs:8:10
     • In the expression: infer ([], [])
diff --git a/testsuite/tests/typecheck/should_fail/all.T b/testsuite/tests/typecheck/should_fail/all.T
index 8745ea9172..b573f9b7f6 100644
--- a/testsuite/tests/typecheck/should_fail/all.T
+++ b/testsuite/tests/typecheck/should_fail/all.T
@@ -633,3 +633,4 @@ test('T19615', normal, compile_fail, [''])
 test('T17817', normal, compile_fail, [''])
 test('T17817_elab', normal, compile_fail, ['-fprint-typechecker-elaboration'])
 test('T19978', normal, compile_fail, [''])
+test('T20122', normal, compile_fail, [''])
\ No newline at end of file
diff --git a/testsuite/tests/typecheck/should_fail/tcfail040.stderr b/testsuite/tests/typecheck/should_fail/tcfail040.stderr
index 6e0da922c5..46e0b86577 100644
--- a/testsuite/tests/typecheck/should_fail/tcfail040.stderr
+++ b/testsuite/tests/typecheck/should_fail/tcfail040.stderr
@@ -1,10 +1,10 @@
 
 tcfail040.hs:19:5: error:
-    Ambiguous type variable ‘a0’ arising from a use of ‘<<’
-    prevents the constraint ‘(ORD a0)’ from being solved.
-    Probable fix: use a type annotation to specify what ‘a0’ should be.
-    These potential instance exist:
-      instance ORD (a -> b) -- Defined at tcfail040.hs:17:10
-    In the first argument of ‘(===)’, namely ‘(<<)’
-    In the expression: (<<) === (<<)
-    In an equation for ‘f’: f = (<<) === (<<)
+    • Ambiguous type variable ‘a0’ arising from a use of ‘<<’
+      prevents the constraint ‘(ORD a0)’ from being solved.
+      Probable fix: use a type annotation to specify what ‘a0’ should be.
+      This potential instance exists:
+        instance ORD (a -> b) -- Defined at tcfail040.hs:17:10
+    • In the first argument of ‘(===)’, namely ‘(<<)’
+      In the expression: (<<) === (<<)
+      In an equation for ‘f’: f = (<<) === (<<)
diff --git a/testsuite/tests/typecheck/should_fail/tcfail043.stderr b/testsuite/tests/typecheck/should_fail/tcfail043.stderr
index c30b5bf854..03aef3db67 100644
--- a/testsuite/tests/typecheck/should_fail/tcfail043.stderr
+++ b/testsuite/tests/typecheck/should_fail/tcfail043.stderr
@@ -1,42 +1,42 @@
 
 tcfail043.hs:38:17: error:
-    Ambiguous type variable ‘a0’ arising from a use of ‘gt’
-    prevents the constraint ‘(Ord_ a0)’ from being solved.
-    Relevant bindings include
-      bs :: [a0] (bound at tcfail043.hs:38:8)
-      a :: a0 (bound at tcfail043.hs:38:6)
-      search :: a0 -> [a0] -> Bool (bound at tcfail043.hs:37:1)
-    Probable fix: use a type annotation to specify what ‘a0’ should be.
-    These potential instance exist:
-      instance Ord_ Int -- Defined at tcfail043.hs:34:10
-    In the expression: gt (hd bs) a
-    In the expression:
-      if gt (hd bs) a then
-          False
-      else
-          if eq a (hd bs) then True else search a (tl bs)
-    In the expression:
-      \ a bs
-        -> if gt (hd bs) a then
-               False
-           else
-               if eq a (hd bs) then True else search a (tl bs)
+    • Ambiguous type variable ‘a0’ arising from a use of ‘gt’
+      prevents the constraint ‘(Ord_ a0)’ from being solved.
+      Relevant bindings include
+        bs :: [a0] (bound at tcfail043.hs:38:8)
+        a :: a0 (bound at tcfail043.hs:38:6)
+        search :: a0 -> [a0] -> Bool (bound at tcfail043.hs:37:1)
+      Probable fix: use a type annotation to specify what ‘a0’ should be.
+      This potential instance exists:
+        instance Ord_ Int -- Defined at tcfail043.hs:34:10
+    • In the expression: gt (hd bs) a
+      In the expression:
+        if gt (hd bs) a then
+            False
+        else
+            if eq a (hd bs) then True else search a (tl bs)
+      In the expression:
+        \ a bs
+          -> if gt (hd bs) a then
+                 False
+             else
+                 if eq a (hd bs) then True else search a (tl bs)
 
 tcfail043.hs:40:25: error:
-    Ambiguous type variable ‘a0’ arising from a use of ‘eq’
-    prevents the constraint ‘(Eq_ a0)’ from being solved.
-    Relevant bindings include
-      bs :: [a0] (bound at tcfail043.hs:38:8)
-      a :: a0 (bound at tcfail043.hs:38:6)
-      search :: a0 -> [a0] -> Bool (bound at tcfail043.hs:37:1)
-    Probable fix: use a type annotation to specify what ‘a0’ should be.
-    These potential instances exist:
-      instance Eq_ Int -- Defined at tcfail043.hs:20:10
-      instance Eq_ a => Eq_ [a] -- Defined at tcfail043.hs:23:10
-    In the expression: eq a (hd bs)
-    In the expression: if eq a (hd bs) then True else search a (tl bs)
-    In the expression:
-      if gt (hd bs) a then
-          False
-      else
-          if eq a (hd bs) then True else search a (tl bs)
+    • Ambiguous type variable ‘a0’ arising from a use of ‘eq’
+      prevents the constraint ‘(Eq_ a0)’ from being solved.
+      Relevant bindings include
+        bs :: [a0] (bound at tcfail043.hs:38:8)
+        a :: a0 (bound at tcfail043.hs:38:6)
+        search :: a0 -> [a0] -> Bool (bound at tcfail043.hs:37:1)
+      Probable fix: use a type annotation to specify what ‘a0’ should be.
+      These potential instances exist:
+        instance Eq_ Int -- Defined at tcfail043.hs:20:10
+        instance Eq_ a => Eq_ [a] -- Defined at tcfail043.hs:23:10
+    • In the expression: eq a (hd bs)
+      In the expression: if eq a (hd bs) then True else search a (tl bs)
+      In the expression:
+        if gt (hd bs) a then
+            False
+        else
+            if eq a (hd bs) then True else search a (tl bs)
diff --git a/testsuite/tests/typecheck/should_fail/tcfail128.stderr b/testsuite/tests/typecheck/should_fail/tcfail128.stderr
index d78c46a191..a63f290418 100644
--- a/testsuite/tests/typecheck/should_fail/tcfail128.stderr
+++ b/testsuite/tests/typecheck/should_fail/tcfail128.stderr
@@ -4,7 +4,7 @@ tcfail128.hs:18:16: error:
       prevents the constraint ‘(Data.Array.Base.MArray
                                   b0 FlatVector IO)’ from being solved.
       Probable fix: use a type annotation to specify what ‘b0’ should be.
-      These potential instance exist:
+      This potential instance exists:
         one instance involving out-of-scope types
         (use -fprint-potential-instances to see them all)
     • In a stmt of a 'do' block: v <- thaw tmp