Improve type-error reporting

This patch does two things:

* When reporting a hole, we now include its kind if the
  kind is not just '*'.  This addresses Trac #14265

* When reporting things like "'a' is a rigid type varaible
  bound by ...", this patch arranges to group the type variables
  together, so we don't repeat the "bound by..." stuff endlessly

16 files changed, 70 insertions, 66 deletions

diff --git a/testsuite/tests/ghci/scripts/T10248.stderr b/testsuite/tests/ghci/scripts/T10248.stderr
index e1ca96c63f..283ccdddc4 100644
--- a/testsuite/tests/ghci/scripts/T10248.stderr
+++ b/testsuite/tests/ghci/scripts/T10248.stderr
@@ -1,10 +1,7 @@
 
 <interactive>:2:10: error:
     • Found hole: _ :: f a
-      Where: ‘f’ is a rigid type variable bound by
-               the inferred type of it :: Functor f => f (Maybe a)
-               at <interactive>:2:1-10
-             ‘a’ is a rigid type variable bound by
+      Where: ‘f’, ‘a’ are rigid type variables bound by
                the inferred type of it :: Functor f => f (Maybe a)
                at <interactive>:2:1-10
     • In the second argument of ‘(<$>)’, namely ‘_’
diff --git a/testsuite/tests/partial-sigs/should_compile/SuperCls.stderr b/testsuite/tests/partial-sigs/should_compile/SuperCls.stderr
index 01651a4136..a11164482c 100644
--- a/testsuite/tests/partial-sigs/should_compile/SuperCls.stderr
+++ b/testsuite/tests/partial-sigs/should_compile/SuperCls.stderr
@@ -1,4 +1,4 @@
 
 SuperCls.hs:4:14: warning: [-Wpartial-type-signatures (in -Wdefault)]
-    • Found type wildcard ‘_’ standing for ‘() :: Constraint’
+    • Found type wildcard ‘_’ standing for ‘()’
     • In the type signature: f :: (Ord a, _) => a -> Bool
diff --git a/testsuite/tests/partial-sigs/should_compile/T10403.stderr b/testsuite/tests/partial-sigs/should_compile/T10403.stderr
index 6ebd844bff..229b9e1df1 100644
--- a/testsuite/tests/partial-sigs/should_compile/T10403.stderr
+++ b/testsuite/tests/partial-sigs/should_compile/T10403.stderr
@@ -1,33 +1,26 @@
 
 T10403.hs:15:7: warning: [-Wpartial-type-signatures (in -Wdefault)]
     • Found type wildcard ‘_’ standing for ‘Functor f’
-      Where: ‘f’ is a rigid type variable bound by
-               the inferred type of h1 :: Functor f => (a -> b) -> f a -> H f
+      Where: ‘f’ is a rigid type variable
+               bound by the inferred type of
+                        h1 :: Functor f => (a -> b) -> f a -> H f
                at T10403.hs:17:1-41
     • In the type signature: h1 :: _ => _
 
 T10403.hs:15:12: warning: [-Wpartial-type-signatures (in -Wdefault)]
     • Found type wildcard ‘_’ standing for ‘(a -> b) -> f a -> H f’
-      Where: ‘b’ is a rigid type variable bound by
-               the inferred type of h1 :: Functor f => (a -> b) -> f a -> H f
-               at T10403.hs:17:1-41
-             ‘a’ is a rigid type variable bound by
-               the inferred type of h1 :: Functor f => (a -> b) -> f a -> H f
-               at T10403.hs:17:1-41
-             ‘f’ is a rigid type variable bound by
-               the inferred type of h1 :: Functor f => (a -> b) -> f a -> H f
+      Where: ‘b’, ‘a’, ‘f’ are rigid type variables
+               bound by the inferred type of
+                        h1 :: Functor f => (a -> b) -> f a -> H f
                at T10403.hs:17:1-41
     • In the type signature: h1 :: _ => _
 
 T10403.hs:19:7: warning: [-Wpartial-type-signatures (in -Wdefault)]
     • Found type wildcard ‘_’ standing for ‘(a -> b) -> f0 a -> H f0’
-      Where: ‘b’ is a rigid type variable bound by
-               the inferred type of h2 :: (a -> b) -> f0 a -> H f0
-               at T10403.hs:22:1-41
-             ‘a’ is a rigid type variable bound by
-               the inferred type of h2 :: (a -> b) -> f0 a -> H f0
+      Where: ‘f0’ is an ambiguous type variable
+             ‘b’, ‘a’ are rigid type variables
+               bound by the inferred type of h2 :: (a -> b) -> f0 a -> H f0
                at T10403.hs:22:1-41
-             ‘f0’ is an ambiguous type variable
     • In the type signature: h2 :: _
 
 T10403.hs:22:15: warning: [-Wdeferred-type-errors (in -Wdefault)]
diff --git a/testsuite/tests/partial-sigs/should_compile/T11016.stderr b/testsuite/tests/partial-sigs/should_compile/T11016.stderr
index 343deabe86..49363fb24c 100644
--- a/testsuite/tests/partial-sigs/should_compile/T11016.stderr
+++ b/testsuite/tests/partial-sigs/should_compile/T11016.stderr
@@ -1,6 +1,6 @@
 
 T11016.hs:5:19: warning: [-Wpartial-type-signatures (in -Wdefault)]
-    • Found type wildcard ‘_’ standing for ‘() :: Constraint’
+    • Found type wildcard ‘_’ standing for ‘()’
     • In the type signature: f1 :: (?x :: Int, _) => Int
 
 T11016.hs:8:22: warning: [-Wpartial-type-signatures (in -Wdefault)]
diff --git a/testsuite/tests/partial-sigs/should_compile/T11192.stderr b/testsuite/tests/partial-sigs/should_compile/T11192.stderr
index 0f2d2e09b6..8030276206 100644
--- a/testsuite/tests/partial-sigs/should_compile/T11192.stderr
+++ b/testsuite/tests/partial-sigs/should_compile/T11192.stderr
@@ -2,7 +2,8 @@
 T11192.hs:7:14: warning: [-Wpartial-type-signatures (in -Wdefault)]
     • Found type wildcard ‘_’ standing for ‘Int -> p -> p’
       Where: ‘p’ is a rigid type variable bound by
-               the inferred type of go :: Int -> p -> p at T11192.hs:8:8-17
+               the inferred type of go :: Int -> p -> p
+               at T11192.hs:8:8-17
     • In the type signature: go :: _
       In the expression:
         let
@@ -19,10 +20,9 @@ T11192.hs:7:14: warning: [-Wpartial-type-signatures (in -Wdefault)]
 
 T11192.hs:13:14: warning: [-Wpartial-type-signatures (in -Wdefault)]
     • Found type wildcard ‘_’ standing for ‘p -> p1 -> p1’
-      Where: ‘p’ is a rigid type variable bound by
-               the inferred type of go :: p -> p1 -> p1 at T11192.hs:14:8-17
-             ‘p1’ is a rigid type variable bound by
-               the inferred type of go :: p -> p1 -> p1 at T11192.hs:14:8-17
+      Where: ‘p’, ‘p1’ are rigid type variables bound by
+               the inferred type of go :: p -> p1 -> p1
+               at T11192.hs:14:8-17
     • In the type signature: go :: _
       In the expression:
         let
diff --git a/testsuite/tests/partial-sigs/should_compile/T12033.stderr b/testsuite/tests/partial-sigs/should_compile/T12033.stderr
index a3b293b0cc..780fb9d41b 100644
--- a/testsuite/tests/partial-sigs/should_compile/T12033.stderr
+++ b/testsuite/tests/partial-sigs/should_compile/T12033.stderr
@@ -1,15 +1,15 @@
 
 T12033.hs:12:22: warning: [-Wpartial-type-signatures (in -Wdefault)]
     • Found type wildcard ‘_’ standing for ‘v -> t’
-      Where: ‘v’ is a rigid type variable bound by
+      Where: ‘t’ is a rigid type variable bound by
+               the inferred types of
+                 makeTuple :: v -> t
+                 makeExpression :: v -> t
+               at T12033.hs:(11,4)-(13,39)
+             ‘v’ is a rigid type variable bound by
                the type signature for:
                  tripleStoreToRuleSet :: forall v. v -> v
                at T12033.hs:6:1-30
-             ‘t’ is a rigid type variable bound by
-               the inferred type of
-               makeTuple :: v -> t
-               makeExpression :: v -> t
-               at T12033.hs:(11,4)-(13,39)
     • In the type signature: makeExpression :: _
       In an equation for ‘tripleStoreToRuleSet’:
           tripleStoreToRuleSet getAtom
diff --git a/testsuite/tests/partial-sigs/should_compile/T12844.stderr b/testsuite/tests/partial-sigs/should_compile/T12844.stderr
index 8ad3777f2a..3846590890 100644
--- a/testsuite/tests/partial-sigs/should_compile/T12844.stderr
+++ b/testsuite/tests/partial-sigs/should_compile/T12844.stderr
@@ -2,24 +2,9 @@
 T12844.hs:12:9: warning: [-Wpartial-type-signatures (in -Wdefault)]
     • Found type wildcard ‘_’
         standing for ‘(Head rngs ~ '(r, r'), Foo rngs)’
-      Where: ‘r’ is a rigid type variable bound by
+      Where: ‘r’, ‘r'’, ‘rngs’, ‘k’, ‘k1’
+               are rigid type variables bound by
                the inferred type of
-               bar :: (Head rngs ~ '(r, r'), Foo rngs) => FooData rngs
-               at T12844.hs:13:1-9
-             ‘r'’ is a rigid type variable bound by
-               the inferred type of
-               bar :: (Head rngs ~ '(r, r'), Foo rngs) => FooData rngs
-               at T12844.hs:13:1-9
-             ‘rngs’ is a rigid type variable bound by
-               the inferred type of
-               bar :: (Head rngs ~ '(r, r'), Foo rngs) => FooData rngs
-               at T12844.hs:13:1-9
-             ‘k’ is a rigid type variable bound by
-               the inferred type of
-               bar :: (Head rngs ~ '(r, r'), Foo rngs) => FooData rngs
-               at T12844.hs:13:1-9
-             ‘k1’ is a rigid type variable bound by
-               the inferred type of
-               bar :: (Head rngs ~ '(r, r'), Foo rngs) => FooData rngs
+                 bar :: (Head rngs ~ '(r, r'), Foo rngs) => FooData rngs
                at T12844.hs:13:1-9
     • In the type signature: bar :: _ => FooData rngs
diff --git a/testsuite/tests/partial-sigs/should_compile/T12845.stderr b/testsuite/tests/partial-sigs/should_compile/T12845.stderr
index b9d7d60a97..a483c84231 100644
--- a/testsuite/tests/partial-sigs/should_compile/T12845.stderr
+++ b/testsuite/tests/partial-sigs/should_compile/T12845.stderr
@@ -1,6 +1,6 @@
 
 T12845.hs:18:70: warning: [-Wpartial-type-signatures (in -Wdefault)]
-    • Found type wildcard ‘_’ standing for ‘() :: Constraint’
+    • Found type wildcard ‘_’ standing for ‘()’
     • In the type signature:
         broken :: forall r r' rngs.
                   ('(r, r') ~ Head rngs, Bar r r' ~  'True, _) =>
diff --git a/testsuite/tests/partial-sigs/should_compile/WarningWildcardInstantiations.stderr b/testsuite/tests/partial-sigs/should_compile/WarningWildcardInstantiations.stderr
index ca815439de..560b74d3b9 100644
--- a/testsuite/tests/partial-sigs/should_compile/WarningWildcardInstantiations.stderr
+++ b/testsuite/tests/partial-sigs/should_compile/WarningWildcardInstantiations.stderr
@@ -34,10 +34,7 @@ WarningWildcardInstantiations.hs:8:8: warning: [-Wpartial-type-signatures (in -W
 
 WarningWildcardInstantiations.hs:8:13: warning: [-Wpartial-type-signatures (in -Wdefault)]
     • Found type wildcard ‘_’ standing for ‘t -> w’
-      Where: ‘t’ is a rigid type variable bound by
-               the inferred type of bar :: t -> (t -> w) -> w
-               at WarningWildcardInstantiations.hs:9:1-13
-             ‘w’ is a rigid type variable bound by
+      Where: ‘t’, ‘w’ are rigid type variables bound by
                the inferred type of bar :: t -> (t -> w) -> w
                at WarningWildcardInstantiations.hs:9:1-13
     • In the type signature: bar :: _ -> _ -> _
diff --git a/testsuite/tests/partial-sigs/should_fail/T10045.stderr b/testsuite/tests/partial-sigs/should_fail/T10045.stderr
index a18ef48b83..e6f6462d5d 100644
--- a/testsuite/tests/partial-sigs/should_fail/T10045.stderr
+++ b/testsuite/tests/partial-sigs/should_fail/T10045.stderr
@@ -1,10 +1,9 @@
 
 T10045.hs:6:18: error:
     • Found type wildcard ‘_’ standing for ‘t1 -> Bool -> t2’
-      Where: ‘t1’ is a rigid type variable bound by
-               the inferred type of copy :: t1 -> Bool -> t2 at T10045.hs:7:10-34
-             ‘t2’ is a rigid type variable bound by
-               the inferred type of copy :: t1 -> Bool -> t2 at T10045.hs:7:10-34
+      Where: ‘t1’, ‘t2’ are rigid type variables bound by
+               the inferred type of copy :: t1 -> Bool -> t2
+               at T10045.hs:7:10-34
       To use the inferred type, enable PartialTypeSignatures
     • In the type signature: copy :: _
       In the expression:
diff --git a/testsuite/tests/partial-sigs/should_fail/T12634.stderr b/testsuite/tests/partial-sigs/should_fail/T12634.stderr
index 7f1d713c4f..dd661a9355 100644
--- a/testsuite/tests/partial-sigs/should_fail/T12634.stderr
+++ b/testsuite/tests/partial-sigs/should_fail/T12634.stderr
@@ -1,6 +1,6 @@
 
 T12634.hs:14:37: error:
-    • Found type wildcard ‘_’ standing for ‘() :: Constraint’
+    • Found type wildcard ‘_’ standing for ‘()’
       To use the inferred type, enable PartialTypeSignatures
     • In the type signature:
         bench_twacePow :: forall t m m' r.
diff --git a/testsuite/tests/partial-sigs/should_fail/WildcardInstantiations.stderr b/testsuite/tests/partial-sigs/should_fail/WildcardInstantiations.stderr
index 440d8722de..aa5e8247f3 100644
--- a/testsuite/tests/partial-sigs/should_fail/WildcardInstantiations.stderr
+++ b/testsuite/tests/partial-sigs/should_fail/WildcardInstantiations.stderr
@@ -30,10 +30,7 @@ WildcardInstantiations.hs:8:8: error:
 
 WildcardInstantiations.hs:8:13: error:
     • Found type wildcard ‘_’ standing for ‘t -> w’
-      Where: ‘t’ is a rigid type variable bound by
-               the inferred type of bar :: t -> (t -> w) -> w
-               at WildcardInstantiations.hs:9:1-13
-             ‘w’ is a rigid type variable bound by
+      Where: ‘t’, ‘w’ are rigid type variables bound by
                the inferred type of bar :: t -> (t -> w) -> w
                at WildcardInstantiations.hs:9:1-13
       To use the inferred type, enable PartialTypeSignatures
diff --git a/testsuite/tests/perf/compiler/T13035.stderr b/testsuite/tests/perf/compiler/T13035.stderr
index 52836d7e3c..fe1f0b2564 100644
--- a/testsuite/tests/perf/compiler/T13035.stderr
+++ b/testsuite/tests/perf/compiler/T13035.stderr
@@ -1,4 +1,4 @@
 
 T13035.hs:141:28: warning: [-Wpartial-type-signatures (in -Wdefault)]
-    • Found type wildcard ‘_’ standing for ‘'['Author]’
+    • Found type wildcard ‘_’ standing for ‘'['Author] :: [Fields]’
     • In the type signature: g :: MyRec RecipeFormatter _
diff --git a/testsuite/tests/polykinds/T14265.hs b/testsuite/tests/polykinds/T14265.hs
new file mode 100644
index 0000000000..84c1a025a1
--- /dev/null
+++ b/testsuite/tests/polykinds/T14265.hs
@@ -0,0 +1,11 @@
+{-# LANGUAGE PolyKinds #-}
+
+module T124265 where
+
+import Control.Monad.Trans.State( StateT )
+
+f :: proxy _ -> ()
+f _ = ()
+
+foo :: StateT _ _ ()
+foo = undefined
diff --git a/testsuite/tests/polykinds/T14265.stderr b/testsuite/tests/polykinds/T14265.stderr
new file mode 100644
index 0000000000..be6868fdc4
--- /dev/null
+++ b/testsuite/tests/polykinds/T14265.stderr
@@ -0,0 +1,24 @@
+
+T14265.hs:7:12: error:
+    • Found type wildcard ‘_’ standing for ‘w :: k’
+      Where: ‘w’, ‘k’ are rigid type variables bound by
+               the inferred type of f :: proxy w -> ()
+               at T14265.hs:8:1-8
+      To use the inferred type, enable PartialTypeSignatures
+    • In the type signature: f :: proxy _ -> ()
+
+T14265.hs:10:15: error:
+    • Found type wildcard ‘_’ standing for ‘w’
+      Where: ‘w’ is a rigid type variable bound by
+               the inferred type of foo :: StateT w w1 ()
+               at T14265.hs:11:1-15
+      To use the inferred type, enable PartialTypeSignatures
+    • In the type signature: foo :: StateT _ _ ()
+
+T14265.hs:10:17: error:
+    • Found type wildcard ‘_’ standing for ‘w1 :: * -> *’
+      Where: ‘w1’ is a rigid type variable bound by
+               the inferred type of foo :: StateT w w1 ()
+               at T14265.hs:11:1-15
+      To use the inferred type, enable PartialTypeSignatures
+    • In the type signature: foo :: StateT _ _ ()
diff --git a/testsuite/tests/polykinds/all.T b/testsuite/tests/polykinds/all.T
index c01b73c98d..78c16975da 100644
--- a/testsuite/tests/polykinds/all.T
+++ b/testsuite/tests/polykinds/all.T
@@ -168,3 +168,4 @@ test('T14110', normal, compile_fail, [''])
 test('BadKindVar', normal, compile_fail, [''])
 test('T13738', normal, compile_fail, [''])
 test('T14209', normal, compile, [''])
+test('T14265', normal, compile_fail, [''])