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 An outdated call to `flattenTys` meant that we missed out on some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     the logic is obsolete and can be removed.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

2 files changed, 16 insertions, 14 deletions

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 = ...