Desugar quoted uses of DerivingVia and expression type signatures properly

The way that `GHC.HsToCore.Quote` desugared quoted `via` types (e.g.,
`deriving via forall a. [a] instance Eq a => Eq (List a)`) and
explicit type annotations in signatures (e.g.,
`f = id @a :: forall a. a -> a`) was completely wrong, as it did not
implement the scoping guidelines laid out in
`Note [Scoped type variables in bindings]`. This is easily fixed.

While I was in town, I did some minor cleanup of related Notes:

* `Note [Scoped type variables in bindings]` and
  `Note [Scoped type variables in class and instance declarations]`
  say very nearly the same thing. I decided to just consolidate the
  two Notes into `Note [Scoped type variables in quotes]`.
* `Note [Don't quantify implicit type variables in quotes]` is
  somewhat outdated, as it predates GHC 8.10, where the
  `forall`-or-nothing rule requires kind variables to be explicitly
  quantified in the presence of an explicit `forall`. As a result,
  the running example in that Note doesn't even compile. I have
  changed the example to something simpler that illustrates the
  same point that the original Note was making.

Fixes #18388.

2 files changed, 30 insertions, 0 deletions

diff --git a/testsuite/tests/th/T18388.hs b/testsuite/tests/th/T18388.hs
new file mode 100644
index 0000000000..d31758004b
--- /dev/null
+++ b/testsuite/tests/th/T18388.hs
@@ -0,0 +1,29 @@
+{-# LANGUAGE DerivingVia #-}
+{-# LANGUAGE FlexibleInstances #-}
+{-# LANGUAGE MultiParamTypeClasses #-}
+{-# LANGUAGE ScopedTypeVariables #-}
+{-# LANGUAGE StandaloneDeriving #-}
+{-# LANGUAGE TemplateHaskell #-}
+{-# LANGUAGE TypeApplications #-}
+module T18388 where
+
+class C x y where
+  m :: x -> y -> y
+
+newtype Tagged x a = MkTagged a
+instance C x (Tagged x a) where
+  m _ = id
+
+$([d| newtype Id1 a = MkId1 a
+        deriving (C x) via forall x. Tagged x a
+
+      newtype Id2 a = MkId2 a
+        deriving (C x) via           Tagged x a
+    |])
+
+newtype List1 a = MkList1 [a]
+newtype List2 a = MkList2 [a]
+$([d| deriving via forall a. [a] instance Eq a => Eq (List1 a) |])
+$([d| deriving via           [a] instance Eq a => Eq (List2 a) |])
+
+$([d| f = id @a :: forall a. a -> a |])
diff --git a/testsuite/tests/th/all.T b/testsuite/tests/th/all.T
index e224641c92..24cc9d9b46 100644
--- a/testsuite/tests/th/all.T
+++ b/testsuite/tests/th/all.T
@@ -510,3 +510,4 @@ test('TH_StringLift', normal, compile, [''])
 test('TH_BytesShowEqOrd', normal, compile_and_run, [''])
 test('T18121', normal, compile, [''])
 test('T18123', normal, compile, [''])
+test('T18388', normal, compile, [''])