Don't quantify implicit type variables when quoting type signatures in TH

Summary:
A bug was introduced in GHC 8.0 in which Template Haskell-quoted type
signatures would quantify _all_ their type variables, even the implicit ones.
This would cause splices like this:

```
$([d| idProxy :: forall proxy (a :: k). proxy a -> proxy a
      idProxy x = x
   |])
```

To splice back in something that was slightly different:

```
idProxy :: forall k proxy (a :: k). proxy a -> proxy a
idProxy x = x
```

Notice that the kind variable `k` is now explicitly quantified! What's worse,
this now requires the `TypeInType` extension to be enabled.

This changes the behavior of Template Haskell quoting to never explicitly
quantify type variables which are implicitly quantified in the source.

There are some other places where this behavior pops up too, including
class methods, type ascriptions, `SPECIALIZE` pragmas, foreign imports,
and pattern synonynms (#13018), so I fixed those too.

Fixes #13018 and #13123.

Test Plan: ./validate

Reviewers: simonpj, goldfire, austin, bgamari

Reviewed By: simonpj, goldfire

Subscribers: simonpj, mpickering, thomie

Differential Revision: https://phabricator.haskell.org/D2974

GHC Trac Issues: #13018, #13123

1 files changed, 36 insertions, 0 deletions

diff --git a/docs/users_guide/8.2.1-notes.rst b/docs/users_guide/8.2.1-notes.rst
index 094fce9eaf..7e75461ac0 100644
--- a/docs/users_guide/8.2.1-notes.rst
+++ b/docs/users_guide/8.2.1-notes.rst
@@ -148,6 +148,42 @@ Template Haskell
    be ambiguous in the presence of :ghc-flag:`-XPolyKinds`.
    (:ghc-ticket:`12646`)
 
+-  Quoted type signatures are more accurate with respect to implicitly
+   quantified type variables. Before, if you quoted this: ::
+
+     [d| id :: a -> a
+         id x = x
+       |]
+
+   then the code that Template Haskell would give back to you would actually be
+   this instead: ::
+
+     id :: forall a. a -> a
+     id x = x
+
+   That is, quoting would explicitly quantify all type variables, even ones
+   that were implicitly quantified in the source. This could be especially
+   harmful if a kind variable was implicitly quantified. For example, if
+   you took this quoted declaration: ::
+
+     [d| idProxy :: forall proxy (b :: k). proxy b -> proxy b
+         idProxy x = x
+       |]
+
+   and tried to splice it back in, you'd get this instead: ::
+
+     idProxy :: forall k proxy (b :: k). proxy b -> proxy b
+     idProxy x = x
+
+   Now ``k`` is explicitly quantified, and that requires turning on
+   :ghc-flag:`-XTypeInType`, whereas the original declaration did not!
+
+   Template Haskell quoting now respects implicit quantification in type
+   signatures, so the quoted declarations above now correctly leave the
+   type variables ``a`` and ``k`` as implicitly quantified.
+   (:ghc-ticket:`13018` and :ghc-ticket:`13123`)
+
+
 Runtime system
 ~~~~~~~~~~~~~~