Parse the (!) type operator and allow type operators in existential context

Summary:
Improve the way `(!)`, `(~)`, and other type operators are handled in the parser,
fixing two issues at once:

1. `(!)` can now be used as a type operator
   that respects fixity and precedence (#15457)
2. Existential context of a data constructor
   no longer needs parentheses (#15675)

In addition to that, with this patch it is now trivial to adjust precedence of
the `{-# UNPACK #-}` pragma, as suggested in
https://ghc.haskell.org/trac/ghc/ticket/14761#comment:7

There was a small change to API Annotations. Before this patch, `(~)` was a
strange special case that produced an annotation unlike any other type
operator. After this patch, when `(~)` or `(!)` are used to specify strictness they
produce AnnTilde and AnnBang annotations respectively, and when they are used
as type operators, they produce no annotations.

Test Plan: Validate

Reviewers: simonpj, bgamari, alanz, RyanGlScott

Reviewed By: RyanGlScott

Subscribers: RyanGlScott, rwbarton, mpickering, carter

GHC Trac Issues: #15457, #15675

Differential Revision: https://phabricator.haskell.org/D5180

1 files changed, 10 insertions, 0 deletions

diff --git a/docs/users_guide/8.8.1-notes.rst b/docs/users_guide/8.8.1-notes.rst
index e3a8d3ea22..00e532c6d1 100644
--- a/docs/users_guide/8.8.1-notes.rst
+++ b/docs/users_guide/8.8.1-notes.rst
@@ -40,6 +40,16 @@ Language
   terminating value of type ``Void``. Accordingly, GHC will not warn about
   ``K2`` (whereas previous versions of GHC would).
 
+- ``(!)`` is now a valid type operator: ::
+
+      type family a ! b
+
+- An existential context no longer requires parenthesization: ::
+
+    class a + b
+    data D1 = forall a b. (a + b) => D1 a b
+    data D2 = forall a b.  a + b  => D2 a b -- now allowed
+
 Compiler
 ~~~~~~~~