Incorporate changes from #11721 into Template Haskell

Summary:
#11721 changed the order of type variables in GADT
constructor type signatures, but these changes weren't reflected in
Template Haskell reification of GADTs. Let's do that.

Along the way, I:

* noticed that the `T13885` test was claiming to test TH reification
  of GADTs, but the reified data type wasn't actually a GADT! Since
  this patch touches that part of the codebase, I decided to fix
  this.
* incorporated some feedback from @simonpj in
  https://phabricator.haskell.org/D3687#113566. (These changes alone
  don't account for any different in behavior.)

Test Plan: make test TEST=T11721_TH

Reviewers: goldfire, austin, bgamari, simonpj

Reviewed By: goldfire, bgamari, simonpj

Subscribers: rwbarton, thomie, simonpj

GHC Trac Issues: #11721

Differential Revision: https://phabricator.haskell.org/D4070

1 files changed, 10 insertions, 0 deletions

diff --git a/docs/users_guide/8.4.1-notes.rst b/docs/users_guide/8.4.1-notes.rst
index dc762602e2..f59d266789 100644
--- a/docs/users_guide/8.4.1-notes.rst
+++ b/docs/users_guide/8.4.1-notes.rst
@@ -197,6 +197,16 @@ Template Haskell
   such as ``data T1 a = (a ~ Int) => MkT1`` being reified as a GADT and
   ``data T2 a where MkT2 :: Show a => T2 a`` *not* being reified as a GADT.
 
+  In addition, reified GADT constructors now more accurately track the order in
+  which users write type variables. Before, if you reified ``MkT`` as below: ::
+
+      data T a where
+        MkT :: forall b a. b -> T a
+
+  Then the reified type signature of ``MkT`` would have been headed by
+  ``ForallC [PlainTV a, PlainTV b]``. Now, reifying ``MkT`` will give a type
+  headed by ``ForallC [PlainTV b, PlainTV a]``, as one would expect.
+
 ``ghc`` library
 ~~~~~~~~~~~~~~~