Clean up TH's treatment of unary tuples (or, #16881 part two)

!1906 left some loose ends in regards to Template Haskell's treatment
of unary tuples. This patch ends to tie up those loose ends:

* In addition to having `TupleT 1` produce unary tuples, `TupE [exp]`
  and `TupP [pat]` also now produce unary tuples.
* I have added various special cases in GHC's pretty-printers to
  ensure that explicit 1-tuples are printed using the `Unit` type.
  See `testsuite/tests/th/T17380`.
* The GHC 8.10.1 release notes entry has been tidied up a little.

Fixes #16881. Fixes #17371. Fixes #17380.

1 files changed, 6 insertions, 3 deletions

diff --git a/docs/users_guide/8.10.1-notes.rst b/docs/users_guide/8.10.1-notes.rst
index b405520c70..5e7356ab2c 100644
--- a/docs/users_guide/8.10.1-notes.rst
+++ b/docs/users_guide/8.10.1-notes.rst
@@ -219,9 +219,12 @@ Template Haskell
   :extension:`DeriveLift` has been simplified to take advantage of expression
   quotations.
 
-- Explicit boxed 1-tuples from ``HsSyn`` are now treated as actual 1-tuples,
-  without flattening. In most of the cases these will be obtained using
-  Template Haskell since it is uncommon to deal with 1-tuples in the source.
+- Using ``TupleT 1``, ``TupE [exp]``, or ``TupP [pat]`` will now produce unary
+  tuples (i.e., involving the ``Unit`` type from ``GHC.Tuple``) instead of
+  silently dropping the parentheses. This brings Template Haskell's treatment
+  of boxed tuples in line with that of unboxed tuples, as ``UnboxedTupleT`,
+  ``UnboxedTupE``, and ``UnboxedTupP`` also produce unary unboxed tuples
+  (i.e., ``Unit#``) when applied to only one argument.
 
 - GHC's constraint solver now solves constraints in each top-level group
   sooner. This has practical consequences for Template Haskell, as TH splices