Special-case record fields ending with hash when deriving Read

Summary:
In commit dbd81f7e86514498218572b9d978373b1699cc5b, a
regression was inadvertently introduced which caused derived `Read`
instances for record data types with fields ending in a `#` symbol
(using `MagicHash`) would no longer parse on valid output. This
is ultimately due to the same reasons as #5041, as we cannot parse
a field name like `foo#` as a single identifier. We fix this issue
by employing the same workaround as in #5041: first parse the
identifier name `foo`, then then symbol `#`.

This is accomplished by the new `readFieldHash` function in
`GHC.Read`. This will likely warrant a `base-4.11.1.0` release.

Test Plan: make test TEST=T14918

Reviewers: tdammers, hvr, bgamari

Reviewed By: bgamari

Subscribers: rwbarton, thomie, carter

GHC Trac Issues: #14918

Differential Revision: https://phabricator.haskell.org/D4502

2 files changed, 90 insertions, 2 deletions

diff --git a/docs/users_guide/8.4.2-notes.rst b/docs/users_guide/8.4.2-notes.rst
new file mode 100644
index 0000000000..7002caae0d
--- /dev/null
+++ b/docs/users_guide/8.4.2-notes.rst
@@ -0,0 +1,88 @@
+.. _release-8-4-2:
+
+Release notes for version 8.4.2
+===============================
+
+TODO
+
+Highlights
+----------
+
+The highlights, since the 8.4.1 release, are:
+
+- TODO
+
+
+Full details
+------------
+
+
+Language
+~~~~~~~~
+
+- Fix a regression in which derived `Read` instances for record data types
+  with field names ending with `#` (by way of :ghc-flag:`-XMagicHash`) would
+  no longer parse valid output.
+
+Compiler
+~~~~~~~~
+
+
+Runtime system
+~~~~~~~~~~~~~~
+
+
+Template Haskell
+~~~~~~~~~~~~~~~~
+
+
+``ghc`` library
+~~~~~~~~~~~~~~~
+
+
+``base`` library
+~~~~~~~~~~~~~~~~
+
+- Add the `readFieldHash` function to `GHC.Read` which behaves like
+  `readField`, but for a field that ends with a `#` symbol.
+
+Build system
+~~~~~~~~~~~~
+
+
+Included libraries
+------------------
+
+The package database provided with this distribution also contains a number of
+packages other than GHC itself. See the changelogs provided with these packages
+for further change information.
+
+.. ghc-package-list::
+
+    libraries/array/array.cabal:             Dependency of ``ghc`` library
+    libraries/base/base.cabal:               Core library
+    libraries/binary/binary.cabal:           Dependency of ``ghc`` library
+    libraries/bytestring/bytestring.cabal:   Deppendency of ``ghc`` library
+    libraries/Cabal/Cabal/Cabal.cabal:       Dependency of ``ghc-pkg`` utility
+    libraries/containers/containers.cabal:   Dependency of ``ghc`` library
+    libraries/deepseq/deepseq.cabal:         Dependency of ``ghc`` library
+    libraries/directory/directory.cabal:     Dependency of ``ghc`` library
+    libraries/filepath/filepath.cabal:       Dependency of ``ghc`` library
+    compiler/ghc.cabal:                      The compiler itself
+    libraries/ghci/ghci.cabal:               The REPL interface
+    libraries/ghc-boot/ghc-boot.cabal:       Internal compiler library
+    libraries/ghc-compact/ghc-compact.cabal: Core library
+    libraries/ghc-prim/ghc-prim.cabal:       Core library
+    libraries/haskeline/haskeline.cabal:     Dependency of ``ghci`` executable
+    libraries/hpc/hpc.cabal:                 Dependency of ``hpc`` executable
+    libraries/integer-gmp/integer-gmp.cabal: Core library
+    libraries/mtl/mtl.cabal:                 Dependency of ``Cabal`` library
+    libraries/parsec/parsec.cabal:           Dependency of ``Cabal`` library
+    libraries/process/process.cabal:         Dependency of ``ghc`` library
+    libraries/template-haskell/template-haskell.cabal:     Core library
+    libraries/text/text.cabal:               Dependency of ``Cabal`` library
+    libraries/time/time.cabal:               Dependency of ``ghc`` library
+    libraries/transformers/transformers.cabal: Dependency of ``ghc`` library
+    libraries/unix/unix.cabal:               Dependency of ``ghc`` library
+    libraries/Win32/Win32.cabal:             Dependency of ``ghc`` library
+    libraries/xhtml/xhtml.cabal:             Dependency of ``haddock`` executable
diff --git a/docs/users_guide/index.rst b/docs/users_guide/index.rst
index b57e37b018..7f42dca802 100644
--- a/docs/users_guide/index.rst
+++ b/docs/users_guide/index.rst
@@ -12,8 +12,8 @@ Contents:
 
    license
    intro
-   8.2.1-notes
-   8.4.1-notes
+   8.4.2-notes
+   8.6.1-notes
    ghci
    runghc
    usage