Fix Show derivation in the presence of RebindableSyntax/OverloadedStrings

Summary:
To fix this issue, we simply disable `RebindableSyntax` whenever we rename
the code generated from a deriving clause.

Fixes #12688.

Test Plan: make test TEST=T12688

Reviewers: simonpj, austin, bgamari

Reviewed By: simonpj, bgamari

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D2591

GHC Trac Issues: #12688

2 files changed, 31 insertions, 0 deletions

diff --git a/docs/users_guide/8.0.2-notes.rst b/docs/users_guide/8.0.2-notes.rst
index 82c214e534..c8e76ed456 100644
--- a/docs/users_guide/8.0.2-notes.rst
+++ b/docs/users_guide/8.0.2-notes.rst
@@ -25,6 +25,10 @@ Language
 -  A bug has been fixed that caused standalone derived ``Ix`` instances to fail
    for GADTs with exactly one constructor (:ghc-ticket:`12583`).
 
+-  A bug has been fixed that caused derived ``Show`` instances to fail in the
+   presence of :ghc-flag:`-XRebindableSyntax` and
+   :ghc-flag:`-XOverloadedStrings` (:ghc-ticket:`12688`).
+
 Compiler
 ~~~~~~~~
 
diff --git a/docs/users_guide/glasgow_exts.rst b/docs/users_guide/glasgow_exts.rst
index e76465af6d..9f0a75502c 100644
--- a/docs/users_guide/glasgow_exts.rst
+++ b/docs/users_guide/glasgow_exts.rst
@@ -1460,6 +1460,33 @@ Be warned: this is an experimental facility, with fewer checks than
 usual. Use ``-dcore-lint`` to typecheck the desugared program. If Core
 Lint is happy you should be all right.
 
+Things unaffected by :ghc-flag:`-XRebindableSyntax`
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+:ghc-flag:`-XRebindableSyntax` does not apply to any code generated from a
+``deriving`` clause or declaration. To see why, consider the following code: ::
+
+    {-# LANGUAGE RebindableSyntax, OverloadedStrings #-}
+    newtype Text = Text String
+
+    fromString :: String -> Text
+    fromString = Text
+
+    data Foo = Foo deriving Show
+
+This will generate code to the effect of: ::
+
+    instance Show Foo where
+      showsPrec _ Foo = showString "Foo"
+
+But because :ghc-flag:`-XRebindableSyntax` and :ghc-flag:`-XOverloadedStrings`
+are enabled, the ``"Foo"`` string literal would now be of type ``Text``, not
+``String``, which ``showString`` doesn't accept! This causes the generated
+``Show`` instance to fail to typecheck. It's hard to imagine any scenario where
+it would be desirable have :ghc-flag:`-XRebindableSyntax` behavior within
+derived code, so GHC simply ignores :ghc-flag:`-XRebindableSyntax` entirely
+when checking derived code.
+
 .. _postfix-operators:
 
 Postfix operators