Suppress -Winaccessible-code in derived code

Summary:
It's rather unfortunate that derived code can produce inaccessible
code warnings (as demonstrated in #8128, #8740, and #15398), since
the programmer has no control over the generated code. This patch
aims to suppress `-Winaccessible-code` in all derived code. It
accomplishes this by doing the following:

* Generalize the `ic_env :: TcLclEnv` field of `Implication` to
  be of type `Env TcGblEnc TcLclEnv` instead. This way, it also
  captures `DynFlags`, which record the flag state at the time
  the `Implication` was created.
* When typechecking derived code, turn off `-Winaccessible-code`.
  This way, any insoluble given `Implication`s that are created when
  typechecking this derived code will remember that
  `-Winaccessible-code` was disabled.
* During error reporting, consult the `DynFlags` of an
  `Implication` before making the decision to report an inaccessible
  code warning.

Test Plan: make test TEST="T8128 T8740 T15398"

Reviewers: simonpj, bgamari

Reviewed By: simonpj

Subscribers: monoidal, rwbarton, thomie, carter

GHC Trac Issues: #8128, #8740, #15398

Differential Revision: https://phabricator.haskell.org/D4993

1 files changed, 20 insertions, 0 deletions

diff --git a/testsuite/tests/deriving/should_compile/T15398.hs b/testsuite/tests/deriving/should_compile/T15398.hs
new file mode 100644
index 0000000000..b78df1fa17
--- /dev/null
+++ b/testsuite/tests/deriving/should_compile/T15398.hs
@@ -0,0 +1,20 @@
+{-# LANGUAGE DeriveAnyClass #-}
+{-# LANGUAGE FlexibleContexts #-}
+{-# LANGUAGE GADTs #-}
+{-# LANGUAGE StandaloneDeriving #-}
+
+module T15398 where
+
+newtype Radius a = Radius a deriving (Eq, Ord)
+
+data CourseLine
+data OpenDistance
+data EndOfSpeedSection
+
+data Zone k a where
+    Point :: (Eq a, Ord a) => Zone CourseLine a
+    Vector :: (Eq a, Ord a) => Zone OpenDistance a
+    Conical :: (Eq a, Ord a) => Radius a -> Zone EndOfSpeedSection a
+
+deriving instance Eq a => Eq (Zone k a)
+deriving instance (Eq a, Ord a) => Ord (Zone k a)