Complete work on new OVERLAPPABLE/OVERLAPPING pragmas (Trac #9242)

* Deprecate -XOverlappingInstances

* Update test suite.  Several tests even had entirely unnecessary
  uses of -XOverlappingInstances

* Update user manual with a careful description of the instance
  resolution story

* Fix an outright bug in the handling of duplidate instances in GHCi,
  which are meant to silently overwrite the earlier duplicate. The
  logic was right for family instances but was both more complicated,
  and plain wrong, for class instances.  (If you are interested, the
  bug was that we were eliminating the duplicate from the InstEnv, but
  not from the [ClsInst] held in tcg_insts.)  Test is ghci044a.

1 files changed, 4 insertions, 2 deletions

diff --git a/testsuite/tests/generics/Uniplate/GUniplate.hs b/testsuite/tests/generics/Uniplate/GUniplate.hs
index 76f387d636..99b0b405a8 100644
--- a/testsuite/tests/generics/Uniplate/GUniplate.hs
+++ b/testsuite/tests/generics/Uniplate/GUniplate.hs
@@ -4,7 +4,8 @@
 {-# LANGUAGE FlexibleContexts           #-}
 {-# LANGUAGE TypeOperators              #-}
 {-# LANGUAGE DefaultSignatures          #-}
-{-# LANGUAGE IncoherentInstances        #-} -- necessary, unfortunately
+{- # LANGUAGE IncoherentInstances        #-} -- necessary, unfortunately
+{-# LANGUAGE OverlappingInstances        #-} 
 
 module GUniplate where
 
@@ -20,7 +21,8 @@ class Uniplate' f b where
 instance Uniplate' U1 a where
   children' U1 = []
 
-instance Uniplate' (K1 i a) a where
+instance {-# OVERLAPPING #-} Uniplate' (K1 i a) a where
+  -- overlaps the (Uniplate' (K1 i a) b) instance
   children' (K1 a) = [a]
 
 instance Uniplate' (K1 i a) b where