Preserve argument order to (==)/eq in nub and nubBy

This makes nub and nubBy behave as specified in the Haskell 98 Report.

This reverts 0ad9def53842e86fb292eccb810190711c42d7c5, and
fixes #3280, #7913 and #2528 (properly).

Before this change, the output of `T2528` was (4x wrong):
```
[A,B]
[1,2]
False
False
```

Reviewed By: dfeuer, ekmett, austin, hvr

Differential Revision: https://phabricator.haskell.org/D238

1 files changed, 27 insertions, 0 deletions

diff --git a/libraries/base/tests/T2528.hs b/libraries/base/tests/T2528.hs
new file mode 100644
index 0000000000..f1568db75a
--- /dev/null
+++ b/libraries/base/tests/T2528.hs
@@ -0,0 +1,27 @@
+module Main where
+
+import qualified Data.List as L
+
+-- USE_REPORT_PRELUDE versions of nub and nubBy, copied from
+-- libraries/base/Data/OldList.hs.
+nub                     :: (Eq a) => [a] -> [a]
+nub                     =  nubBy (==)
+
+nubBy                   :: (a -> a -> Bool) -> [a] -> [a]
+nubBy eq []             =  []
+nubBy eq (x:xs)         =  x : nubBy eq (filter (\ y -> not (eq x y)) xs)
+
+data Asymmetric = A | B deriving Show
+
+instance Eq Asymmetric where
+  A == _ = True
+  B == _ = False
+
+main :: IO()
+main = do
+  print $ L.nub [A,B]
+  print $ L.nubBy (<) [1,2]
+  -- The implementation from Data.List and the one from the Prelude defined in
+  -- the Haskell 98 report should have the same behavior.
+  print $ L.nub [A,B] == nub [A,B]
+  print $ L.nubBy (<) [1,2] == nubBy (<) [1,2]