Make dropDerivedSimples restore [WD] constraints

I'd forgotten to turn [W] + [D] constraints back into [WD]
in dropDerivedSimples; and that led to Trac #12936.

Fortunately the fix is simple.

1 files changed, 30 insertions, 0 deletions

diff --git a/testsuite/tests/typecheck/should_compile/T12936.hs b/testsuite/tests/typecheck/should_compile/T12936.hs
new file mode 100644
index 0000000000..c4f966093f
--- /dev/null
+++ b/testsuite/tests/typecheck/should_compile/T12936.hs
@@ -0,0 +1,30 @@
+{-# LANGUAGE FlexibleContexts #-}
+{-# LANGUAGE FlexibleInstances #-}
+{-# LANGUAGE FunctionalDependencies #-}
+{-# LANGUAGE MonomorphismRestriction #-}
+{-# LANGUAGE MultiParamTypeClasses #-}
+{-# LANGUAGE ScopedTypeVariables #-}
+module Token where
+
+class S s t | s -> t
+
+m :: forall s t . S s t => s
+m = undefined
+
+o :: forall s t . S s t => s -> s
+o = undefined
+
+c :: forall s . s -> s -> s
+c = undefined
+
+p :: forall s . S s () => s -> s
+p d = f
+  where
+
+    -- declaring either of these type signatures will cause the bug to go away
+
+    -- f :: s
+    f = c d (o e)
+
+    -- e :: s
+    e = c m m