Warn about simplifiable class constraints

Provoked by Trac #11948, this patch adds a new warning to GHC

  -Wsimplifiable-class-constraints

It warns if you write a class constraint in a type signature that
can be simplified by an existing instance declaration.  Almost always
this means you should simplify it right now; type inference is very
fragile without it, as #11948 shows.

I've put the warning as on-by-default, but I suppose that if there are
howls of protest we can move it out (as happened for -Wredundant-constraints.

It actually found an example of an over-complicated context in CmmNode.

Quite a few tests use these weird contexts to trigger something else,
so I had to suppress the warning in those.

The 'haskeline' library has a few occurrences of the warning (which
I think should be fixed), so I switched it off for that library in
warnings.mk.

The warning itself is done in TcValidity.check_class_pred.

HOWEVER, when type inference fails we get a type error; and the error
suppresses the (informative) warning.  So as things stand, the warning
only happens when it doesn't cause a problem.  Not sure what to do
about this, but this patch takes us forward, I think.

1 files changed, 2 insertions, 1 deletions

diff --git a/testsuite/tests/th/T3100.hs b/testsuite/tests/th/T3100.hs
index 9e529f13db..40adf6a7b6 100644
--- a/testsuite/tests/th/T3100.hs
+++ b/testsuite/tests/th/T3100.hs
@@ -1,4 +1,4 @@
-{-# OPTIONS_GHC -fno-warn-redundant-constraints #-}
+{-# OPTIONS_GHC -Wno-redundant-constraints -Wno-simplifiable-class-constraints #-}
 {-# LANGUAGE RankNTypes, FlexibleContexts, ImplicitParams, TemplateHaskell #-}
 
 -- This test makes sure TH understands types where
@@ -12,6 +12,7 @@ module T3100 where
 import Language.Haskell.TH
 
 flop :: Ord Int => Int -> Int
+-- Weird test case: (Ord Int) is simplifiable and redundant
 flop x = x
 
 $(do { t <- reify 'flop; return [] })