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, 1 insertions, 1 deletions

diff --git a/compiler/cmm/CmmNode.hs b/compiler/cmm/CmmNode.hs
index 40bb5a0a33..c93639c3c0 100644
--- a/compiler/cmm/CmmNode.hs
+++ b/compiler/cmm/CmmNode.hs
@@ -335,7 +335,7 @@ instance UserOfRegs GlobalReg (CmmNode e x) where
                        (b -> GlobalReg -> b) -> b -> a -> b
           fold f z n = foldRegsUsed dflags f z n
 
-instance (Ord r, UserOfRegs r CmmExpr) => UserOfRegs r ForeignTarget where
+instance (Ord r, UserOfRegs r CmmReg) => UserOfRegs r ForeignTarget where
   -- The (Ord r) in the context is necessary here
   -- See Note [Recursive superclasses] in TcInstDcls
   foldRegsUsed _      _ z (PrimTarget _)      = z