Fix 'SPECIALISE instance'

Trac #12944 showed that the DsBinds code that implemented a
SPECIALISE pragma was inadequate if the constraints solving
added let-bindings for dictionaries.  The result was that
we ended up with an unbound dictionary in a DFunUnfolding -- and
Lint didn't even check for that!

Fixing this was not entirely straightforward

* In DsBinds.dsSpec we use a new function
     TcEvidence.collectHsWrapBinders
  to pick off the lambda binders from the HsWapper

* dsWrapper now returns a (CoreExpr -> CoreExpr) function

* CoreUnfold.specUnfolding now takes a (CoreExpr -> CoreExpr)
  function it can use to specialise the unfolding.

On the whole the code is simpler than before.

1 files changed, 7 insertions, 4 deletions

diff --git a/compiler/deSugar/DsExpr.hs b/compiler/deSugar/DsExpr.hs
index a08c3ac7cb..214cb0bb32 100644
--- a/compiler/deSugar/DsExpr.hs
+++ b/compiler/deSugar/DsExpr.hs
@@ -214,8 +214,9 @@ dsExpr (HsOverLit lit)        = dsOverLit lit
 
 dsExpr (HsWrap co_fn e)
   = do { e' <- dsExpr e
-       ; wrapped_e <- dsHsWrapper co_fn e'
+       ; wrap' <- dsHsWrapper co_fn
        ; dflags <- getDynFlags
+       ; let wrapped_e = wrap' e'
        ; warnAboutIdentities dflags e' (exprType wrapped_e)
        ; return wrapped_e }
 
@@ -748,9 +749,11 @@ dsSyntaxExpr (SyntaxExpr { syn_expr      = expr
                          , syn_arg_wraps = arg_wraps
                          , syn_res_wrap  = res_wrap })
              arg_exprs
-  = do { args <- zipWithM dsHsWrapper arg_wraps arg_exprs
-       ; fun  <- dsExpr expr
-       ; dsHsWrapper res_wrap $ mkApps fun args }
+  = do { fun            <- dsExpr expr
+       ; core_arg_wraps <- mapM dsHsWrapper arg_wraps
+       ; core_res_wrap  <- dsHsWrapper res_wrap
+       ; let wrapped_args = zipWith ($) core_arg_wraps arg_exprs
+       ; return (core_res_wrap (mkApps fun wrapped_args)) }
 
 findField :: [LHsRecField Id arg] -> Name -> [arg]
 findField rbinds sel