Ensure exprIsCheap/exprIsExpandable deal with Cast properly

This bug was causing a Lint error on the stable branch. For some
reason it doesn't show up in HEAD, but it's still worth fixing.

The point is that ((f `cast` co) a) is cheap if f has arity>1.
This was being gratuitously missed before.

diff --git a/compiler/coreSyn/CoreUtils.lhs b/compiler/coreSyn/CoreUtils.lhs
index b50cb01..c901fc2 100644 (file)
--- a/compiler/coreSyn/CoreUtils.lhs
+++ b/compiler/coreSyn/CoreUtils.lhs
@@ -563,6 +563,7 @@ exprIsCheap' good_app other_expr    -- Applications and variables
   = go other_expr []
   where
        -- Accumulate value arguments, then decide
   = go other_expr []
   where
        -- Accumulate value arguments, then decide

+    go (Cast e _) val_args                 = go e val_args

     go (App f a) val_args | isRuntimeArg a = go f (a:val_args)
                          | otherwise      = go f val_args
 
     go (App f a) val_args | isRuntimeArg a = go f (a:val_args)
                          | otherwise      = go f val_args