Error about wrong missing instance

Here's a puzzling one. I've made a mistake in the second clause of `leftSkewed`. The second argument should be `Leaf n`, not `n`. Therefore I really want the error "No instance for `Num Tree`", pointing to that mistake. Instead I get the error "No instance for `Eq Tree`" pointing to the `0` match in the first clause!

I think what has happened here is that the type checker has deduced that `n :: Tree`, and given that we're passing `n - 1` to `leftSkewed` the argument to `leftSkewed` must be `:: Tree`. Then the `0` pattern requires an `Eq Tree` constraint. But I wish it would just bail out earlier and say that it can't find a `Num` instance for `Tree`!

```haskell
data Tree = Branch Tree Tree | Leaf Int

leftSkewed 0 = Leaf 0
leftSkewed n = Branch (leftSkewed (n - 1)) n
```