Parse erorr on input `->`

From [stackoverflow](https://stackoverflow.com/questions/69122820):

```haskell
concat [[a]] -> [a]
concat xss = [x | xs <- xss, x <- xs]
```

Produces the error:

```
Test.hs:1:14: error: parse error on input ‘->’
|
1 | concat [[a]] -> [a]
| ^^
```

Which is not very descriptive. Perhaps GHC knows enough to be able to suggest adding `::`? Maybe the `->` token can be used to infer that the user probably meant to write a type signature?

Actually, I think this is very similar to #14 in the sense that the compiler starts parsing a top-level splice. But in this case the suggestion to use ` = ` is not right.