The constraint ... is no smaller than the instance head... (Use UndecidableInstances ...)

Example:

```
{-# language FlexibleInstances #-}

module Main where

class X x where
x :: x

class Y y where
y :: y

instance X a => Y a where
y = undefined
```

The error is:

```
/home/noon/tmp/err/src/Main.hs:11:10: error:
• The constraint ‘X a’ is no smaller than the instance head ‘Y a’
(Use UndecidableInstances to permit this)
• In the instance declaration for ‘Y a’
|
11 | instance X a => Y a where
```

Of course, `UndecidableInstances` is perhaps one fix, but a "better" one, as I understand it, [is the newtype wrapper](https://stackoverflow.com/a/7198951).

Maybe this error message could change to make this suggestion?