Odd error about 'DatatypeContexts'

`DatatypeContexts` is deprecated anyway and the following file will yield an error, while it compiles perfectly with ghc-8.0.2:

```hs
module Foo where

data Show a => Foo a = Foo a
```

Error:

```
src/Foo.hs
	error: 3:6 Illegal datatype context (use DatatypeContexts): Show a =>
```