Matthew Pickering authored Feb 07, 2017 and Ben Gamari committed Feb 07, 2017

Correct behaviour of push_bang_into_newtype when the pattern match has
no arguments. A user can write

```
newtype T = T Int

f :: T -> ()
f !(T {}) = ()
```
in which case we have to push the bang inwards through the newtype in
order to achieve the desired strictness properties. This patch fixes
this special case where the pattern match has no arguments to push the
bang onto. We now make up a wildcard pattern which is wrapped in the
bang pattern.

```
f (T !_) = ()
```

Reviewers: simonpj, austin, bgamari

Reviewed By: simonpj

Subscribers: simonpj, thomie

Differential Revision: https://phabricator.haskell.org/D3057