Ningning Xie authored Jul 29, 2018 and Krzysztof Gogolewski committed Jul 29, 2018

Summary:
Given

```
co1 = \/ tv1 : eta1. r1
co2 = \/ tv2 : eta2. r2
```

We would like to optimize `co1; co2` so we push transitivity inside forall.
It should be

```
\/tv1 : (eta1;eta2).  (r1; r2[tv2 |-> tv1 |> eta1])
```

It is implemented in the ForAllCo case in opt_trans_rule in OptCoercion.
However current implementation is not right:

```
r2' = substCoWithUnchecked [tv2] [TyVarTy tv1] r2 -- ill-kinded!
```

This patch corrects it to be

```
r2' = substCoWithUnchecked [tv2] [mkCastTy (TyVarTy tv1) eta1] r2
```

Test Plan: validate

Reviewers: bgamari, goldfire, RyanGlScott

Reviewed By: RyanGlScott

Subscribers: rwbarton, thomie, carter

GHC Trac Issues: #15453

Differential Revision: https://phabricator.haskell.org/D5018