Ryan Scott authored 5 years ago and Marge Bot committed 5 years ago

GHC used to reject programs of this form:

```
newtype Age = MkAge Int
  deriving Eq via Const Int a
```

That's because an earlier implementation of `DerivingVia` would
generate the following instance:

```
instance Eq Age where
  (==) = coerce @(Const Int a -> Const Int a -> Bool)
                @(Age         -> Age         -> Bool)
                (==)
```

Note that the `a` in `Const Int a` is not bound anywhere, which
causes all sorts of issues. I figured that no one would ever want to
write code like this anyway, so I simply banned "floating" `via` type
variables like `a`, checking for their presence in the aptly named
`reportFloatingViaTvs` function.

`reportFloatingViaTvs` ended up being implemented in a subtly
incorrect way, as #15831 demonstrates. Following counsel with the
sage of gold fire, I decided to abandon `reportFloatingViaTvs`
entirely and opt for a different approach that would _accept_
the instance above. This is because GHC now generates this instance
instead:

```
instance forall a. Eq Age where
  (==) = coerce @(Const Int a -> Const Int a -> Bool)
                @(Age         -> Age         -> Bool)
                (==)
```

Notice that we now explicitly quantify the `a` in
`instance forall a. Eq Age`, so everything is peachy scoping-wise.
See `Note [Floating `via` type variables]` in `TcDeriv` for the full
scoop.

A pleasant benefit of this refactoring is that it made it much easier
to catch the problem observed in #16181, so this patch fixes that
issue too.

Fixes #15831. Fixes #16181.