Inconsistent precedence of ~
The following compiles as expected:
```hs
{-# LANGUAGE TypeFamilies #-}
type family Foo a b
f :: (Foo a b ~ Int) => a -> b -> b
f = error ""
```
but this fails:
```hs
{-# LANGUAGE TypeFamilies #-}
type family a \\ b
f :: (a \\ b ~ Int) => a -> b -> b
f = error ""
```
with the error
> "The second argument of `(\\)` should have kind `*`, but `b ~ Int` has kind `Constraint`."
Thus the first example is being parsed as `(Foo a b) ~ Int`, while the second is parsed as `a \\ (b ~ Int)`. I believe the second example should compile, i.e. `(\\)` and `Foo` should have the same precedence, both of which are higher than `(~)`.
<details><summary>Trac metadata</summary>
| Trac field | Value |
| ---------------------- | ----------------- |
| Version | 7.8.4 |
| Type | Bug |
| TypeOfFailure | OtherFailure |
| Priority | normal |
| Resolution | Unresolved |
| Component | Compiler (Parser) |
| Test case | |
| Differential revisions | |
| BlockedBy | |
| Related | |
| Blocking | |
| CC | |
| Operating system | |
| Architecture | |
</details>
<!-- {"blocked_by":[],"summary":"Inconsistent precedence of ~","status":"New","operating_system":"","component":"Compiler (Parser)","related":[],"milestone":"","resolution":"Unresolved","owner":{"tag":"Unowned"},"version":"7.8.4","keywords":[],"differentials":[],"test_case":"","architecture":"","cc":[""],"type":"Bug","description":"The following compiles as expected:\r\n\r\n{{{#!hs\r\n{-# LANGUAGE TypeFamilies #-}\r\ntype family Foo a b\r\n\r\nf :: (Foo a b ~ Int) => a -> b -> b\r\nf = error \"\"\r\n}}}\r\n\r\nbut this fails:\r\n{{{#!hs\r\n{-# LANGUAGE TypeFamilies #-}\r\ntype family a \\\\ b\r\n\r\nf :: (a \\\\ b ~ Int) => a -> b -> b\r\nf = error \"\"\r\n}}}\r\n\r\nwith the error \r\n> \"The second argument of `(\\\\)` should have kind `*`, but `b ~ Int` has kind `Constraint`.\"\r\n\r\nThus the first example is being parsed as `(Foo a b) ~ Int`, while the second is parsed as `a \\\\ (b ~ Int)`. I believe the second example should compile, i.e. `(\\\\)` and `Foo` should have the same precedence, both of which are higher than `(~)`.","type_of_failure":"OtherFailure","blocking":[]} -->
issue