GHC unexpectedly refines explicit kind signatures (#9201) · Issues · Glasgow Haskell Compiler / GHC

GHC unexpectedly refines explicit kind signatures

The following program should be rejected by the type checker:

{-# LANGUAGE PolyKinds, FunctionalDependencies, MultiParamTypeClasses #-}

class MonoidalCCC (f :: x -> y) (d :: y -> y -> *) | f -> d where
  ret :: d a (f a)

Instead it is accepted, but the kind variables specified above are 'corrected' during typechecking to:

>>> :browse
class MonoidalCCC (f :: x -> x) (d :: x -> x -> *) | f -> d where
  ret :: d a (f a)

This may be similar in root cause to issue #9200 (closed), but there a valid program is rejected, and here an invalid program is accepted, so the fixes may be quite different.

It seems these kind variables are just being allowed to unify rather than being checked for subsumption.

Trac metadata

Trac field	Value
Version	7.8.2
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler (Type checker)
Test case
Differential revisions
BlockedBy
Related
Blocking
CC	ekmett
Operating system
Architecture

The following program should be rejected by the type checker:

```
{-# LANGUAGE PolyKinds, FunctionalDependencies, MultiParamTypeClasses #-}

class MonoidalCCC (f :: x -> y) (d :: y -> y -> *) | f -> d where
  ret :: d a (f a)
```

Instead it is accepted, but the kind variables specified above are 'corrected' during typechecking to:

```
>>> :browse
class MonoidalCCC (f :: x -> x) (d :: x -> x -> *) | f -> d where
  ret :: d a (f a)
```

This may be similar in root cause to issue #9200, but there a valid program is rejected, and here an invalid program is accepted, so the fixes may be quite different.

It seems these kind variables are just being allowed to unify rather than being checked for subsumption.

<details><summary>Trac metadata</summary>

| Trac field             | Value                   |
| ---------------------- | ----------------------- |
| Version                | 7.8.2                   |
| Type                   | Bug                     |
| TypeOfFailure          | OtherFailure            |
| Priority               | normal                  |
| Resolution             | Unresolved              |
| Component              | Compiler (Type checker) |
| Test case              |                         |
| Differential revisions |                         |
| BlockedBy              |                         |
| Related                |                         |
| Blocking               |                         |
| CC                     | ekmett                  |
| Operating system       |                         |
| Architecture           |                         |

</details>