Ordering of declarations can cause typechecking to fail
The following will successfully typecheck:
```hs
{-# LANGUAGE DataKinds #-}
{-# LANGUAGE KindSignatures #-}
{-# LANGUAGE TypeInType #-}
{-# LANGUAGE TypeFamilies #-}
data CInst
data G (b :: ()) = G
class C a where
type family F a
class (C a) => C' a where
type family F' a (b :: F a)
-- data CInst
instance C CInst where
type F CInst = ()
instance C' CInst where
type F' CInst (b :: F CInst) = G b
```
But if the `data CInst` declaration is moved to where it is currently commented out, typechecking fails with this error:
```
Test.hs:23:18: error:
• Expected kind ‘F CInst’, but ‘b’ has kind ‘()’
• In the second argument of ‘F'’, namely ‘(b :: F CInst)’
In the type instance declaration for ‘F'’
In the instance declaration for ‘C' CInst’
|
23 | type F' CInst (b :: F CInst) = G b
|
```
However, the data declaration //can// be in the lower position if the kind annotation for its argument is instead written as `data G (b :: F CInst) = G`.
This behavior is also exhibited when G is a type family (I believe the sort of type family does not matter, but I know for sure closed and open type families).
I was using GHC 8.2.2 when I discovered this, but user `erisco` on `#haskell` confirmed for 8.2.1 as well.
<details><summary>Trac metadata</summary>
| Trac field | Value |
| ---------------------- | ----------------------- |
| Version | 8.2.1 |
| Type | Bug |
| TypeOfFailure | OtherFailure |
| Priority | normal |
| Resolution | Unresolved |
| Component | Compiler (Type checker) |
| Test case | |
| Differential revisions | |
| BlockedBy | |
| Related | |
| Blocking | |
| CC | |
| Operating system | |
| Architecture | |
</details>
<!-- {"blocked_by":[],"summary":"Ordering of declarations can cause typechecking to fail","status":"New","operating_system":"","component":"Compiler (Type checker)","related":[],"milestone":"","resolution":"Unresolved","owner":{"tag":"Unowned"},"version":"8.2.1","keywords":[],"differentials":[],"test_case":"","architecture":"","cc":[""],"type":"Bug","description":"The following will successfully typecheck:\r\n\r\n{{{#!hs\r\n{-# LANGUAGE DataKinds #-}\r\n{-# LANGUAGE KindSignatures #-}\r\n{-# LANGUAGE TypeInType #-}\r\n{-# LANGUAGE TypeFamilies #-}\r\n\r\ndata CInst\r\n\r\ndata G (b :: ()) = G \r\n\r\nclass C a where\r\n type family F a\r\n \r\nclass (C a) => C' a where\r\n type family F' a (b :: F a)\r\n\r\n-- data CInst\r\n\r\ninstance C CInst where\r\n type F CInst = ()\r\n\r\ninstance C' CInst where\r\ntype F' CInst (b :: F CInst) = G b\r\n}}}\r\n\r\nBut if the `data CInst` declaration is moved to where it is currently commented out, typechecking fails with this error: \r\n\r\n{{{\r\nTest.hs:23:18: error:\r\n • Expected kind ‘F CInst’, but ‘b’ has kind ‘()’\r\n • In the second argument of ‘F'’, namely ‘(b :: F CInst)’\r\n In the type instance declaration for ‘F'’\r\n In the instance declaration for ‘C' CInst’\r\n |\r\n23 | type F' CInst (b :: F CInst) = G b\r\n | \r\n}}}\r\n\r\nHowever, the data declaration //can// be in the lower position if the kind annotation for its argument is instead written as `data G (b :: F CInst) = G`.\r\n\r\nThis behavior is also exhibited when G is a type family (I believe the sort of type family does not matter, but I know for sure closed and open type families).\r\n\r\nI was using GHC 8.2.2 when I discovered this, but user `erisco` on `#haskell` confirmed for 8.2.1 as well.","type_of_failure":"OtherFailure","blocking":[]} -->
issue