Unable to instantiate data members of kind Nat in backpack signatures. (#15138) · Issues · Glasgow Haskell Compiler / GHC

Unable to instantiate data members of kind Nat in backpack signatures.

This is in context with backpack signatures and their instantiation with concrete implementation. Consider the signature Abstract which contains a data NatType of kind Nat.

{- skipped relevant language extensions -}

signature Abstract where
     import GHC.TypeLits
     data NatType :: Nat  -- comment this out

Concrete implementations are unable to instantiate this abstract class during linking

   module Concrete where
     type NatType = 42

This is neither working with explicit .bkp files nor actual cabal package. I have isolated a minimum example into a repository a cabal packages as well as a single bkp file. The urls above also have the associated ghc log messages.

The version of ghc I have tested with is 8.4.1

Trac metadata

Trac field	Value
Version	8.4.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
Test case
Differential revisions
BlockedBy
Related
Blocking
CC
Operating system
Architecture

This is in context with backpack signatures and their instantiation with concrete implementation. Consider the signature `Abstract` which contains a
data `NatType` of kind `Nat`.

```hs
{- skipped relevant language extensions -}

signature Abstract where
     import GHC.TypeLits
     data NatType :: Nat  -- comment this out

```

Concrete implementations are unable to instantiate this abstract class during
linking

```hs
   module Concrete where
     type NatType = 42
```

This is neither working with explicit .bkp files nor actual cabal package.
I have isolated a minimum example into a repository [a cabal packages](https://github.com/piyush-kurur/backpack-nat) as well as a [single bkp file](https://github.com/piyush-kurur/backpack-nat/blob/master/as-bkpfiles/backpack-nat.bkp). The urls above also have the associated
ghc log messages.

The version of ghc I have tested with is 8.4.1

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

| Trac field             | Value        |
| ---------------------- | ------------ |
| Version                | 8.4.1        |
| Type                   | Bug          |
| TypeOfFailure          | OtherFailure |
| Priority               | normal       |
| Resolution             | Unresolved   |
| Component              | Compiler     |
| Test case              |              |
| Differential revisions |              |
| BlockedBy              |              |
| Related                |              |
| Blocking               |              |
| CC                     |              |
| Operating system       |              |
| Architecture           |              |

</details>