GHC 8.2 regression: "Can't find interface-file declaration" for promoted data family instance (#13915) · Issues · Glasgow Haskell Compiler / GHC

GHC 8.2 regression: "Can't find interface-file declaration" for promoted data family instance

Due to #12088, you can't define a data family instance and promote it in the same module. One could, up until GHC 8.2, work around this using (somewhat obscure) wisdom: define the data family instance in a separate module from where it's promoted. For example, Bug typechecks in GHC 8.0.1 and 8.0.2:

{-# LANGUAGE TypeFamilies #-}
module Foo where

data family T a
data instance T Int = MkT

{-# LANGUAGE TypeInType #-}
module Bug where

import Foo

data Proxy (a :: k)
data S = MkS (Proxy 'MkT)

However, this stopped typechecking in GHC 8.2:

$ /opt/ghc/8.2.1/bin/ghci Bug.hs
GHCi, version 8.2.0.20170623: http://www.haskell.org/ghc/  :? for help
Loaded GHCi configuration from /home/rgscott/.ghci
[1 of 2] Compiling Foo              ( Foo.hs, interpreted )
[2 of 2] Compiling Bug              ( Bug.hs, interpreted )

Bug.hs:1:1: error:
    Can't find interface-file declaration for variable Foo.$tc'MkT
      Probable cause: bug in .hi-boot file, or inconsistent .hi file
      Use -ddump-if-trace to get an idea of which file caused the error
  |
1 | {-# LANGUAGE TypeInType #-}
  | ^

Trac metadata

Trac field	Value
Version	8.2.1-rc2
Type	Bug
TypeOfFailure	OtherFailure
Priority	high
Resolution	Unresolved
Component	Compiler
Test case
Differential revisions
BlockedBy
Related
Blocking
CC
Operating system
Architecture

```hs
{-# LANGUAGE TypeFamilies #-}
module Foo where

data family T a
data instance T Int = MkT
```

```hs
{-# LANGUAGE TypeInType #-}
module Bug where

import Foo

data Proxy (a :: k)
data S = MkS (Proxy 'MkT)
```

However, this stopped typechecking in GHC 8.2:

```
$ /opt/ghc/8.2.1/bin/ghci Bug.hs
GHCi, version 8.2.0.20170623: http://www.haskell.org/ghc/  :? for help
Loaded GHCi configuration from /home/rgscott/.ghci
[1 of 2] Compiling Foo              ( Foo.hs, interpreted )
[2 of 2] Compiling Bug              ( Bug.hs, interpreted )

Bug.hs:1:1: error:
    Can't find interface-file declaration for variable Foo.$tc'MkT
      Probable cause: bug in .hi-boot file, or inconsistent .hi file
      Use -ddump-if-trace to get an idea of which file caused the error
  |
1 | {-# LANGUAGE TypeInType #-}
  | ^
```

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

| Trac field             | Value        |
| ---------------------- | ------------ |
| Version                | 8.2.1-rc2    |
| Type                   | Bug          |
| TypeOfFailure          | OtherFailure |
| Priority               | high         |
| Resolution             | Unresolved   |
| Component              | Compiler     |
| Test case              |              |
| Differential revisions |              |
| BlockedBy              |              |
| Related                |              |
| Blocking               |              |
| CC                     |              |
| Operating system       |              |
| Architecture           |              |

</details>