Possible type inference regression: "Expected a type, but 'a' has kind 'k0'" (#12069) · Issues · Glasgow Haskell Compiler / GHC

Possible type inference regression: "Expected a type, but 'a' has kind 'k0'"

The following program causes GHC HEAD to give an error:

{-# LANGUAGE ScopedTypeVariables #-}
module Foo where

foo (_ :: p a) = [] :: [a]

[1 of 1] Compiling Foo              ( kind.hs, kind.o )

kind.hs:4:25: error:
    • Expected a type, but ‘a’ has kind ‘k0’
    • In an expression type signature: [a]
      In the expression: [] :: [a]
      In an equation for ‘foo’: foo (_ :: p a) = [] :: [a]

GHC 7.10.1 compiles it fine, and infers the type foo :: p a -> [a].

Trac metadata

Trac field	Value
Version	8.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler (Type checker)
Test case
Differential revisions
BlockedBy
Related
Blocking
CC
Operating system
Architecture

The following program causes GHC HEAD to give an error:

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

foo (_ :: p a) = [] :: [a]
```

```
[1 of 1] Compiling Foo              ( kind.hs, kind.o )

kind.hs:4:25: error:
    • Expected a type, but ‘a’ has kind ‘k0’
    • In an expression type signature: [a]
      In the expression: [] :: [a]
      In an equation for ‘foo’: foo (_ :: p a) = [] :: [a]
```

GHC 7.10.1 compiles it fine, and infers the type `foo :: p a -> [a]`.

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

| Trac field             | Value                   |
| ---------------------- | ----------------------- |
| Version                | 8.1                     |
| Type                   | Bug                     |
| TypeOfFailure          | OtherFailure            |
| Priority               | normal                  |
| Resolution             | Unresolved              |
| Component              | Compiler (Type checker) |
| Test case              |                         |
| Differential revisions |                         |
| BlockedBy              |                         |
| Related                |                         |
| Blocking               |                         |
| CC                     |                         |
| Operating system       |                         |
| Architecture           |                         |

</details>