Poor error message around CUSKs (#13777) · Issues · Glasgow Haskell Compiler / GHC

Poor error message around CUSKs

While typing up ticket:13761#comment:137502, I came across a poor error message around CUSKs.

data Proxy (a :: k) = P
data S :: forall k. Proxy k -> Type where
  MkS :: S (P :: Proxy Maybe)

produces

    You have written a *complete user-suppled kind signature*,
    but the following variable is undetermined: k0 :: *
    Perhaps add a kind signature.
    Inferred kinds of user-written variables:

That promised list of the kinds of user-written variables is empty. Either GHC should find something to print (like k :: k0, perhaps) or omit the header.

Trac metadata

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

While typing up [ticket:13761\#comment:137502](https://gitlab.haskell.org//ghc/ghc/issues/13761#note_137502), I came across a poor error message around CUSKs.

```
data Proxy (a :: k) = P
data S :: forall k. Proxy k -> Type where
  MkS :: S (P :: Proxy Maybe)
```

produces

```
    You have written a *complete user-suppled kind signature*,
    but the following variable is undetermined: k0 :: *
    Perhaps add a kind signature.
    Inferred kinds of user-written variables:
```

That promised list of the kinds of user-written variables is empty. Either GHC should find something to print (like `k :: k0`, perhaps) or omit the header.

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

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

</details>