Function with type error accepted (#9636) · Issues · Glasgow Haskell Compiler / GHC

Function with type error accepted

The following program is accepted by ghc even though it is clearly wrong. The type family T is closed so T Bool cannot ever be reduced, so it must be a type error.

{-# LANGUAGE TypeFamilies #-}
module Err62 where

type family T a where
    T Int = ()

x :: T Bool
x = undefined

f :: T Bool -> Bool
f _ = True

y :: Bool
y = f x

Trac metadata

Trac field	Value
Version	7.8.3
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
Test case
Differential revisions
BlockedBy
Related
Blocking
CC
Operating system
Architecture

The following program is accepted by ghc even though it is clearly wrong.
The type family T is closed so T Bool cannot ever be reduced, so it must be a type error.

```hs
{-# LANGUAGE TypeFamilies #-}
module Err62 where

type family T a where
    T Int = ()

x :: T Bool
x = undefined

f :: T Bool -> Bool
f _ = True

y :: Bool
y = f x
```

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

| Trac field             | Value        |
| ---------------------- | ------------ |
| Version                | 7.8.3        |
| Type                   | Bug          |
| TypeOfFailure          | OtherFailure |
| Priority               | normal       |
| Resolution             | Unresolved   |
| Component              | Compiler     |
| Test case              |              |
| Differential revisions |              |
| BlockedBy              |              |
| Related                |              |
| Blocking               |              |
| CC                     |              |
| Operating system       |              |
| Architecture           |              |

</details>