Template Haskell ignores bad type family definitions (#15362) · Issues · Glasgow Haskell Compiler / GHC

Template Haskell ignores bad type family definitions

This garbage is accepted:

data Nat = Zero | Succ Nat

$( [d| type family a + b where
         Maybe Zero b = b
         Succ a + b = Succ (a + b) |] )

Note the Maybe photo-bombing in the first equation. The problem is that TH uses the symbol declared in the family head for all the equations, ignoring what's actually there. This is sad.

Trac metadata

Trac field	Value
Version	8.4.3
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Template Haskell
Test case
Differential revisions
BlockedBy
Related
Blocking
CC
Operating system
Architecture

This garbage is accepted:

```hs
data Nat = Zero | Succ Nat

$( [d| type family a + b where
         Maybe Zero b = b
         Succ a + b = Succ (a + b) |] )
```

Note the `Maybe` photo-bombing in the first equation. The problem is that TH uses the symbol declared in the family head for all the equations, ignoring what's actually there. This is sad.

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

| Trac field             | Value            |
| ---------------------- | ---------------- |
| Version                | 8.4.3            |
| Type                   | Bug              |
| TypeOfFailure          | OtherFailure     |
| Priority               | normal           |
| Resolution             | Unresolved       |
| Component              | Template Haskell |
| Test case              |                  |
| Differential revisions |                  |
| BlockedBy              |                  |
| Related                |                  |
| Blocking               |                  |
| CC                     |                  |
| Operating system       |                  |
| Architecture           |                  |

</details>