Deriving Generic1 interacts poorly with TypeInType
From @RyanGlScott, ticket:11357#comment:117922:
Vanilla datatypes and data family instances are still inconsistent w.r.t. which type variables are considered "instantiated" in a Generic1 instance. For instance, this is rejected:
λ> data Proxy k (a :: k) = ProxyCon deriving Generic1<interactive>:32:43: error:
• Can't make a derived instance of ‘Generic1 (Proxy *)’:
Proxy must not be instantiated; try deriving `Proxy k a' instead
• In the data declaration for ‘Proxy’And rightfully so, since the visible kind binder k is instantiated to *. But now it's possible to have an equivalent instance for a data family that squeaks past this check!
λ> data family ProxyFam (a :: y) (b :: z)
λ> data instance ProxyFam k (a :: k) = ProxyFamCon deriving Generic1
==================== Derived instances ====================
Derived instances:
instance GHC.Generics.Generic1 (Ghci13.ProxyFam *) where
...[Ryan needs] to investigate further to see why this is the case.
Trac metadata
| Trac field | Value |
|---|---|
| Version | 8.1 |
| Type | Bug |
| TypeOfFailure | OtherFailure |
| Priority | normal |
| Resolution | Unresolved |
| Component | Compiler |
| Test case | |
| Differential revisions | |
| BlockedBy | |
| Related | |
| Blocking | |
| CC | |
| Operating system | |
| Architecture |