Impredicativity is still sneaking in
This is erroneously accepted:
{-# LANGUAGE RankNTypes #-}
module Bug where
foo :: forall a. (Show a => a -> a) -> ()
foo = undefinedundefined is being instantiated at (Show a => a -> a) -> (), which requires impredicativity. GHC does the right thing (reject!) if we try to instantiate with a type involving proper foralls.
I imagine this is a simple missing check.
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 |