Explicit foralls in associated type family equations are oblivious to class-bound variables
This doesn't typecheck on GHC HEAD:
{-# LANGUAGE ScopedTypeVariables #-}
{-# LANGUAGE TypeFamilies #-}
module Bug where
class C a where
type T a b
instance C (Maybe a) where
type forall b. T (Maybe a) b = Either a b
$ ~/Software/ghc/inplace/bin/ghc-stage2 Bug.hs
[1 of 1] Compiling Bug ( Bug.hs, Bug.o )
Bug.hs:9:3: error:
The RHS of an associated type declaration mentions out-of-scope variable ‘a’
All such variables must be bound on the LHS
|
9 | type forall b. T (Maybe a) b = Either a b
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
But it ought to. a
isn't out of scope at all—it's bound by the class head instance C (Maybe a)
.
Trac metadata
Trac field | Value |
---|---|
Version | 8.7 |
Type | Bug |
TypeOfFailure | OtherFailure |
Priority | normal |
Resolution | Unresolved |
Component | Compiler |
Test case | |
Differential revisions | |
BlockedBy | |
Related | |
Blocking | |
CC | mayac |
Operating system | |
Architecture |