Impredicativity behavior in `:k` command in GHCi

In GHCi, the following command works:

Prelude> :set -XRankNTypes
Prelude> :k (Maybe (forall a. a -> a))
(Maybe (forall a. a -> a)) :: *

Note here the type argument of Maybe is a polymorphic type.

However the following would be (correctly) rejected:

Prelude> let f :: (Maybe (forall a. a -> a)) -> Int; f _ = 1

<interactive>:4:10: error:
    • Illegal polymorphic type: forall a. a -> a
      GHC doesn't yet support impredicative polymorphism
    • In the type signature: f :: (Maybe (forall a. a -> a)) -> Int

Question: why does :k behave differently and is it what is expected?

I think if we have #14859 (closed) those are both acceptable though?

Trac metadata

Trac field	Value
Version	8.6.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
Test case
Differential revisions
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Related
Blocking
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Operating system
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changed weight to 5

added Tbug Trac import labels

I think the :k should be rejected to -- maybe we are missing a call to checkValidType somewhere?

Yes if we had explicit impredicativity we should be ok

assigned to @trac-ningning

Yes that's exactly the reason! Just pushed a patch.

Trac metadata

Trac field	Value
Differential revisions	- → D5265

changed milestone to %8.6.2

added 1 deleted label

mentioned in commit c4a876d5

closed

Merged to ghc-8.6 with 804518f7.

Trac metadata

Trac field	Value
Resolution	Unresolved → ResolvedFixed

removed 1 deleted label

mentioned in issue #16170 (closed)

mentioned in commit 804518f7

added Pnormal label