Impredicativity flag needed more often

I don't know if this is a bug or a deliberate change, but HEAD requires -XImpredicativeTypes for this program while 7.6.3 does not.

{-# LANGUAGE RankNTypes #-}

f :: (forall a. a) -> b
f = undefined

Trac metadata

Trac field	Value
Version	7.6.3
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler (Type checker)
Test case
Differential revisions
BlockedBy
Related
Blocking
CC
Operating system
Architecture

mentioned in issue #7887 (closed)

changed weight to 5

added Tbug Trac import typechecker labels

mentioned in commit 7a7ace72

mentioned in commit a18ea4f2

closed

There really is a reason for this. The elaborated definition would be

f :: forall b. (forall a. a) -> b
  = /\b. undefined @((forall a. a) -> b)

So we need to instantiate undefined at a polytype, which isn't usually allowed. But it is for error and there's no reason not to allow undefined too.

See the commit comments above. Both for error and undefined, it's a bit of a hack.

Simon

Trac metadata

Trac field	Value
Resolution	Unresolved → ResolvedFixed
Test case	- → typecheck/should_compile/T7888

This is at odds with #11431 (closed). I'm going with #11431 (closed) as that's more consistent, as discussed on that ticket.

added error messages label

added Pnormal label