Polytype accepted in RHS of default associated type

This hogwash is accepted:

class C a where
  type F a
  type F a = forall m. m a

I will fix.

Trac metadata

Trac field	Value
Version	7.10.2
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
Test case
Differential revisions
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Related
Blocking
CC
Operating system
Architecture