"equirecursive" type family leads to stack overflow in ghci
Summary
Trying to typecheck a term using a (silly) type family such that T ~ Maybe T leads to a stack overflow (instead of a proper type error from reaching the max reduction depth, if this is to be an error at all).
Steps to reproduce
> :set -XTypeFamilies -XUndecidableInstances
> type family T where T = Maybe T
> :t Nothing @T
Nothing @T*** Exception: stack overflow(where the exception takes a few seconds to appear)
Compare that output to this variant that immediately produces a proper type error:
> :t Nothing @T :: T
<interactive>:5:1 error:
...Expected behavior
Either a success (Nothing @T has type Maybe T without requiring any type conversion), or a type error instead of an internal exception.
Environment
- GHC version used: 8.10.1