Add explanations for unsolved Typeable constraints

Mrjtjmnrequested to merge
meritamen/ghc:wip/mrjtjmn/add-explanations-for-typeable into master

Ticket: #26532 (closed)

This MR adds explanations for unsolved 'Typeable' constraints.

GHC will now provide additional explanations for an unsolved constraint of the form Typeable ty, explain why GHC did not solve Typeable constraint, there are five cases:

  • ty is a polymorphic type (e.g. forall a. a -> a)
  • ty is a qualified type (e.g. Eq Int => Int)
  • ty is an unboxed sum type
  • ty is an unreduced type family application
  • ty whose kind is not typeable
Edited by Mrjtjmn

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