UndecidableInstances also allows flexible contexts

When I say

module Bug where

data T

instance Eq Int => Eq T

I get

Bug.hs:5:10: error:
    • Non type-variable argument in the constraint: Eq Int
      (Use FlexibleContexts to permit this)
    • In the instance declaration for ‘Eq T’

Good so far. Now, just because I never like to be predictable, I add {-# LANGUAGE UndecidableInstances #-}. Lo, the program is accepted.

Why? I think we should indeed require FlexibleContexts here.

Once we nail the design, the fix is easy: just remove undecidable_ok from this line in GHC.Tc.Validity.check_class_pred:

        InstDeclCtxt {} -> checkValidClsArgs (flexible_contexts || undecidable_ok) cls tys