T9222.stderr 945 Bytes
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T9222.hs:13:3:
    Couldn't match type ‘b0’ with ‘b’
      ‘b0’ is untouchable
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        inside the constraints: a ~ '(b0, c0)
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        bound by the type of the constructor ‘Want’:
                   (a ~ '(b0, c0)) => Proxy b0
        at T9222.hs:13:3
      ‘b’ is a rigid type variable bound by
          the type of the constructor ‘Want’:
            ((a ~ '(b, c)) => Proxy b) -> Want a
          at T9222.hs:13:3
    Expected type: '(b, c)
      Actual type: a
    In the ambiguity check for the type of the constructor ‘Want’:
      Want :: forall (k :: BOX)
                     (k1 :: BOX)
                     (a :: (,) k k1)
                     (b :: k)
                     (c :: k1).
              ((a ~ '(b, c)) => Proxy b) -> Want a
    To defer the ambiguity check to use sites, enable AllowAmbiguousTypes
    In the definition of data constructor ‘Want’
    In the data declaration for ‘Want’