Non-local bug reporting around levity polymorphism

From ticket:13105#comment:141886.

In HEAD (or, at least, my version, on the wip/rae branch), this code

class Unbox (t :: *) (r :: TYPE k) | t -> r, r -> t where
  unbox :: t -> r
  box :: r -> t
  
instance Unbox Int Int# where
  unbox (I# i) = i
  box i = I# i

instance Unbox Char Char# where
  unbox (C# c) = c
  box c = C# c

instance (Unbox a a', Unbox b b') => Unbox (a,b) (# a', b' #) where
  unbox (a,b) = (# unbox a, unbox b #)
  box (# a, b #) = (box a, box b)

testInt :: Int
testInt = box (unbox 1)

testTup :: (Int, Char)
testTup = box (unbox (1, 'a'))

fails with

Bug.hs:27:11: error:
    • Couldn't match a lifted type with an unlifted type
      When matching types
        a' :: *
        Int# :: TYPE 'IntRep
    • In the expression: box (unbox 1)
      In an equation for ‘testInt’: testInt = box (unbox 1)
   |
27 | testInt = box (unbox 1)
   |           ^^^^^^^^^^^^^

Bug.hs:27:16: error:
    • Couldn't match a lifted type with an unlifted type
      When matching types
        a' :: *
        Int# :: TYPE 'IntRep
    • In the first argument of ‘box’, namely ‘(unbox 1)’
      In the expression: box (unbox 1)
      In an equation for ‘testInt’: testInt = box (unbox 1)
   |
27 | testInt = box (unbox 1)
   |                ^^^^^^^

Bug.hs:42:11: error:
    • Couldn't match a lifted type with an unlifted type
      When matching types
        a' :: *
        Int# :: TYPE 'IntRep
    • In the expression: box (unbox (1, 'a'))
      In an equation for ‘testTup’: testTup = box (unbox (1, 'a'))
   |
42 | testTup = box (unbox (1, 'a'))
   |           ^^^^^^^^^^^^^^^^^^^^

I think it should succeed. Worse, when I comment out the testTup definition, the file succeeds... but note that two of the errors above are in testInt, which compiles fine on its own.

Trac metadata

Trac field	Value
Version	8.3
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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