When I say

class C a b | a -> b where
  foo :: a -> b

instance C Int Bool where
  foo = (>0)

blah :: C Int Double => Int -> _a
blah = foo

I get

    • Couldn't match type ‘Double’ with ‘Bool’
        arising from a functional dependency between constraints:
          ‘C Int Bool’ arising from a use of ‘foo’ at Scratch.hs:51:8-10
          ‘C Int Double’ arising from UnkSkol at Scratch.hs:51:1-10
    • In the expression: foo
      In an equation for ‘blah’: blah = foo

There are actually two problems:

1. If we replace _a with Double, GHC accepts the program. I'm not sure it should. But that's a larger problem than...
2. ... when it prints the error, it says UnkSkol. It probably shouldn't.

This ticket is about the second problem, because the first one require more fundep love than is usually on offer around here.

Erroring here is reasonable enoug