GHC doesn't use fixity in type instances

If I say

data a * b
infixl 7 *

type family a + b
infixl 6 +

type instance Int * Bool + Double = Float

(with -XNoStarIsType -- * is not the issue here)

I get

    • Illegal family instance for ‘*’
        (* is not an indexed type family)
    • In the type instance declaration for ‘*’

But that's wrong. The * should bind tighter than the +, meaning this is an instance for +, not *. This also fails for closed type families:

data a * b
infixl 7 *

type family a + b where
  Int * Bool + Double = Float
infixl 6 +

Interestingly, a class instance works here.

This did not bite me "in the wild", but came up while reading the GHC source code. Still, it is a real bug.

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