(~) should be infix 0

Summary

(~) has no fixity declaration, so is treated as having maximal fixity. This breaks equality constraints where one side uses an infix operator (i.e. of the form (x+y ~ z)), requiring seemingly unnecessary parentheses around x+y.

Steps to reproduce

GHCi 8.6.5:

Prelude> import GHC.TypeNats
Prelude GHC.TypeNats> :set -XDataKinds -XTypeOperators 
Prelude GHC.TypeNats> :k (2 ~ 3 + 5)

<interactive>:1:2: error:
    • Expected kind ‘Nat’, but ‘2 ~ 3’ has kind ‘Constraint’
    • In the first argument of ‘(+)’, namely ‘2 ~ 3’
      In the type ‘(2 ~ 3 + 5)’

Expected behavior

(2 ~ 3 + 5) :: Constraint

Environment

• GHC version used: 8.6.5