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The docs claims

In types, an operator symbol like (+) is normally treated as a type variable, just like a. Thus in Haskell 98 you can say

type T (+) = ((+), (+))
-- Just like: type T a = (a,a)

but that does not seem to work:

~ $ ghci-8.10
GHCi, version 8.10.2: https://www.haskell.org/ghc/  :? for help
Prelude> type Test (+) = ((+), (+))

<interactive>:1:11: error:
    Unexpected type ‘(+)’
    In the type declaration for ‘Test’
    A type declaration should have form
      type Test a = ...
Prelude> type Test a = (a, a)
Prelude> :set -XHaskell98 
Prelude> type Test (+) = ((+), (+))

<interactive>:4:11: error:
    Unexpected type ‘(+)’
    In the type declaration for ‘Test’
    A type declaration should have form
      type Test a = ...