The `(~)` type operator is currently considered built-in syntax, with the following consequences: 1. The user is not allowed to define their own type constructors with this name: ``` ghci> class a ~ b <interactive>:1:1: error: Illegal binding of built-in syntax: ~ ``` 2. Writing `a ~ b` does not require enabling `-XTypeOperators`. 3. The `~` type operator is always in scope. I created [GHC Proposal #371](https://github.com/ghc-proposals/ghc-proposals/pull/371) to discuss making it an entirely ordinary type operator. ~~However, that would be a breaking change.~~ ~~This ticket is to discuss a less intrusive change: export the `(~)` from `Data.Type.Equality`, but also let the user define their own. In the implementation, this just means that if we failed to look up `~`, default to `Data.Type.Equality.~`.~~ This ticket is to track the implementation of that (now accepted) proposal. As the result: * Users can define their own, entirely ordinary `~`~~, while the built-in `~` remains magical.~~ * Haddock can display the documentation for the built-in `~` (currently it is unable to do so).