Defining methods on Updates<MySelection>

[rcarver]

Hey @stephencelis, we made some fixes for @Selection in 0.26.0 and somehow I didn't end up updating the app to use them. I'm doing that now and hit a snag.

From that change:

This is the simplest way I could come up with to fix the nested update issue. Underscored type for now since I'm not sure folks should be using it, especially if we come up with a better way to handle things in the future. But if we come up with more use cases for this type, we could consider making a proper public API.

Originally posted by @stephencelis in #233

The issue is that I've defined methods on selection types in the same way that you'd do so for table types, via Updates<MyTable> for example:

@Selection struct NestedFields {
  var honestCount: Int = 0
  var optionalCount: Int?
}

extension Updates<NestedFields> {
  mutating func modify(count: Int) {
    self.honestCount = count
    self.optionalCount = Optional(count)
  }
}

@Test func mutateSelectionFields() {
  assertInlineSnapshot(
    of: Root.update {
      $0.fields.modify(count: 4)
    },
    as: .sql
  )
}

Now it looks like I'd have to define it like this:

extension Updates<TableAlias<NestedFields, _TableAliasName<Root>>> {
  mutating func modify(count: Int) {
    self.honestCount = count
    self.optionalCount = Optional(count)
  }
}

Which is unfortunate since now modify(count:) can only be used via Root and not other embedding types. We can clean up the syntax like this, but the coupling of selection+root remains.

public typealias UpdatesOf<S, B> = Updates<TableAlias<S, _TableAliasName<B>>> where S: Table & QueryExpression, B: Table

So I guess the question is: it possible to define selection updates on a type with one generic?

[stephencelis]

Can you constrain the helper itself with generics?

extension Updates {
  mutating func modify<S, B>(count: Int) where /* constraints */ {
    …
  }
}

Since this doesn't appear to be a bug I'm going to convert to a discussion.