I don't see any error at all until the call site in Swift 3, but either way we should fix this.

Comment by Ryan Collins (JIRA)

@belkadan chriseidhof (JIRA User)

I'm having trouble getting the compiler to throw this error at all. Do you have an example project?

My test project to try to get the compiler to throw that error is here: https://github.com/rymcol/SR2354 (using DEVELOPMENT-SNAPSHOT-2016-08-23-a)

I think that must have just recently changed. We still reject it at the call site, though, so we probably should be rejecting it at the declaration site.

struct X {
    subscript(x: inout Int) -> () {
      x += 1
    }
}

func test(_ x: X) {
  var y = 1
  print(x[&y])
  print(y)
}
test(X())

Resolved by #9147.