How do I declare that a computed property ‘throws’ in Swift?

This functionality is added for read-only computed properties in Swift 5.5 as part of SE-0310 (included in Xcode 13).

Based on SE-0310, the syntax would be:

class SomeClass {
  var someProperty: Int {
    get throws {
      throw Err("SNAFU")
    }
  }
}

Here is the previous answer for versions of Swift prior to 5.5:

You cannot throw from a computed property. You must use a function if you want to throw. The Declarations section of the Language Reference part at the end of The Swift Programming Language only lists throws (and rethrows) as a keyword for function and initializer declarations.