Dynamic member lookup subscript should support `file:line:` default parameters

[stephencelis]

Is your feature request related to a problem? Please describe.

Dynamic member lookup subscript should support file:line: default parameters. Knowing the calling context of the lookup can be important for surfacing where the source origin of a problem is.

Describe the solution you'd like

The following code should compile:

@dynamicMemberLookup
struct S<T> {
  var wrappedValue: T
  subscript<U>(
    dynamicMember keyPath: KeyPath<T, U>
    // Comment the next line out and all is well.
    , file: StaticString = #file, line: UInt = #line
  ) -> U {
    self.wrappedValue[keyPath: keyPath]
  }
}

[filip-sakel]

If this wasn’t mentioned in the key-path dynamic member lookup proposal, I think it has to go through Swift evolution.

[stephencelis]

@filip-sakel Posted https://forums.swift.org/t/add-default-parameter-support-e-g-file-to-dynamic-member-lookup/58490, but despite some hearts, not a ton of discussion.