IdentifiedArray.replaceSubrange causes a stack overflow

[peterkovacs]

Describe the bug
Calling replaceSubrange on an identified array causes a stack overflow.

To Reproduce

  func testReplaceSubrange() {
    struct User: Equatable, Identifiable {
      let id: Int
      var name: String
    }

    var array: IdentifiedArray = [
      User(id: 3, name: "Blob Sr."),
      User(id: 2, name: "Blob Jr."),
      User(id: 1, name: "Blob"),
    ]

    array.replaceSubrange(0...1, with: [
      User(id: 4, name: "Flob IV"),
      User(id: 5, name: "Flob V")
    ])

    XCTAssertEqual(
      array,
      [User(id: 4, name: "Flob IV"), User(id: 5, name: "Flob V"), User(id: 1, name: "Blob")]
    )
  }

This results in repeated calls to

#69778	0x0000000111c933ed in protocol witness for RangeReplaceableCollection.replaceSubrange<A>(_:with:) in conformance <> IdentifiedArray<A, B> ()
#69779	0x00007fff5116a0f7 in RangeReplaceableCollection.replaceSubrange<A, B>(_:with:) ()

Environment

• Xcode 11.5
• Swift 5.2.4

[stephencelis]

Fixed by #126.