struct S {
  var x = 0
  struct R {
    func foo() {
      let x = 0
      x += 1
    }
  }
}

main.swift:11:9: error: left side of mutating operator isn't mutable: 'x' is a 'let' constant [cannot_apply_lvalue_binop_to_subelement]
 x += 1
 ~ ^
main.swift:11:9: note: add explicit 'self.' to refer to mutable property of 'S.R' [masked_instance_variable]
 x += 1
 ^
 self.