stdlib: csv: Add types for Array#to_csv and String#parse_csv

refs:

* https://www.rubydoc.info/gems/csv/Array#to_csv-instance_method
* https://www.rubydoc.info/gems/csv/String#parse_csv-instance_method

Author: tk0miya

stdlib/csv/0/csv.rbs

@@ -3814,3 +3814,21 @@ class CSV::Table[out Elem] < Object
   #
   def values_at: (*untyped indices_or_headers) -> untyped
 end
+
+%a{annotate:rdoc:skip}
+class Array[unchecked out Elem] < Object
+  # Equivalent to CSV::generate_line(self, options)
+  #
+  #   ["CSV", "data"].to_csv
+  #     #=> "CSV,data\n"
+  def to_csv: (**untyped options) -> String
+end
+
+%a{annotate:rdoc:skip}
+class String
+  # Equivalent to CSV::parse_line(self, options)
+  #
+  #   "CSV,data".parse_csv
+  #     #=> ["CSV", "data"]
+  def parse_csv: (**untyped options) -> ::Array[String?]?
+end

test/stdlib/CSV_test.rb

@@ -96,3 +96,31 @@ def test_headers
                      csv, :headers
   end
 end
+
+class CSVArrayTest < Test::Unit::TestCase
+  include TestHelper
+
+  library "csv"
+  testing "Array[untyped]"
+
+  def test_to_csv_with_array
+    assert_send_type "() -> String",
+                     [1, 2, 3], :to_csv
+    assert_send_type "(**untyped) -> String",
+                     [1, 2, 3], :to_csv, col_sep: '\t'
+  end
+end
+
+class CSVStringTest < Test::Unit::TestCase
+  include TestHelper
+
+  library "csv"
+  testing "String"
+
+  def test_parse_csv_with_string
+    assert_send_type "() -> Array[String?]",
+                     "1,2,3", :parse_csv
+    assert_send_type "(**untyped) -> Array[String?]",
+                     "1,2,3", :parse_csv, col_sep: '\t'
+  end
+end