diff --git a/test/stdlib/StringScanner_test.rb b/test/stdlib/StringScanner_test.rb
new file mode 100644
index 000000000..1a8b7cdce
--- /dev/null
+++ b/test/stdlib/StringScanner_test.rb
@@ -0,0 +1,382 @@
+require_relative "test_helper"
+require "strscan"
+
+class StringScannerSingletonTest < Test::Unit::TestCase
+  include TestHelper
+
+  library "strscan"
+  testing "singleton(::StringScanner)"
+
+  def test_new
+    assert_send_type "(::String) -> ::StringScanner",
+                     StringScanner, :new, "foo"
+    assert_send_type "(::String, fixed_anchor: bool) -> ::StringScanner",
+                     StringScanner, :new, "foo", fixed_anchor: true
+  end
+
+  def test_must_C_version
+    assert_send_type "() -> singleton(::StringScanner)",
+                     StringScanner, :must_C_version
+  end
+end
+
+class StringScannerTest < Test::Unit::TestCase
+  include TestHelper
+
+  library "strscan"
+  testing "::StringScanner"
+
+  # A scanner that has already matched a named-capture regexp; every capture
+  # produced a non-nil String. Use this when you want the "all matched" branch.
+  def matched_named_scanner
+    s = StringScanner.new("Fri Dec 12 1975 14:39")
+    s.scan(/(?<wday>\w+) (?<month>\w+) (?<day>\d+) /)
+    s
+  end
+
+  # A scanner where a named-capture regexp matched, but some captures were
+  # optional and resolved to nil. Use this when you want String? value cells.
+  def partial_named_scanner
+    s = StringScanner.new("Fri")
+    s.scan(/(?<wday>\w+)(?<month> \w+)?/)
+    s
+  end
+
+  # A fresh scanner that has not attempted any match yet.
+  def unmatched_scanner
+    StringScanner.new("Fri Dec 12 1975 14:39")
+  end
+
+  def test_inspect
+    assert_send_type "() -> ::String",
+                     StringScanner.new("foo"), :inspect
+  end
+
+  def test_initialize_copy
+    assert_send_type "(::StringScanner) -> void",
+                     StringScanner.new("foo"), :initialize_copy, StringScanner.new("bar")
+  end
+
+  def test_left_shift
+    assert_send_type "(::String) -> ::StringScanner",
+                     StringScanner.new("foo"), :<<, "bar"
+  end
+
+  def test_square_bracket
+    s = matched_named_scanner
+    assert_send_type "(::Integer) -> ::String",
+                     s, :[], 0
+    assert_send_type "(::String) -> ::String",
+                     s, :[], "wday"
+    assert_send_type "(::Symbol) -> ::String",
+                     s, :[], :wday
+    assert_send_type "(::Integer) -> nil",
+                     unmatched_scanner, :[], 0
+  end
+
+  def test_beginning_of_line?
+    assert_send_type "() -> bool",
+                     StringScanner.new("foo"), :beginning_of_line?
+  end
+
+  def test_bol?
+    assert_send_type "() -> bool",
+                     StringScanner.new("foo"), :bol?
+  end
+
+  def test_captures
+    assert_send_type "() -> ::Array[::String]",
+                     matched_named_scanner, :captures
+    assert_send_type "() -> nil",
+                     unmatched_scanner, :captures
+  end
+
+  def test_charpos
+    assert_send_type "() -> ::Integer",
+                     StringScanner.new("foo"), :charpos
+  end
+
+  def test_check
+    assert_send_type "(::Regexp) -> ::String",
+                     StringScanner.new("foo"), :check, /foo/
+    assert_send_type "(::String) -> ::String",
+                     StringScanner.new("foo"), :check, "foo"
+    assert_send_type "(::Regexp) -> nil",
+                     StringScanner.new("foo"), :check, /nope/
+  end
+
+  def test_check_until
+    assert_send_type "(::Regexp) -> ::String",
+                     StringScanner.new("foobar"), :check_until, /bar/
+    assert_send_type "(::String) -> ::String",
+                     StringScanner.new("foobar"), :check_until, "bar"
+    assert_send_type "(::Regexp) -> nil",
+                     StringScanner.new("foobar"), :check_until, /nope/
+  end
+
+  def test_concat
+    assert_send_type "(::String) -> ::StringScanner",
+                     StringScanner.new("foo"), :concat, "bar"
+  end
+
+  def test_eos?
+    assert_send_type "() -> bool",
+                     StringScanner.new("foo"), :eos?
+  end
+
+  def test_exist?
+    assert_send_type "(::Regexp) -> ::Integer",
+                     StringScanner.new("foobar"), :exist?, /bar/
+    assert_send_type "(::String) -> ::Integer",
+                     StringScanner.new("foobar"), :exist?, "bar"
+    assert_send_type "(::Regexp) -> nil",
+                     StringScanner.new("foobar"), :exist?, /nope/
+  end
+
+  def test_fixed_anchor?
+    assert_send_type "() -> bool",
+                     StringScanner.new("foo"), :fixed_anchor?
+  end
+
+  def test_get_byte
+    assert_send_type "() -> ::String",
+                     StringScanner.new("foo"), :get_byte
+    assert_send_type "() -> nil",
+                     StringScanner.new(""), :get_byte
+  end
+
+  def test_getch
+    assert_send_type "() -> ::String",
+                     StringScanner.new("foo"), :getch
+    assert_send_type "() -> nil",
+                     StringScanner.new(""), :getch
+  end
+
+  def test_match?
+    assert_send_type "(::Regexp) -> ::Integer",
+                     StringScanner.new("foo"), :match?, /foo/
+    assert_send_type "(::String) -> ::Integer",
+                     StringScanner.new("foo"), :match?, "foo"
+    assert_send_type "(::Regexp) -> nil",
+                     StringScanner.new("foo"), :match?, /nope/
+  end
+
+  def test_matched
+    assert_send_type "() -> ::String",
+                     matched_named_scanner, :matched
+    assert_send_type "() -> nil",
+                     unmatched_scanner, :matched
+  end
+
+  def test_matched?
+    assert_send_type "() -> bool",
+                     matched_named_scanner, :matched?
+    assert_send_type "() -> bool",
+                     unmatched_scanner, :matched?
+  end
+
+  def test_matched_size
+    assert_send_type "() -> ::Integer",
+                     matched_named_scanner, :matched_size
+    assert_send_type "() -> nil",
+                     unmatched_scanner, :matched_size
+  end
+
+  def test_named_captures
+    assert_send_type "() -> ::Hash[::String, ::String]",
+                     matched_named_scanner, :named_captures
+    assert_send_type "() -> ::Hash[::String, ::String?]",
+                     partial_named_scanner, :named_captures
+  end
+
+  def test_peek
+    assert_send_type "(::Integer) -> ::String",
+                     StringScanner.new("foo"), :peek, 3
+  end
+
+  def test_peek_byte
+    assert_send_type "() -> ::Integer",
+                     StringScanner.new("foo"), :peek_byte
+    assert_send_type "() -> nil",
+                     StringScanner.new(""), :peek_byte
+  end
+
+  def test_pointer
+    assert_send_type "() -> ::Integer",
+                     StringScanner.new("foo"), :pointer
+  end
+
+  def test_pointer=
+    assert_send_type "(::Integer) -> ::Integer",
+                     StringScanner.new("foo"), :pointer=, 0
+  end
+
+  def test_pos
+    assert_send_type "() -> ::Integer",
+                     StringScanner.new("foo"), :pos
+  end
+
+  def test_pos=
+    assert_send_type "(::Integer) -> ::Integer",
+                     StringScanner.new("foo"), :pos=, 0
+  end
+
+  def test_post_match
+    assert_send_type "() -> ::String",
+                     matched_named_scanner, :post_match
+    assert_send_type "() -> nil",
+                     unmatched_scanner, :post_match
+  end
+
+  def test_pre_match
+    assert_send_type "() -> ::String",
+                     matched_named_scanner, :pre_match
+    assert_send_type "() -> nil",
+                     unmatched_scanner, :pre_match
+  end
+
+  def test_reset
+    assert_send_type "() -> ::StringScanner",
+                     StringScanner.new("foo"), :reset
+  end
+
+  def test_rest
+    assert_send_type "() -> ::String",
+                     StringScanner.new("foo"), :rest
+  end
+
+  def test_rest?
+    assert_send_type "() -> bool",
+                     StringScanner.new("foo"), :rest?
+  end
+
+  def test_rest_size
+    assert_send_type "() -> ::Integer",
+                     StringScanner.new("foo"), :rest_size
+  end
+
+  def test_scan
+    assert_send_type "(::Regexp) -> ::String",
+                     StringScanner.new("foo"), :scan, /foo/
+    assert_send_type "(::String) -> ::String",
+                     StringScanner.new("foo"), :scan, "foo"
+    assert_send_type "(::Regexp) -> nil",
+                     StringScanner.new("foo"), :scan, /nope/
+  end
+
+  def test_scan_byte
+    assert_send_type "() -> ::Integer",
+                     StringScanner.new("foo"), :scan_byte
+    assert_send_type "() -> nil",
+                     StringScanner.new(""), :scan_byte
+  end
+
+  def test_scan_full
+    assert_send_type "(::Regexp, bool, true) -> ::String",
+                     StringScanner.new("foo"), :scan_full, /foo/, true, true
+    assert_send_type "(::String, bool, true) -> ::String",
+                     StringScanner.new("foo"), :scan_full, "foo", false, true
+    assert_send_type "(::Regexp, bool, true) -> nil",
+                     StringScanner.new("foo"), :scan_full, /nope/, true, true
+    assert_send_type "(::Regexp, bool, false) -> ::Integer",
+                     StringScanner.new("foo"), :scan_full, /foo/, true, false
+    assert_send_type "(::String, bool, false) -> ::Integer",
+                     StringScanner.new("foo"), :scan_full, "foo", false, false
+    assert_send_type "(::Regexp, bool, false) -> nil",
+                     StringScanner.new("foo"), :scan_full, /nope/, true, false
+  end
+
+  def test_scan_integer
+    assert_send_type "() -> ::Integer",
+                     StringScanner.new("123abc"), :scan_integer
+    assert_send_type "(base: ::Integer) -> ::Integer",
+                     StringScanner.new("0xff"), :scan_integer, base: 16
+    assert_send_type "() -> nil",
+                     StringScanner.new("abc"), :scan_integer
+  end
+
+  def test_scan_until
+    assert_send_type "(::Regexp) -> ::String",
+                     StringScanner.new("foobar"), :scan_until, /bar/
+    assert_send_type "(::String) -> ::String",
+                     StringScanner.new("foobar"), :scan_until, "bar"
+    assert_send_type "(::Regexp) -> nil",
+                     StringScanner.new("foobar"), :scan_until, /nope/
+  end
+
+  def test_search_full
+    assert_send_type "(::Regexp, bool, true) -> ::String",
+                     StringScanner.new("foobar"), :search_full, /bar/, true, true
+    assert_send_type "(::String, bool, true) -> ::String",
+                     StringScanner.new("foobar"), :search_full, "bar", false, true
+    assert_send_type "(::Regexp, bool, true) -> nil",
+                     StringScanner.new("foobar"), :search_full, /nope/, true, true
+    assert_send_type "(::Regexp, bool, false) -> ::Integer",
+                     StringScanner.new("foobar"), :search_full, /bar/, true, false
+    assert_send_type "(::String, bool, false) -> ::Integer",
+                     StringScanner.new("foobar"), :search_full, "bar", false, false
+    assert_send_type "(::Regexp, bool, false) -> nil",
+                     StringScanner.new("foobar"), :search_full, /nope/, true, false
+  end
+
+  def test_size
+    assert_send_type "() -> ::Integer",
+                     matched_named_scanner, :size
+    assert_send_type "() -> nil",
+                     unmatched_scanner, :size
+  end
+
+  def test_skip
+    assert_send_type "(::Regexp) -> ::Integer",
+                     StringScanner.new("foo"), :skip, /foo/
+    assert_send_type "(::String) -> ::Integer",
+                     StringScanner.new("foo"), :skip, "foo"
+    assert_send_type "(::Regexp) -> nil",
+                     StringScanner.new("foo"), :skip, /nope/
+  end
+
+  def test_skip_until
+    assert_send_type "(::Regexp) -> ::Integer",
+                     StringScanner.new("foobar"), :skip_until, /bar/
+    assert_send_type "(::String) -> ::Integer",
+                     StringScanner.new("foobar"), :skip_until, "bar"
+    assert_send_type "(::Regexp) -> nil",
+                     StringScanner.new("foobar"), :skip_until, /nope/
+  end
+
+  def test_string
+    assert_send_type "() -> ::String",
+                     StringScanner.new("foo"), :string
+  end
+
+  def test_string=
+    assert_send_type "(::String) -> ::String",
+                     StringScanner.new("foo"), :string=, "bar"
+  end
+
+  def test_terminate
+    assert_send_type "() -> ::StringScanner",
+                     StringScanner.new("foo"), :terminate
+  end
+
+  def test_unscan
+    # unscan raises StringScanner::Error unless there was a prior match,
+    # so we have to scan something first.
+    s = StringScanner.new("foo")
+    s.scan(/foo/)
+    assert_send_type "() -> ::StringScanner",
+                     s, :unscan
+  end
+
+  def test_values_at
+    s = matched_named_scanner
+    assert_send_type "(::Integer) -> ::Array[::String]",
+                     s, :values_at, 0
+    assert_send_type "(::String) -> ::Array[::String]",
+                     s, :values_at, "wday"
+    assert_send_type "(::Symbol) -> ::Array[::String]",
+                     s, :values_at, :wday
+    assert_send_type "(::Integer) -> nil",
+                     unmatched_scanner, :values_at, 0
+  end
+end