Support %U/%u/%W/%w/%V/%g/%G formats in Time.strptime

Most of these formats were documented as supported, but were not
actually supported. Document that %g and %G are supported.

If %U/%W is specified without yday and mon/mday are not specified,
then Date.strptime is used to get the appropriate yday.

If cwyear is specifier without the year, or cwday and cweek are
specified without mday and mon, then use Date.strptime and convert
the resulting value to Time, since Time.make_time cannot handle
those conversions

Fixes [Bug #9836]
Fixes [Bug #14241]

lib/time.rb

@@ -397,6 +397,9 @@ class Time
     # %D :: Date (%m/%d/%y)
     # %e :: Day of the month, blank-padded ( 1..31)
     # %F :: Equivalent to %Y-%m-%d (the ISO 8601 date format)
+    # %g :: The last two digits of the commercial year
+    # %G :: The week-based year according to ISO-8601 (week 1 starts on Monday
+    #       and includes January 4)
     # %h :: Equivalent to %b
     # %H :: Hour of the day, 24-hour clock (00..23)
     # %I :: Hour of the day, 12-hour clock (01..12)
@@ -456,7 +459,15 @@ class Time
       else
         year = d[:year]
         year = yield(year) if year && block_given?
-        t = make_time(date, year, d[:yday], d[:mon], d[:mday], d[:hour], d[:min], d[:sec], d[:sec_fraction], d[:zone], now)
+        yday = d[:yday]
+        if (d[:cwyear] && !year) || ((d[:cwday] || d[:cweek]) && !(d[:mon] && d[:mday]))
+          # make_time doesn't deal with cwyear/cwday/cweek
+          return Date.strptime(date, format).to_time
+        end
+        if (d[:wnum0] || d[:wnum1]) && !yday && !(d[:mon] && d[:mday])
+          yday = Date.strptime(date, format).yday
+        end
+        t = make_time(date, year, yday, d[:mon], d[:mday], d[:hour], d[:min], d[:sec], d[:sec_fraction], d[:zone], now)
       end
       t
     end

test/test_time.rb

@@ -528,6 +528,17 @@ class TestTimeExtension < Test::Unit::TestCase # :nodoc:
     assert_equal(15, t.hour)
   end
 
+  def test_strptime_wuvg
+    assert_equal(Time.local(2019, 1, 30), Time.strptime("3 4 2019", "%w %W %Y"))
+    assert_equal(Time.local(2019, 2, 7), Time.strptime("4 5 2019", "%u %U %Y"))
+    assert_equal(Time.local(2019, 1, 28), Time.strptime("4 2019", "%W %Y"))
+    assert_equal(Time.local(2019, 2, 3), Time.strptime("5 2019", "%U %Y"))
+    assert_equal(Time.local(2019, 1, 1), Time.strptime("1 2 2019", "%V %w %G"))
+    assert_equal(Time.local(2016, 1, 1), Time.strptime("53 5 15", "%V %w %g"))
+    assert_equal(Time.local(2018, 12, 31), Time.strptime("1 2019", "%V %G"))
+    assert_equal(Time.local(2015, 12, 28), Time.strptime("53 15", "%V %g"))
+  end
+
   def test_nsec
     assert_equal(123456789, Time.parse("2000-01-01T00:00:00.123456789+00:00").tv_nsec)
   end