Fix parsing of fractional durations (#13832)

The parsing of fractional durations checked for non-negativity by
testing second > 0, which reports false for not only negative integers
but also for 0.

Note that changing `if second > 0` to `if second >= 0` would fix
behaviour for "PT0,6S", but would break "PT-0,6S".

lib/elixir/lib/calendar/iso.ex

@@ -715,7 +715,15 @@ defmodule Calendar.ISO do
       {second, <<delimiter, _::binary>> = rest} when delimiter in [?., ?,] ->
         case parse_microsecond(rest) do
           {{ms, precision}, "S"} ->
-            ms = if second > 0, do: ms, else: -ms
+            ms =
+              case string do
+                "-" <> _ ->
+                  -ms
+
+                _ ->
+                  ms
+              end
+
             {:ok, [second: second, microsecond: {ms, precision}] ++ acc}
 
           _ ->

lib/elixir/test/elixir/calendar/duration_test.exs

@@ -237,8 +237,10 @@ defmodule DurationTest do
     assert Duration.from_iso8601("P4Y2W3Y") == {:error, :invalid_date_component}
     assert Duration.from_iso8601("P5HT4MT3S") == {:error, :invalid_date_component}
     assert Duration.from_iso8601("P5H3HT4M") == {:error, :invalid_date_component}
+    assert Duration.from_iso8601("P0.5Y") == {:error, :invalid_date_component}
     assert Duration.from_iso8601("PT1D") == {:error, :invalid_time_component}
     assert Duration.from_iso8601("PT.6S") == {:error, :invalid_time_component}
+    assert Duration.from_iso8601("PT0.5H") == {:error, :invalid_time_component}
     assert Duration.from_iso8601("invalid") == {:error, :invalid_duration}
   end
 
@@ -262,6 +264,9 @@ defmodule DurationTest do
     assert Duration.from_iso8601!("PT6S") == %Duration{second: 6}
     assert Duration.from_iso8601!("PT1,6S") == %Duration{second: 1, microsecond: {600_000, 1}}
     assert Duration.from_iso8601!("PT-1.6S") == %Duration{second: -1, microsecond: {-600_000, 1}}
+    assert Duration.from_iso8601!("PT0,6S") == %Duration{second: 0, microsecond: {600_000, 1}}
+    assert Duration.from_iso8601!("PT-0,6S") == %Duration{second: 0, microsecond: {-600_000, 1}}
+    assert Duration.from_iso8601!("-PT-0,6S") == %Duration{second: 0, microsecond: {600_000, 1}}
     assert Duration.from_iso8601!("-P10DT4H") == %Duration{day: -10, hour: -4}
     assert Duration.from_iso8601!("-P10DT-4H") == %Duration{day: -10, hour: 4}
     assert Duration.from_iso8601!("P-10D") == %Duration{day: -10}