Update ICU to 58.1

source/i18n/unicode/dtitvfmt.h

Index: source/i18n/unicode/dtitvfmt.h
diff --git a/source/i18n/unicode/dtitvfmt.h b/source/i18n/unicode/dtitvfmt.h
index d163bad60f4c127bd382f171667fa5e9b23e25c7..68360b87dffaf5ba234400d0f52325707c4b635c 100644
--- a/source/i18n/unicode/dtitvfmt.h
+++ b/source/i18n/unicode/dtitvfmt.h
@@ -1,5 +1,7 @@
+// Copyright (C) 2016 and later: Unicode, Inc. and others.
+// License & terms of use: http://www.unicode.org/copyright.html
 /********************************************************************************
-* Copyright (C) 2008-2013,2015, International Business Machines Corporation and
+* Copyright (C) 2008-2016, International Business Machines Corporation and
 * others. All Rights Reserved.
 *******************************************************************************
 *
@@ -504,7 +506,13 @@ public:
 
 
     /**
-     * Gets the date formatter
+     * Gets the date formatter. The DateIntervalFormat instance continues to own
+     * the returned DateFormatter object, and will use and possibly modify it
+     * during format operations. In a multi-threaded environment, the returned
+     * DateFormat can only be used if it is certain that no other threads are
+     * concurrently using this DateIntervalFormatter, even for nominally const
+     * functions.
+     *
      * @return the date formatter associated with this date interval formatter.
      * @stable ICU 4.0
      */
@@ -609,7 +617,7 @@ private:
 
     /**
      * default constructor
-     * @internal ICU 4.0
+     * @internal (private)
      */
     DateIntervalFormat();
 
@@ -685,6 +693,8 @@ private:
      * The full pattern used in this fall-back format is the
      * full pattern of the date formatter.
      *
+     * gFormatterMutex must already be locked when calling this function.
+     *
      * @param fromCalendar      calendar set to the from date in date interval
      *                          to be formatted into date interval string
      * @param toCalendar        calendar set to the to date in date interval
@@ -697,6 +707,7 @@ private:
      *                          On output: the offsets of the alignment field.
      * @param status            output param set to success/failure code on exit
      * @return                  Reference to 'appendTo' parameter.
+     * @internal (private)
      */
     UnicodeString& fallbackFormat(Calendar& fromCalendar,
                                   Calendar& toCalendar,
@@ -952,6 +963,37 @@ private:
                         const UnicodeString* secondPart,
                         UBool laterDateFirst);
 
+    /**
+     * Format 2 Calendars to produce a string.
+     * Implementation of the similar public format function.
+     * Must be called with gFormatterMutex already locked.
+     *
+     * Note: "fromCalendar" and "toCalendar" are not const,
+     * since calendar is not const in  SimpleDateFormat::format(Calendar&),
+     *
+     * @param fromCalendar      calendar set to the from date in date interval
+     *                          to be formatted into date interval string
+     * @param toCalendar        calendar set to the to date in date interval
+     *                          to be formatted into date interval string
+     * @param appendTo          Output parameter to receive result.
+     *                          Result is appended to existing contents.
+     * @param fieldPosition     On input: an alignment field, if desired.
+     *                          On output: the offsets of the alignment field.
+     *                          There may be multiple instances of a given field type
+     *                          in an interval format; in this case the fieldPosition
+     *                          offsets refer to the first instance.
+     * @param status            Output param filled with success/failure status.
+     *                          Caller needs to make sure it is SUCCESS
+     *                          at the function entrance
+     * @return                  Reference to 'appendTo' parameter.
+     * @internal (private)
+     */
+    UnicodeString& formatImpl(Calendar& fromCalendar,
+                              Calendar& toCalendar,
+                              UnicodeString& appendTo,
+                              FieldPosition& fieldPosition,
+                              UErrorCode& status) const ;
+
 
     // from calendar field to pattern letter
     static const UChar fgCalendarFieldToPatternLetter[];