ICU-20558 Fix regression in DateTimePatternGenerator

Backporting to ICU 63. This is manually cherry-picked from
commit: 693adf3

This fixes a regression introduced by commit
b12a927 for issue ICU-13778.

The above commit improved the error checking in the
DateTimePatternGenerator class, adding checks for errors/failures
where there previously was none at all. This was done in order to
catch catastrophic errors like out-of-memory (OOM), and properly
report them to the caller, rather than ignoring/hiding these errors.

However, in doing so it exposed a case where the code was depending
on ignoring errors in order to fall-back to the Gregorian calendar
when the default ICU locale is set to root.

This restores the previous behavior, by allowing the error of
U_MISSING_RESOURCE_ERROR to fall-though and continue without
reporting back an error to the caller.

Note: This regression was technically introduced in ICU 63, and
also effects ICU 64 as well.

icu4c/source/i18n/dtptngen.cpp

@@ -756,6 +756,7 @@ void
 DateTimePatternGenerator::getCalendarTypeToUse(const Locale& locale, CharString& destination, UErrorCode& err) {
     destination.clear().append(DT_DateTimeGregorianTag, -1, err); // initial default
     if ( U_SUCCESS(err) ) {
+        UErrorCode localStatus = U_ZERO_ERROR;
         char localeWithCalendarKey[ULOC_LOCALE_IDENTIFIER_CAPACITY];
         // obtain a locale that always has the calendar key value that should be used
         ures_getFunctionalEquivalent(
@@ -767,8 +768,7 @@ DateTimePatternGenerator::getCalendarTypeToUse(const Locale& locale, CharString&
             locale.getName(),
             nullptr,
             FALSE,
-            &err);
-        if (U_FAILURE(err)) { return; }
+            &localStatus);
         localeWithCalendarKey[ULOC_LOCALE_IDENTIFIER_CAPACITY-1] = 0; // ensure null termination
         // now get the calendar key value from that locale
         char calendarType[ULOC_KEYWORDS_CAPACITY];
@@ -777,13 +777,17 @@ DateTimePatternGenerator::getCalendarTypeToUse(const Locale& locale, CharString&
             "calendar",
             calendarType,
             ULOC_KEYWORDS_CAPACITY,
-            &err);
-        if (U_FAILURE(err)) { return; }
+            &localStatus);
+        // If the input locale was invalid, don't fail with missing resource error, instead
+        // continue with default of Gregorian.
+        if (U_FAILURE(localStatus) && localStatus != U_MISSING_RESOURCE_ERROR) {
+            err = localStatus;
+            return;
+        }
         if (calendarTypeLen < ULOC_KEYWORDS_CAPACITY) {
             destination.clear().append(calendarType, -1, err);
             if (U_FAILURE(err)) { return; }
         }
-        err = U_ZERO_ERROR;
     }
 }
 

icu4c/source/test/intltest/dtptngts.cpp

@@ -38,6 +38,7 @@ void IntlTestDateTimePatternGeneratorAPI::runIndexedTest( int32_t index, UBool e
         TESTCASE(4, testC);
         TESTCASE(5, testSkeletonsWithDayPeriods);
         TESTCASE(6, testGetFieldDisplayNames);
+        TESTCASE(7, testFallbackWithDefaultRootLocale);
         default: name = ""; break;
     }
 }
@@ -1259,4 +1260,31 @@ void IntlTestDateTimePatternGeneratorAPI::testGetFieldDisplayNames() {
     }
 }
 
+void IntlTestDateTimePatternGeneratorAPI::testFallbackWithDefaultRootLocale() {
+    UErrorCode status = U_ZERO_ERROR;
+    char original[ULOC_FULLNAME_CAPACITY];
+
+    uprv_strcpy(original, uloc_getDefault());
+    uloc_setDefault("root", &status);
+    if (U_FAILURE(status)) {
+        errln("ERROR: Failed to change the default locale to root! Default is: %s\n", uloc_getDefault());
+    }
+
+    DateTimePatternGenerator* dtpg = icu::DateTimePatternGenerator::createInstance("abcdedf", status);
+
+    if (U_FAILURE(status)) {
+        errln("ERROR: expected createInstance with invalid locale to succeed. Status: %s", u_errorName(status));
+    }
+    if (status != U_USING_DEFAULT_WARNING) {
+        errln("ERROR: expected createInstance to return U_USING_DEFAULT_WARNING for invalid locale and default root locale. Status: %s", u_errorName(status));
+    }
+
+    delete dtpg;
+
+    uloc_setDefault(original, &status);
+    if (U_FAILURE(status)) {
+        errln("ERROR: Failed to change the default locale back to %s\n", original);
+    }
+}
+
 #endif /* #if !UCONFIG_NO_FORMATTING */

icu4c/source/test/intltest/dtptngts.h

@@ -32,6 +32,7 @@ class IntlTestDateTimePatternGeneratorAPI : public IntlTest {
     void testC();
     void testSkeletonsWithDayPeriods();
     void testGetFieldDisplayNames();
+    void testFallbackWithDefaultRootLocale();
 };
 
 #endif /* #if !UCONFIG_NO_FORMATTING */