ICU-22983 Fix DecimalQuantity::shiftLeft ubsan issue

ICU-22983 Add java tests and comments

Author: FrankYFTang

icu4c/source/i18n/number_decimalquantity.cpp

@@ -1133,7 +1133,7 @@ void DecimalQuantity::setDigitPos(int32_t position, int8_t value) {
 }
 
 void DecimalQuantity::shiftLeft(int32_t numDigits) {
-    if (!usingBytes && precision + numDigits > 16) {
+    if (!usingBytes && precision + numDigits >= 16) {
         switchStorage();
     }
     if (usingBytes) {

icu4c/source/test/intltest/numfmtst.cpp

@@ -253,6 +253,7 @@ void NumberFormatTest::runIndexedTest( int32_t index, UBool exec, const char* &n
   TESTCASE_AUTO(Test10997_FormatCurrency);
   TESTCASE_AUTO(Test21556_CurrencyAsDecimal);
   TESTCASE_AUTO(Test22088_Ethiopic);
+  TESTCASE_AUTO(Test22983_LongFraction);
   TESTCASE_AUTO_END;
 }
 
@@ -10161,5 +10162,9 @@ void NumberFormatTest::Test22088_Ethiopic() {
         assertEquals("Wrong result with UNUM_NUMBERING_SYSTEM and English", u"123", nf3->format(123, result));
     }
 }
+void NumberFormatTest::Test22983_LongFraction() {
+    IcuTestErrorCode status(*this, "Test22983_LongFraction");
+    DecimalFormat df(u"0.0000000000000001", status);
+}
 
 #endif /* #if !UCONFIG_NO_FORMATTING */

icu4c/source/test/intltest/numfmtst.h

@@ -309,6 +309,7 @@ class NumberFormatTest: public CalendarTimeZoneTest {
     void Test10997_FormatCurrency();
     void Test21556_CurrencyAsDecimal();
     void Test22088_Ethiopic();
+    void Test22983_LongFraction();
 
  private:
     UBool testFormattableAsUFormattable(const char *file, int line, Formattable &f);

icu4j/main/common_tests/src/test/java/com/ibm/icu/dev/test/format/NumberRegressionTests.java

@@ -1835,6 +1835,11 @@ public void test4241880() {
         }
         Locale.setDefault(savedLocale);
     }
+
+    @Test
+    public void test22983LongFraction() {
+        DecimalFormat format = new DecimalFormat("0.0000000000000001");
+    }
 }
 
 class myformat implements Serializable

icu4j/main/core/src/main/java/com/ibm/icu/impl/number/DecimalQuantity_DualStorageBCD.java

@@ -167,7 +167,14 @@ protected void setDigitPos(int position, byte value) {
 
     @Override
     protected void shiftLeft(int numDigits) {
-        if (!usingBytes && precision + numDigits > 16) {
+        // https://docs.oracle.com/javase/specs/jls/se8/html/jls-15.html#jls-15.19
+        // If the promoted type of the left-hand operand is long, then only the
+        // six lowest-order bits of the right-hand operand are used as the shift
+        // distance. It is as if the right-hand operand were subjected to a bitwise
+        // logical AND operator & (§15.22.1) with the mask value 0x3f (0b111111).
+        // The shift distance actually used is therefore always in the range 0 to
+        // 63, inclusive.
+        if (!usingBytes && precision + numDigits >= 16) {
             switchStorage();
         }
         if (usingBytes) {