feat: add unit test for alibaba#2558

core/src/test/java/com/alibaba/fastjson2/issues_2500/Issue2558.java

@@ -0,0 +1,23 @@
+package com.alibaba.fastjson2.issues_2500;
+
+import com.alibaba.fastjson2.util.TypeUtils;
+import org.bson.types.Decimal128;
+import org.junit.jupiter.api.Test;
+
+import java.math.BigDecimal;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+public class Issue2558 {
+    @Test
+    public void test() {
+        BigDecimal decimal = new BigDecimal("123.45");
+        Decimal128 decimal128 = new Decimal128(decimal);
+        Double doubleValue = decimal.doubleValue();
+
+        assertEquals(
+                doubleValue,
+                TypeUtils.cast(decimal128, Double.class)
+        );
+    }
+}