fix: Preserve integral Number type in toJsonTree for byte/short/int

The BYTE, SHORT, and INTEGER type adapters' write methods called
value(byteValue()), value(shortValue()), and value(intValue())
respectively. Since JsonWriter only has value(long), Java auto-widened
the primitive to long, causing JsonTreeWriter.value(long) to store a
Long in the JsonPrimitive instead of the original Byte/Short/Integer.

Fix by casting the narrowed primitive to Number, which triggers
autoboxing to the correct type and resolves to value(Number) instead
of value(long).

Fixes #2680

Author: daguimu

gson/src/main/java/com/google/gson/internal/bind/TypeAdapters.java

@@ -214,7 +214,7 @@ public void write(JsonWriter out, Number value) throws IOException {
           if (value == null) {
             out.nullValue();
           } else {
-            out.value(value.byteValue());
+            out.value((Number) value.byteValue());
           }
         }
       };
@@ -249,7 +249,7 @@ public void write(JsonWriter out, Number value) throws IOException {
           if (value == null) {
             out.nullValue();
           } else {
-            out.value(value.shortValue());
+            out.value((Number) value.shortValue());
           }
         }
       };
@@ -277,7 +277,7 @@ public void write(JsonWriter out, Number value) throws IOException {
           if (value == null) {
             out.nullValue();
           } else {
-            out.value(value.intValue());
+            out.value((Number) value.intValue());
           }
         }
       };

gson/src/test/java/com/google/gson/functional/PrimitiveTest.java

@@ -161,6 +161,33 @@ public void testIntSerialization() {
     assertThat(gson.toJson(1.5, Integer.class)).isEqualTo("1");
   }
 
+  /** Regression test for https://github.com/google/gson/issues/2680 */
+  @Test
+  public void testToJsonTreePreservesIntegralType() {
+    // toJsonTree should preserve the Number type, not widen byte/short/int to Long
+    assertThat(gson.toJsonTree((byte) 1, byte.class).getAsNumber().getClass())
+        .isEqualTo(Byte.class);
+    assertThat(gson.toJsonTree((byte) 1, Byte.class).getAsNumber().getClass())
+        .isEqualTo(Byte.class);
+
+    assertThat(gson.toJsonTree((short) 1, short.class).getAsNumber().getClass())
+        .isEqualTo(Short.class);
+    assertThat(gson.toJsonTree((short) 1, Short.class).getAsNumber().getClass())
+        .isEqualTo(Short.class);
+
+    assertThat(gson.toJsonTree(1, int.class).getAsNumber().getClass()).isEqualTo(Integer.class);
+    assertThat(gson.toJsonTree(1, Integer.class).getAsNumber().getClass()).isEqualTo(Integer.class);
+
+    // Long should remain Long
+    assertThat(gson.toJsonTree(1L, long.class).getAsNumber().getClass()).isEqualTo(Long.class);
+    assertThat(gson.toJsonTree(1L, Long.class).getAsNumber().getClass()).isEqualTo(Long.class);
+
+    // Narrowing conversion should produce the target type
+    assertThat(gson.toJsonTree(1.5, Integer.class).getAsNumber().getClass())
+        .isEqualTo(Integer.class);
+    assertThat(gson.toJsonTree(1L, Byte.class).getAsNumber().getClass()).isEqualTo(Byte.class);
+  }
+
   @Test
   public void testLongSerialization() {
     assertThat(gson.toJson(1L, long.class)).isEqualTo("1");