Allow registering adapters for JsonElement again

gson/src/main/java/com/google/gson/GsonBuilder.java

@@ -710,7 +710,7 @@ public GsonBuilder registerTypeAdapter(Type type, Object typeAdapter) {
             || typeAdapter instanceof InstanceCreator<?>
             || typeAdapter instanceof TypeAdapter<?>);
 
-    if (isTypeObjectOrJsonElement(type)) {
+    if (hasNonOverridableAdapter(type)) {
       throw new IllegalArgumentException("Cannot override built-in adapter for " + type);
     }
 
@@ -730,9 +730,14 @@ public GsonBuilder registerTypeAdapter(Type type, Object typeAdapter) {
     return this;
   }
 
-  private static boolean isTypeObjectOrJsonElement(Type type) {
-    return type instanceof Class
-        && (type == Object.class || JsonElement.class.isAssignableFrom((Class<?>) type));
+  /** Whether the type has a built-in adapter which cannot be overridden. */
+  private static boolean hasNonOverridableAdapter(Type type) {
+    return type == Object.class;
+    // This should also cover `JsonElement.class.isAssignableFrom(type)`, however for backward
+    // compatibility this is not covered here because really old Gson versions had no built-in
+    // adapter for JsonElement so users registered custom adapters. These adapters don't have any
+    // effect in recent Gson versions. See
+    // https://github.com/google/gson/issues/2787#issuecomment-2581568157
   }
 
   /**
@@ -778,10 +783,6 @@ public GsonBuilder registerTypeHierarchyAdapter(Class<?> baseType, Object typeAd
             || typeAdapter instanceof JsonDeserializer<?>
             || typeAdapter instanceof TypeAdapter<?>);
 
-    if (JsonElement.class.isAssignableFrom(baseType)) {
-      throw new IllegalArgumentException("Cannot override built-in adapter for " + baseType);
-    }
-
     if (typeAdapter instanceof JsonDeserializer || typeAdapter instanceof JsonSerializer) {
       hierarchyFactories.add(TreeTypeAdapter.newTypeHierarchyFactory(baseType, typeAdapter));
     }

gson/src/test/java/com/google/gson/GsonBuilderTest.java

@@ -28,6 +28,7 @@
 import java.lang.reflect.Type;
 import java.text.DateFormat;
 import java.util.Date;
+import org.junit.Ignore;
 import org.junit.Test;
 
 /**
@@ -251,7 +252,10 @@ public void testSetStrictness() throws IOException {
   public void testRegisterTypeAdapterForObjectAndJsonElements() {
     String errorMessage = "Cannot override built-in adapter for ";
     Type[] types = {
-      Object.class, JsonElement.class, JsonArray.class,
+      Object.class,
+      // TODO: Registering adapter for JsonElement is allowed (for now) for backward compatibility,
+      //   see https://github.com/google/gson/issues/2787
+      // JsonElement.class, JsonArray.class,
     };
     GsonBuilder gsonBuilder = new GsonBuilder();
     for (Type type : types) {
@@ -263,6 +267,22 @@ public void testRegisterTypeAdapterForObjectAndJsonElements() {
     }
   }
 
+  /**
+   * Verifies that (for now) registering adapter for {@link JsonElement} and subclasses is possible,
+   * but has no effect. See {@link #testRegisterTypeAdapterForObjectAndJsonElements()}.
+   */
+  @Test
+  public void testRegisterTypeAdapterForJsonElements() {
+    Gson gson = new GsonBuilder().registerTypeAdapter(JsonArray.class, NULL_TYPE_ADAPTER).create();
+    TypeAdapter<JsonArray> adapter = gson.getAdapter(JsonArray.class);
+    // Does not use registered adapter
+    assertThat(adapter).isNotSameInstanceAs(NULL_TYPE_ADAPTER);
+    assertThat(adapter.toJson(new JsonArray())).isEqualTo("[]");
+  }
+
+  @Ignore(
+      "Registering adapter for JsonElement is allowed (for now) for backward compatibility, see"
+          + " https://github.com/google/gson/issues/2787")
   @Test
   public void testRegisterTypeHierarchyAdapterJsonElements() {
     String errorMessage = "Cannot override built-in adapter for ";
@@ -282,6 +302,20 @@ public void testRegisterTypeHierarchyAdapterJsonElements() {
     gsonBuilder.registerTypeHierarchyAdapter(Object.class, NULL_TYPE_ADAPTER);
   }
 
+  /**
+   * Verifies that (for now) registering hierarchy adapter for {@link JsonElement} and subclasses is
+   * possible, but has no effect. See {@link #testRegisterTypeHierarchyAdapterJsonElements()}.
+   */
+  @Test
+  public void testRegisterTypeHierarchyAdapterJsonElements_Allowed() {
+    Gson gson =
+        new GsonBuilder().registerTypeHierarchyAdapter(JsonArray.class, NULL_TYPE_ADAPTER).create();
+    TypeAdapter<JsonArray> adapter = gson.getAdapter(JsonArray.class);
+    // Does not use registered adapter
+    assertThat(adapter).isNotSameInstanceAs(NULL_TYPE_ADAPTER);
+    assertThat(adapter.toJson(new JsonArray())).isEqualTo("[]");
+  }
+
   @Test
   public void testSetDateFormatWithInvalidPattern() {
     GsonBuilder builder = new GsonBuilder();