Add #[ReturnTypeWillChange] to internal PHP methods

Suppress PHP 8.0+ deprecation notices from internal methods missing
explicit return types. This copies "laravel/framework" 8.x file
Illuminate\Database\Eloquent\Model.

.github/workflows/ci.yml

@@ -9,7 +9,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        php: [7.1, 7.2, 7.3, 8.0]
+        php: [7.1, 7.2, 7.3, 8.0, 8.1]
     steps:
     - uses: actions/checkout@v2
     - uses: shivammathur/setup-php@v2

.travis.yml

@@ -7,6 +7,7 @@ php:
   - 7.2
   - 7.3
   - 8.0
+  - 8.1
 
 sudo: false
 

src/Model.php

@@ -441,6 +441,7 @@ public function toJson($options = 0)
      *
      * @return array
      */
+    #[\ReturnTypeWillChange]
     public function jsonSerialize()
     {
         return $this->toArray();
@@ -863,6 +864,7 @@ public function __set($key, $value)
      * @param  mixed  $offset
      * @return bool
      */
+    #[\ReturnTypeWillChange]
     public function offsetExists($offset)
     {
         return isset($this->$offset);
@@ -874,6 +876,7 @@ public function offsetExists($offset)
      * @param  mixed  $offset
      * @return mixed
      */
+    #[\ReturnTypeWillChange]
     public function offsetGet($offset)
     {
         return $this->$offset;
@@ -886,6 +889,7 @@ public function offsetGet($offset)
      * @param  mixed  $value
      * @return void
      */
+    #[\ReturnTypeWillChange]
     public function offsetSet($offset, $value)
     {
         $this->$offset = $value;
@@ -897,6 +901,7 @@ public function offsetSet($offset, $value)
      * @param  mixed  $offset
      * @return void
      */
+    #[\ReturnTypeWillChange]
     public function offsetUnset($offset)
     {
         unset($this->$offset);