[Downgrade] Fix parameter type widening issue, when method lives on the ancestor's interface (#4877)

Co-authored-by: rector-bot <tomas@getrector.org>

rules/downgrade-php72/src/Rector/ClassMethod/DowngradeParameterTypeWideningRector.php

@@ -150,8 +150,21 @@ CODE_SAMPLE
         foreach ($ancestorAndInterfaceClassNames as $ancestorClassOrInterface) {
             /** @var string */
             $parentClassName = $ancestorClassOrInterface->getAttribute(AttributeKey::CLASS_NAME);
-            /** @var ClassMethod */
             $classMethod = $this->nodeRepository->findClassMethod($parentClassName, $methodName);
+            /**
+             * If it doesn't find the method, it's because the method
+             * lives somewhere else.
+             * For instance, in test "interface_on_parent_class.php.inc",
+             * the ancestor abstract class is also retrieved
+             * as containing the method, but it does not: it is
+             * in its implemented interface. That happens because
+             * `ReflectionMethod` doesn't allow to do do the distinction.
+             * The interface is also retrieve though, so that method
+             * will eventually be refactored.
+             */
+            if ($classMethod === null) {
+                continue;
+            }
             $this->removeParamTypeFromMethod($ancestorClassOrInterface, $position, $classMethod);
             $this->removeParamTypeFromMethodForChildren($parentClassName, $methodName, $position);
         }

rules/downgrade-php72/tests/Rector/ClassMethod/DowngradeParameterTypeWideningRector/Fixture/interface_on_parent_class.php.inc

@@ -0,0 +1,48 @@
+<?php
+
+namespace Rector\DowngradePhp72\Tests\Rector\ClassMethod\DowngradeParameterTypeWideningRector\Fixture;
+
+interface SomeInterface
+{
+    public function test(string $input);
+}
+
+abstract class AbstractSomeAncestorClass implements SomeInterface
+{
+}
+
+class SomeChildClass extends AbstractSomeAncestorClass
+{
+    public function test($input) // type omitted for $input
+    {
+        /* ... */
+    }
+}
+
+?>
+-----
+<?php
+
+namespace Rector\DowngradePhp72\Tests\Rector\ClassMethod\DowngradeParameterTypeWideningRector\Fixture;
+
+interface SomeInterface
+{
+    /**
+     * @param string $input
+     */
+    public function test($input);
+}
+
+abstract class AbstractSomeAncestorClass implements SomeInterface
+{
+}
+
+class SomeChildClass extends AbstractSomeAncestorClass
+{
+    public function test($input) // type omitted for $input
+    {
+        /* ... */
+    }
+}
+
+?>