Allow abstract classes to be implemented in NoSingleInterfaceImplementerRule

src/Collector/ImplementedInterfaceCollector.php

@@ -18,12 +18,17 @@ public function getNodeType(): string
 
     /**
      * @param Class_ $node
-     * @return string[]
+     * @return string[]|null
      */
-    public function processNode(Node $node, Scope $scope): array
+    public function processNode(Node $node, Scope $scope): ?array
     {
         $implementedInterfaceNames = [];
 
+        // skip abstract classes, as they can enforce child implementations
+        if ($node->isAbstract()) {
+            return null;
+        }
+
         foreach ($node->implements as $implement) {
             $implementedInterfaceNames[] = $implement->toString();
         }

tests/Rules/NoSingleInterfaceImplementerRule/Fixture/AllowAbstract.php

@@ -0,0 +1,9 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Symplify\PHPStanRules\Tests\Rules\NoSingleInterfaceImplementerRule\Fixture;
+
+abstract class AllowAbstract implements SimpleInterface
+{
+}

tests/Rules/NoSingleInterfaceImplementerRule/NoSingleInterfaceImplementerRuleTest.php

@@ -31,6 +31,7 @@ public function testRule(array $filePaths, array $expectedErrorMessagesWithLines
     public static function provideData(): Iterator
     {
         yield [[__DIR__ . '/Fixture/SimpleInterface.php'], []];
+        yield [[__DIR__ . '/Fixture/AllowAbstract.php', __DIR__ . '/Fixture/SimpleInterface.php'], []];
 
         yield [
             [