gracefully handle cases when no resolver is set

Exception/LogicException.php

@@ -0,0 +1,16 @@
+<?php
+
+/*
+ * This file is part of the Symfony package.
+ *
+ * (c) Fabien Potencier <fabien@symfony.com>
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+namespace Symfony\Component\Config\Exception;
+
+class LogicException extends \LogicException
+{
+}

Loader/Loader.php

@@ -12,6 +12,7 @@
 namespace Symfony\Component\Config\Loader;
 
 use Symfony\Component\Config\Exception\LoaderLoadException;
+use Symfony\Component\Config\Exception\LogicException;
 
 /**
  * Loader is the abstract class used by all built-in loaders.
@@ -20,7 +21,7 @@
  */
 abstract class Loader implements LoaderInterface
 {
-    protected LoaderResolverInterface $resolver;
+    protected ?LoaderResolverInterface $resolver = null;
     protected ?string $env;
 
     public function __construct(?string $env = null)
@@ -30,6 +31,10 @@ public function __construct(?string $env = null)
 
     public function getResolver(): LoaderResolverInterface
     {
+        if (null === $this->resolver) {
+            throw new LogicException('Cannot get a resolver if none was set.');
+        }
+
         return $this->resolver;
     }
 

Tests/Loader/LoaderTest.php

@@ -13,6 +13,7 @@
 
 use PHPUnit\Framework\TestCase;
 use Symfony\Component\Config\Exception\LoaderLoadException;
+use Symfony\Component\Config\Exception\LogicException;
 use Symfony\Component\Config\Loader\Loader;
 use Symfony\Component\Config\Loader\LoaderInterface;
 use Symfony\Component\Config\Loader\LoaderResolverInterface;
@@ -29,6 +30,14 @@ public function testGetSetResolver()
         $this->assertSame($resolver, $loader->getResolver(), '->setResolver() sets the resolver loader');
     }
 
+    public function testGetResolverWithoutSetResolver()
+    {
+        $this->expectException(LogicException::class);
+
+        $loader = new ProjectLoader1();
+        $loader->getResolver();
+    }
+
     public function testResolve()
     {
         $resolvedLoader = $this->createMock(LoaderInterface::class);
@@ -46,6 +55,14 @@ public function testResolve()
         $this->assertSame($resolvedLoader, $loader->resolve('foo.xml'), '->resolve() finds a loader');
     }
 
+    public function testResolveWithoutSetResolver()
+    {
+        $this->expectException(LoaderLoadException::class);
+
+        $loader = new ProjectLoader1();
+        $loader->resolve('foo.xml');
+    }
+
     public function testResolveWhenResolverCannotFindLoader()
     {
         $resolver = $this->createMock(LoaderResolverInterface::class);