diff --git a/src/Psalm/Visitor/DependencyFinderVisitor.php b/src/Psalm/Visitor/DependencyFinderVisitor.php
index a3ae8f47e..d86993e2c 100644
--- a/src/Psalm/Visitor/DependencyFinderVisitor.php
+++ b/src/Psalm/Visitor/DependencyFinderVisitor.php
@@ -514,6 +514,28 @@ class DependencyFinderVisitor extends PhpParser\NodeVisitorAbstract implements P
                     $this->file_storage->declaring_constants[$const->name->name] = $this->file_path;
                 }
             }
+        } elseif ($this->codebase->register_global_functions && $node instanceof PhpParser\Node\Stmt\If_) {
+            if ($node->cond instanceof PhpParser\Node\Expr\BooleanNot) {
+                if ($node->cond->expr instanceof PhpParser\Node\Expr\FuncCall
+                    && $node->cond->expr->name instanceof PhpParser\Node\Name
+                ) {
+                    if ($node->cond->expr->name->parts === ['function_exists']
+                        && isset($node->cond->expr->args[0])
+                        && $node->cond->expr->args[0]->value instanceof PhpParser\Node\Scalar\String_
+                        && function_exists($node->cond->expr->args[0]->value->value)
+                    ) {
+                        return PhpParser\NodeTraverser::DONT_TRAVERSE_CHILDREN;
+                    }
+
+                    if ($node->cond->expr->name->parts === ['class_exists']
+                        && isset($node->cond->expr->args[0])
+                        && $node->cond->expr->args[0]->value instanceof PhpParser\Node\Scalar\String_
+                        && class_exists($node->cond->expr->args[0]->value->value, false)
+                    ) {
+                        return PhpParser\NodeTraverser::DONT_TRAVERSE_CHILDREN;
+                    }
+                }
+            }
         }
     }
 
diff --git a/tests/AnnotationTest.php b/tests/AnnotationTest.php
index b8a9e5e7f..82af918eb 100644
--- a/tests/AnnotationTest.php
+++ b/tests/AnnotationTest.php
@@ -894,7 +894,7 @@ class AnnotationTest extends TestCase
 
                     $arr["a"]()',
             ],
-            'magicMethodAnnotation' => [
+            'magicMethodValidAnnotations' => [
                 '<?php
                     class Parent {
                         public function __call() {}
@@ -915,6 +915,7 @@ class AnnotationTest extends TestCase
 
                     $a = $child->getString();
                     $child->setInteger(4);
+                    /** @psalm-suppress MixedAssignment */
                     $b = $child->setString(5);
                     $c = $child->getBool("hello");
                     $d = $child->getArray();
@@ -927,7 +928,6 @@ class AnnotationTest extends TestCase
                     '$d' => 'array<mixed, string|int>',
                     '$e' => 'callable():string',
                 ],
-                'error_levels' => ['MixedAssignment'],
             ],
         ];
     }
diff --git a/tests/ConfigTest.php b/tests/ConfigTest.php
index 1fca7cce3..67cedef8f 100644
--- a/tests/ConfigTest.php
+++ b/tests/ConfigTest.php
@@ -19,6 +19,14 @@ class ConfigTest extends TestCase
     public static function setUpBeforeClass()
     {
         self::$config = new TestConfig();
+
+        if (!defined('PSALM_VERSION')) {
+            define('PSALM_VERSION', '2.0.0');
+        }
+
+        if (!defined('PHP_PARSER_VERSION')) {
+            define('PHP_PARSER_VERSION', '4.0.0');
+        }
     }
 
     /**
@@ -437,6 +445,38 @@ class ConfigTest extends TestCase
         $this->analyzeFile($file_path, new Context());
     }
 
+    /**
+     * @return void
+     */
+    public function testPolyfilledFunction()
+    {
+        $this->project_checker = $this->getProjectCheckerWithConfig(
+            TestConfig::loadFromXML(
+                dirname(__DIR__),
+                '<?xml version="1.0"?>
+                <psalm>
+                    <projectFiles>
+                        <directory name="src" />
+                    </projectFiles>
+
+                    <stubs>
+                        <file name="tests/stubs/polyfill.php" />
+                    </stubs>
+                </psalm>'
+            )
+        );
+
+        $file_path = getcwd() . '/src/somefile.php';
+
+        $this->addFile(
+            $file_path,
+            '<?php
+                $a = random_bytes(16);'
+        );
+
+        $this->analyzeFile($file_path, new Context());
+    }
+
     /**
      * @return void
      */
diff --git a/tests/stubs/polyfill.php b/tests/stubs/polyfill.php
new file mode 100644
index 000000000..bff9ce6db
--- /dev/null
+++ b/tests/stubs/polyfill.php
@@ -0,0 +1,11 @@
+<?php
+if (!function_exists("random_bytes")) {
+    /**
+     * @param int $bytes
+     * @return void
+     */
+    function random_bytes($bytes)
+    {
+        throw new \Exception("bad");
+    }
+}