Don't autoload classes during covariant type check against mixed

mixed should be behaving the same way as no type here, and not
require X to be autoloaded. Everything apart from "void" is trivially
covariant to "mixed".

Author: nikic

Zend/tests/type_declarations/variance/mixed_return_type.phpt

@@ -0,0 +1,20 @@
+--TEST--
+Everything is trivially a subtype of mixed
+--FILE--
+<?php
+
+spl_autoload_register(function($class) {
+    echo "Loading $class\n";
+});
+
+class A {
+    public function test(): mixed {}
+}
+class B extends A {
+    public function test(): X {}
+}
+
+?>
+===DONE===
+--EXPECT--
+===DONE===

Zend/zend_inheritance.c

@@ -429,6 +429,13 @@ static inheritance_status zend_perform_covariant_type_check(
 {
 	ZEND_ASSERT(ZEND_TYPE_IS_SET(fe_type) && ZEND_TYPE_IS_SET(proto_type));
 
+	/* Apart from void, everything is trivially covariant to the mixed type.
+	 * Handle this case separately to ensure it never requires class loading. */
+	if (ZEND_TYPE_PURE_MASK(proto_type) == MAY_BE_ANY &&
+			!ZEND_TYPE_CONTAINS_CODE(fe_type, IS_VOID)) {
+		return INHERITANCE_SUCCESS;
+	}
+
 	/* Builtin types may be removed, but not added */
 	uint32_t fe_type_mask = ZEND_TYPE_PURE_MASK(fe_type);
 	uint32_t proto_type_mask = ZEND_TYPE_PURE_MASK(proto_type);