Deprecate return by ref from void function

Part of https://wiki.php.net/rfc/deprecations_php_8_1.

Author: nikic

UPGRADING

@@ -330,7 +330,9 @@ PHP 8.1 UPGRADE NOTES
     e.g. a truncation from 1.9 to 1, is deprecated. This affects array keys,
     int parameter and return types, and operators working on integers.
     RFC: https://wiki.php.net/rfc/implicit-float-int-deprecate
-  . returning a non-array from __sleep will raise a warning
+  . Returning a non-array from __sleep will raise a warning
+  . Returning by reference from a void function is deprecated.
+    RFC: https://wiki.php.net/rfc/deprecations_php_8_1
 
 - Date:
   . The date_sunrise() and date_sunset() functions have been deprecated in

Zend/tests/return_by_ref_from_void_function.phpt

@@ -0,0 +1,17 @@
+--TEST--
+Returning by reference from void function is deprecated
+--FILE--
+<?php
+
+function &test(): void {
+}
+
+$r =& test();
+var_dump($r);
+
+?>
+--EXPECTF--
+Deprecated: Returning by reference from a void function is deprecated in %s on line %d
+
+Notice: Only variable references should be returned by reference in %s on line %d
+NULL

Zend/zend_compile.c

@@ -6532,6 +6532,11 @@ void zend_compile_params(zend_ast *ast, zend_ast *return_type_ast, uint32_t fall
 		}
 		arg_infos++;
 		op_array->fn_flags |= ZEND_ACC_HAS_RETURN_TYPE;
+
+		if (ZEND_TYPE_CONTAINS_CODE(arg_infos[-1].type, IS_VOID)
+				&& (op_array->fn_flags & ZEND_ACC_RETURN_REFERENCE)) {
+			zend_error(E_DEPRECATED, "Returning by reference from a void function is deprecated");
+		}
 	} else {
 		if (list->children == 0) {
 			return;