Implement nullable return types.

Zend/tests/return_types/030.phpt

@@ -0,0 +1,23 @@
+--TEST--
+Nullable return value 
+--FILE--
+<?php
+function foo($x) : ?array {
+	return $x;
+}
+
+foo([]);
+echo "ok\n";
+foo(null);
+echo "ok\n";
+foo(0);
+?>
+--EXPECTF--
+ok
+ok
+
+Fatal error: Uncaught TypeError: Return value of foo() must be of the type array, integer returned in %s030.php:3
+Stack trace:
+#0 %s030.php(10): foo(0)
+#1 {main}
+  thrown in %s030.php on line 3

Zend/tests/return_types/031.phpt

@@ -0,0 +1,14 @@
+--TEST--
+Nullable return type inheritance rules (non-nullable and nullable)
+--FILE--
+<?php
+class A {
+	function foo(): int {}
+}
+class B extends A {
+	function foo(): ?int {}
+}
+?>
+DONE
+--EXPECTF--
+Fatal error: Declaration of B::foo(): ?int must be compatible with A::foo(): int in %s031.php on line 7

Zend/tests/return_types/032.phpt

@@ -0,0 +1,14 @@
+--TEST--
+Nullable return type inheritance rules (nullable and non-nullable)
+--FILE--
+<?php
+class A {
+	function foo(): ?int {}
+}
+class B extends A {
+	function foo(): int {}
+}
+?>
+DONE
+--EXPECT--
+DONE

Zend/zend_compile.c

@@ -4844,6 +4844,11 @@ void zend_compile_params(zend_ast *ast, zend_ast *return_type_ast) /* {{{ */
 		arg_infos->allow_null = 0;
 		arg_infos->class_name = NULL;
 
+		if (return_type_ast->attr & ZEND_TYPE_NULLABLE) {
+			arg_infos->allow_null = 1;
+			return_type_ast->attr &= ~ZEND_TYPE_NULLABLE;
+		}
+
 		zend_compile_typename(return_type_ast, arg_infos);
 
 		arg_infos++;

Zend/zend_compile.h

@@ -835,6 +835,8 @@ ZEND_API void zend_assert_valid_class_name(const zend_string *const_name);
 #define ZEND_NAME_NOT_FQ   1
 #define ZEND_NAME_RELATIVE 2
 
+#define ZEND_TYPE_NULLABLE (1<<8)
+
 /* var status for backpatching */
 #define BP_VAR_R			0
 #define BP_VAR_W			1

Zend/zend_inheritance.c

@@ -341,6 +341,10 @@ static zend_bool zend_do_perform_implementation_check(const zend_function *fe, c
 		if (!zend_do_perform_type_hint_check(fe, fe->common.arg_info - 1, proto, proto->common.arg_info - 1)) {
 			return 0;
 		}
+
+		if (fe->common.arg_info[-1].allow_null && !proto->common.arg_info[-1].allow_null) {
+			return 0;
+		}
 	}
 	return 1;
 }
@@ -503,6 +507,9 @@ static ZEND_COLD zend_string *zend_get_function_declaration(const zend_function
 
 	if (fptr->common.fn_flags & ZEND_ACC_HAS_RETURN_TYPE) {
 		smart_str_appends(&str, ": ");
+		if (fptr->common.arg_info[-1].allow_null) {
+			smart_str_appendc(&str, '?');
+		}
 		zend_append_type_hint(&str, fptr, fptr->common.arg_info - 1, 1);
 	}
 	smart_str_0(&str);
@@ -585,7 +592,8 @@ static void do_inheritance_check_on_method(zend_function *child, zend_function *
 			error_verb = "must";
 		} else if ((parent->common.fn_flags & ZEND_ACC_HAS_RETURN_TYPE) &&
                    (!(child->common.fn_flags & ZEND_ACC_HAS_RETURN_TYPE) ||
-		            !zend_do_perform_type_hint_check(child, child->common.arg_info - 1, parent, parent->common.arg_info - 1))) {
+		            !zend_do_perform_type_hint_check(child, child->common.arg_info - 1, parent, parent->common.arg_info - 1) ||
+		            (child->common.arg_info[-1].allow_null && !parent->common.arg_info[-1].allow_null))) {
 			error_level = E_COMPILE_ERROR;
 			error_verb = "must";
 		} else {

Zend/zend_language_parser.y

@@ -651,6 +651,7 @@ type:
 return_type:
 		/* empty */	{ $$ = NULL; }
 	|	':' type	{ $$ = $2; }
+	|	':' '?' type	{ $$ = $3; $$->attr |= ZEND_TYPE_NULLABLE; }
 ;
 
 argument_list: