Handle explicitly nullable types in optional-before-required deprecation

The exception for null default values here exists to keep compatibility
with PHP < 7.1 where "Foo $bar = null" was the canonical way to create
a nullable parameter. If the parameter is actually "?Foo $bar = null",
then clearly compatibility with PHP < 7.1 is not a concern, and we
can throw a deprecation notice.

Author: nikic

Zend/tests/required_param_after_optional.phpt

@@ -5,7 +5,7 @@ Required parameter after optional is deprecated
 
 function test($testA = null, $testB = null, $testC) {}
 function test2(Type $test2A = null, $test2B = null, $test2C) {}
-function test3(Type $test3A = null, Type2 $test3B = null, $test3C) {}
+function test3(Type $test3A = null, ?Type2 $test3B = null, $test3C) {}
 
 ?>
 --EXPECTF--
@@ -14,3 +14,5 @@ Deprecated: Optional parameter $testA declared before required parameter $testC
 Deprecated: Optional parameter $testB declared before required parameter $testC is implicitly treated as a required parameter in %s on line %d
 
 Deprecated: Optional parameter $test2B declared before required parameter $test2C is implicitly treated as a required parameter in %s on line %d
+
+Deprecated: Optional parameter $test3B declared before required parameter $test3C is implicitly treated as a required parameter in %s on line %d

Zend/zend_compile.c

@@ -6558,7 +6558,8 @@ void zend_compile_params(zend_ast *ast, zend_ast *return_type_ast, uint32_t fall
 				/* Ignore parameters of the form "Type $param = null".
 				 * This is the PHP 5 style way of writing "?Type $param", so allow it for now. */
 				bool is_implicit_nullable =
-					type_ast && Z_TYPE(default_node.u.constant) == IS_NULL;
+					type_ast && !(type_ast->attr & ZEND_TYPE_NULLABLE)
+					&& Z_TYPE(default_node.u.constant) == IS_NULL;
 				if (!is_implicit_nullable) {
 					zend_ast *required_param_ast = list->child[last_required_param];
 					zend_error(E_DEPRECATED,