Convert Implicitly nullable pure intersection types to DNF

If we don't then Reflection would give us a ReflectionIntersectionType even if the type is currently displayed as (X&Y)|null

Author: Girgias

Zend/zend_compile.c

@@ -6527,10 +6527,24 @@ static zend_type zend_compile_typename(
 
 		ZEND_ASSERT(list->children == type_list->num_types);
 
-		ZEND_TYPE_SET_LIST(type, type_list);
 		ZEND_TYPE_FULL_MASK(type) |= _ZEND_TYPE_ARENA_BIT;
-		/* Inform that the type list is an intersection type */
-		ZEND_TYPE_FULL_MASK(type) |= _ZEND_TYPE_INTERSECTION_BIT;
+		/* An implicitly nullable intersection type needs to be converted to a DNF type */
+		if (force_allow_null) {
+			zend_type intersection_type = ZEND_TYPE_INIT_NONE(0);
+			ZEND_TYPE_SET_LIST(intersection_type, type_list);
+			ZEND_TYPE_FULL_MASK(intersection_type) |= _ZEND_TYPE_INTERSECTION_BIT;
+
+			zend_type_list *dnf_type_list = zend_arena_alloc(&CG(arena), ZEND_TYPE_LIST_SIZE(list->children));
+			dnf_type_list->num_types = 1;
+			dnf_type_list->types[0] = intersection_type;
+			ZEND_TYPE_SET_LIST(type, dnf_type_list);
+			/* Inform that the type list is a DNF type */
+			ZEND_TYPE_FULL_MASK(type) |= _ZEND_TYPE_UNION_BIT;
+		} else {
+			ZEND_TYPE_SET_LIST(type, type_list);
+			/* Inform that the type list is an intersection type */
+			ZEND_TYPE_FULL_MASK(type) |= _ZEND_TYPE_INTERSECTION_BIT;
+		}
 	} else {
 		type = zend_compile_single_typename(ast);
 	}

ext/reflection/tests/types/pure_intersection_type_implicitly_nullable.phpt

@@ -0,0 +1,37 @@
+--TEST--
+Intersection type which is implicitly nullable should be a DNF type in reflection
+--FILE--
+<?php
+
+function dumpType(ReflectionType $rt, int $indent = 0) {
+    $str_indent = str_repeat(' ', 2 * $indent);
+    echo $str_indent . "Type $rt is " . $rt::class . ":\n";
+    echo $str_indent . "Allows Null: " . json_encode($rt->allowsNull()) . "\n";
+    foreach ($rt->getTypes() as $type) {
+        if ($type instanceof ReflectionNamedType) {
+            echo $str_indent . "  Name: " . $type->getName() . "\n";
+            echo $str_indent . "  String: " . (string) $type . "\n";
+        } else {
+            dumpType($type, $indent+1);
+        }
+    }
+}
+
+function foo(X&Y $foo = null) {
+    var_dump($foo);
+}
+
+dumpType((new ReflectionFunction('foo'))->getParameters()[0]->getType());
+
+?>
+--EXPECT--
+Type (X&Y)|null is ReflectionUnionType:
+Allows Null: true
+  Type X&Y is ReflectionIntersectionType:
+  Allows Null: false
+    Name: X
+    String: X
+    Name: Y
+    String: Y
+  Name: null
+  String: null