Improve TypeCombinator::unionWithSubtractedType

src/Type/TypeCombinator.php

@@ -540,6 +540,18 @@ private static function unionWithSubtractedType(
 			return $type;
 		}
 
+		if ($subtractedType instanceof SubtractableType) {
+			$withoutSubtracted = $subtractedType->getTypeWithoutSubtractedType();
+			if ($withoutSubtracted->isSuperTypeOf($type)->yes()) {
+				$subtractedSubtractedType = $subtractedType->getSubtractedType();
+				if ($subtractedSubtractedType === null) {
+					return new NeverType();
+				}
+
+				return self::intersect($type, $subtractedSubtractedType);
+			}
+		}
+
 		if ($type instanceof SubtractableType) {
 			$subtractedType = $type->getSubtractedType() === null
 				? $subtractedType

tests/PHPStan/Analyser/nsrt/subtracted.php

@@ -0,0 +1,31 @@
+<?php declare(strict_types = 1);
+
+namespace Substracted;
+
+
+use function PHPStan\Testing\assertType;
+
+class HelloWorld
+{
+	/**
+	 * @param mixed $date
+	 * @param bool $foo
+	 */
+	public function sayHello($date, $foo): void
+	{
+		if(is_object($date)){
+
+		} else {
+			assertType('mixed~object', $date);
+
+			if ($foo) {
+				$date = new \stdClass();
+			}
+			assertType('mixed~object~stdClass', $date);
+
+			if (is_object($date)) {
+				assertType('stdClass', $date);
+			}
+		}
+	}
+}

tests/PHPStan/Type/TypeCombinatorTest.php

@@ -4143,6 +4143,14 @@ public static function dataIntersect(): iterable
 				ObjectWithoutClassType::class,
 				'object',
 			],
+			[
+				[
+					new MixedType(subtractedType: new ObjectWithoutClassType(new ObjectType('stdClass'))),
+					new ObjectWithoutClassType(),
+				],
+				ObjectType::class,
+				'stdClass',
+			],
 		];
 
 		if (PHP_VERSION_ID < 80100) {