Fix Filter Assert message on callable without return type

src/Assert/Filter.php

@@ -10,6 +10,7 @@
 use ReflectionIntersectionType;
 use ReflectionMethod;
 use ReflectionNamedType;
+use ReflectionType;
 use ReflectionUnionType;
 use Webmozart\Assert\Assert;
 
@@ -37,6 +38,9 @@ public static function boolean(callable $filter): void
         };
 
         $returnType = $reflection->getReturnType();
+        if (! $returnType instanceof ReflectionType) {
+            throw new InvalidArgumentException(sprintf(self::MESSAGE, 'none'));
+        }
 
         if ($returnType instanceof ReflectionUnionType || $returnType instanceof ReflectionIntersectionType) {
             $separator = $returnType instanceof ReflectionUnionType ? '|' : '&';
@@ -68,12 +72,9 @@ function (ReflectionNamedType|ReflectionIntersectionType $reflectionType): strin
             );
         }
 
-        if (! $returnType instanceof ReflectionNamedType) {
-            throw new InvalidArgumentException(sprintf(self::MESSAGE, 'mixed'));
-        }
+        Assert::isInstanceOf($returnType, ReflectionNamedType::class);
 
         $returnTypeName = $returnType->getName();
-
         if ($returnType->allowsNull()) {
             throw new InvalidArgumentException(sprintf(
                 self::MESSAGE,

tests/FilterTest.php

@@ -37,7 +37,7 @@ public function testOnceWithStringFilter(): void
     public function testWithoutReturnTypeCallable(): void
     {
         $this->expectException(InvalidArgumentException::class);
-        $this->expectExceptionMessage('Expected a bool return type on callable filter, mixed given');
+        $this->expectExceptionMessage('Expected a bool return type on callable filter, none given');
 
         $data = [1, 2, 3];