Fix nullable message

Author: samsonasik

src/Assert/Filter.php

@@ -60,11 +60,19 @@ public static function boolean(callable $filter): void
         }
 
         $returnTypeName = $returnType->getName();
+
         if ($returnTypeName !== 'bool') {
             throw new InvalidArgumentException(sprintf(
                 self::MESSAGE,
                 $returnTypeName
             ));
         }
+
+        if ($returnType->allowsNull()) {
+            throw new InvalidArgumentException(sprintf(
+                self::MESSAGE,
+                '?' . $returnTypeName
+            ));
+        }
     }
 }

tests/FilterTest.php

@@ -69,6 +69,22 @@ public function __invoke(int $datum): string
         AtLeast::once($data, $filter);
     }
 
+    public function testWithNullableReturnTypeCallable(): void
+    {
+        $this->expectException(InvalidArgumentException::class);
+        $this->expectExceptionMessage('Expected a bool return type on callable filter, ?bool given');
+
+        $data   = [1, 2, 3];
+        $filter = new class {
+            public function __invoke(int $datum): ?bool
+            {
+                return true;
+            }
+        };
+
+        AtLeast::once($data, $filter);
+    }
+
     public function testWithUnionReturnTypeCallable(): void
     {
         $this->expectException(InvalidArgumentException::class);