Method getFullClassNameFromRelativeFileName() should return class-string<CommandExpression>|null but returns class-string

[zer0uno]

I have the following method:

/** 
 * @return class-string<CommandExpression>
 */
private static function getFullClassNameFromRelativeFileName(string $relativeFileName): ?string
{
    $relativeClassName = str_replace(self::COMMAND_EXPRESSION_FILE_EXTENSION, '', $relativeFileName);

    $fullClassName = __NAMESPACE__ . '\\' . $relativeClassName;
    if (
        !class_exists($fullClassName)
    ) {
        return null;
    }

    if (
        !array_key_exists(CommandExpression::class, class_implements($fullClassName))
    ) {
        return null;
    }

    return $fullClassName;
}

but I'm getting the following error:

Method getFullClassNameFromRelativeFileName() should return class-string<CommandExpression>|null but returns class-string. 

How is that possible? I have the expression array_key_exists(CommandExpression::class, class_implements($fullClassName)) that should avoid to return a class-string that doesn't implement CommandExpression.

[ondrejmirtes]

Please reproduce it on phpstan.org/try and I'll show you how to fix it.

[zer0uno]

@ondrejmirtes Here you can reproduce it https://phpstan.org/r/cf0ebada-f8c2-4b0d-8ef4-c9699663ac8a

[ondrejmirtes]

Use is_a() https://phpstan.org/r/66c3871f-d959-4243-a850-2a7178ba5734