psalm/docs/running_psalm/issues/UnresolvableConstant.md

UnresolvableConstant

Emitted when Psalm cannot resolve a constant used in key-of or value-of. Note that static::CONST is considered unresolvable for key-of and value-of, since the literal keys and values can't be resolved due to the possibility of being overridden by child classes.

<?php

class Foo
{
    public const BAR = ['bar'];

    /**
     * @return value-of<self::BAT>
     */
    public function bar(): string
    {
        return self::BAR[0];
    }
}