Psalm seems to think of "set" as a type - #1722

Hmm, I don't understand what the issue is.. Psalm isn't complaining about it at all, and this PR only increases the amount of psalm-errors?

Hi @tvdijen , it is hard to show within the SSP project because of its psalm error levels and which things are set to just info.
I created minimal repo to show the psalm error.
There is a super simple test case

    $config = Configuration::loadFromArray([
        'option' => 'value'
    ]);


    $value = $config->getOptionalString('someOption', null);
    /**
     * Psalm wrongly complains about this since it thinks $value can never be null
     */
    $this->assertNull($value);

and for phpunit it passes fine. However when running psalm it reports

ERROR: DocblockTypeContradiction - tests/ConfigurationForPsalmStuffTest.php:16:16 - Cannot resolve types for $value - docblock-defined type string does not contain null (see https://psalm.dev/155) $this->assertNull($value);

It is basically saying that the return type of $config->getOptionalString can never be null. We know it can be though, so why is psalm confused? I think it is because of @psalm-return ($default is set ? string|null : string) on getOptionalString. According to psalms conditional types docs the format is (<template param> is <union type> ? <union type> : <union type>). SSP is using set as the union type , which isn't a type in the docs (from what I can see).

So I think it should be ($default is null ? string|null : string) or ($default is string ? string: string|null)

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