Why does not defining array key type end up as mixed type?

[balintcodes]

Hi there,

After a version bump of dependencies in a Symfony project I noticed that since December symfony/type-info throws an InvalidArgumentException when it faces a mixed key type (see update here).

I ended up at ApiPlatform\Metadata\Util\PropertyInfoToTypeInfoHelper::createTypeFromLegacyValues(). At some point it does the following:

$variableTypes[] = \is_array($collectionKeyTypes) ? Type::mixed() : Type::union(Type::int(), Type::string()); // @phpstan-ignore-line

Because of this, PHPDoc annotations like so throw an error in entities:

/*
 *@return array<string>
 */
private array $foo = [];

Can anyone help me wrap my head around why this is, and if I understand it right, why a mixed type is returned in every scenario, since $collectionKeyTypes is always an array?
Fixing such annotations in now a big deal, I'm more interesed in the whys:)