Better comparison would be:

(new UnionType([new IntegerType(), new FloatType()]))->isSuperTypeOf($itemType)->yes()

dtto

I don't think it makes sense to modify this.

For an array of string, $itemType is a stringType.
So TypeCombinator::intersect($itemType, new UnionType([new IntegerType(), new FloatType()]) is nothing.
That's why the dynamic return type return new ConstantIntegerType(0).

The intersect seems to be the reason of the bug.

But on the other hand, this intersect allows us to specify that when we only have array<int>, the result is going to be int.

Isn't this handle by the previous check

if ($intUnionFloat->isSuperTypeOf($itemType)->yes()) {
    return TypeCombinator::union(new ConstantIntegerType(0), $itemType);
}

?