psalm-if-this-is enhancements

[klimick]

This example raises many issues: https://psalm.dev/r/8f7f7985a6

All issues can be fixed if:

1. $this variables will be inferred from psalm-if-this-is
2. template params will be resolved from psalm-if-this-is

[psalm-github-bot]

I found these snippets:

https://psalm.dev/r/8f7f7985a6

<?php

/**
 * @template T
 */
final class Option
{
    /**
     * @return T|null
     */
    public function unwrap()
    {
        throw new RuntimeException('???');
    }
}

/**
 * @template L
 * @template R
 */
final class Either
{
    /**
     * @return R|null
     */
    public function unwrap()
    {
        throw new RuntimeException('???');
    }
}

/**
 * @template T
 */
final class ArrayList
{
    /** @var list<T> */
    private $items;

    /**
     * @param list<T> $items
     */
    public function __construct(array $items)
    {
        $this->items = $items;
    }

    /**
     * @template A
     * @template B
     * @template TOption of Option<A>
     * @template TEither of Either<mixed, B>
     * @psalm-if-this-is ArrayList<TOption|TEither>
     * @return ArrayList<A|B>
     */
    public function compact(): ArrayList
    {
        $values = [];

        foreach ($this->items as $item) {
            $value = $item->unwrap();

            if (null !== $value) {
                $values[] = $value;
            }
        }

        return new self($values);
    }
}

/** @var ArrayList<Either<Exception, int>|Option<int>> $list */
$list = new ArrayList([]);

/** @psalm-trace $numbers */
$numbers = $list->compact();

Psalm output (using commit 51ba96c):

ERROR: IfThisIsMismatch - 76:19 - Class type must be ArrayList<(TEither:fn-arraylist::compact as Either<mixed, B:fn-arraylist::compact as mixed>)|(TOption:fn-arraylist::compact as Option<A:fn-arraylist::compact as mixed>)> current type ArrayList<Either<Exception, int>|Option<int>>

INFO: Trace - 76:1 - $numbers: ArrayList<empty>

INFO: MixedMethodCall - 61:29 - Cannot determine the type of $item when calling method unwrap

INFO: MixedAssignment - 61:13 - Unable to determine the type that $value is being assigned to

INFO: MixedAssignment - 64:17 - Unable to determine the type of this assignment