Using union with generic types in a parameter results in wrong return type

[shira-374]

I'm trying to implement a kind of "auto-boxing" of generic result objects. If a non-result value gets passed, it is automatically wrapped:

/**
 * @template TNextValue
 * @template TNextError
 *
 * @param Result<TNextValue, TNextError>|TNextValue $result
 * @return Result<TValue|TNextValue, TNextError>
 */
abstract function orElse($result): Result;

However, I'm getting a wrong return type when I actually call this method:

https://psalm.dev/r/40be01250b

It seems to work fine in PHPStan: https://phpstan.org/r/c9779525-79a6-4a8f-b5f5-9d1c652c204f

I don't think the |TNextValue part should be considered in this case, because Result<TNextValue, TNextError> is more specific.

---

For completness, my actual code involves closures (and exhibits the same issue):

https://psalm.dev/r/9880a39426

Also works in PHPStan: https://phpstan.org/r/cd2248d1-2862-477d-ba0a-65ab69445f25

[psalm-github-bot]

I found these snippets:

https://psalm.dev/r/40be01250b

<?php declare(strict_types=1);

/**
 * @template-covariant TValue
 * @template-covariant TError
 * @psalm-suppress InvalidTemplateParam not relevant
 */
abstract class Result
{
    /**
     * @template TNextValue
     * @template TNextError
     *
     * @param Result<TNextValue, TNextError>|TNextValue $result
     * @return Result<TValue|TNextValue, TNextError>
     */
    abstract function orElse($result): Result;
}

/**
 * @param Result<int, string> $result
 * @param Result<never, float> $elseResult
 * @psalm-suppress UnusedVariable not relevant
 */
function example(Result $result, Result $elseResult): void
{
    /**
     * @psalm-trace $x
     * @psalm-check-type $x = Result<int, float>
     */
    $x = $result->orElse($elseResult);
}

Psalm output (using commit 765dcbf):

INFO: Trace - 31:5 - $x: Result<Result<never, float>|int, float>

ERROR: CheckType - 31:5 - Checked variable $x = Result<int, float> does not match $x = Result<Result<never, float>|int, float>

https://psalm.dev/r/9880a39426

<?php declare(strict_types=1);

/**
 * @template-covariant TValue
 * @template-covariant TError
 * @psalm-suppress InvalidTemplateParam not relevant
 */
abstract class Result
{
    /**
     * @template TNextValue
     * @template TNextError
     *
     * @param Closure(TError, mixed...):(Result<TNextValue, TNextError>|TNextValue) $callback
     * @return Result<TValue|TNextValue, TNextError>
     */
    abstract function orElse(\Closure $callback): Result;
}

/**
 * @param Result<int, string> $result
 * @param Closure():Result<never, float> $callback
 * @psalm-suppress UnusedVariable not relevant
 */
function example(Result $result, Closure $callback): void
{
    /**
     * @psalm-trace $x
     * @psalm-check-type $x = Result<int, float>
     */
    $x = $result->orElse($callback);
}

Psalm output (using commit 765dcbf):

INFO: Trace - 31:5 - $x: Result<Result<never, float>|int, float>

ERROR: CheckType - 31:5 - Checked variable $x = Result<int, float> does not match $x = Result<Result<never, float>|int, float>