Incorrect return value is not detected for methods with a template return type since 5.3

[vudaltsov]

This code has an error on line 23 (should return int, not string).

https://psalm.dev/r/aa26301af0

Removing the mixed return type fixes the issue: https://psalm.dev/r/6bf5cdea8a.

Regression occurred in #8926.

Commit da001c4 is the last one, when Psalm reports the problem (correct behavior).

[psalm-github-bot]

I found these snippets:

https://psalm.dev/r/aa26301af0

<?php

declare(strict_types=1);

/**
 * @template T
 */
interface I
{
    /**
     * @return T
     */
    public function process(): mixed;
}

/**
 * @implements I<int>
 */
final class B implements I
{
    public function process(): mixed
    {
        return '';
    }
}

Psalm output (using commit 4eacb2f):

No issues!

https://psalm.dev/r/6bf5cdea8a

<?php

declare(strict_types=1);

/**
 * @template T
 */
interface I
{
    /**
     * @return T
     */
    public function process();
}

/**
 * @implements I<int>
 */
final class B implements I
{
    public function process()
    {
        return '';
    }
}

Psalm output (using commit 4eacb2f):

ERROR: InvalidReturnStatement - 23:16 - The inferred type '''' does not match the declared return type 'int' for B::process

ERROR: InvalidReturnType - 21:21 - The declared return type 'int' for B::process is incorrect, got ''''

[orklah]

@danog could you take a look please?