Bug with partially templated callable?

[thomasvargiu]

Related to #4485

There is a problem when I don't explicity set a template for a callable return type.

https://psalm.dev/r/ec02d9ad54

If I ignore the return type template for the callable input, even if I'm not using it, checks fail.
If I put a template, it works as expected.

In this case I don't care about callable return type, so I shouldn't need to specify a template.

[psalm-github-bot]

I found these snippets:

https://psalm.dev/r/ec02d9ad54

<?php

/**
 * @template TP
 * @template TR
 * @psalm-param callable(TP): TR $func
 * @psalm-return callable(TP): TP
 */
function tee(callable $func): callable {
    return function ($a) use ($func) {
        $func($a);

        return $a;
    };
}

/**
 * @template TP
 * @psalm-param callable(TP): mixed $func
 * @psalm-return callable(TP): TP
 */
function tee2(callable $func): callable {
    return function ($a) use ($func) {
        $func($a);

        return $a;
    };
}

/** @psalm-trace $f */
$f = tee(fn (int $a): string => $a . 'foo');

/** @psalm-trace $f */
$f = tee2(fn (int $a): string => $a . 'foo');

Psalm output (using commit 5a5cbb2):

INFO: Trace - 31:1 - $f: callable(int):int

INFO: Trace - 34:1 - $f: callable(empty):empty

[muglug]

There's no feasible way for Psalm to infer this type correctly, so it chooses the empty type (similar to a bottom type).

[muglug]

Correction: there is a way, and now it does.