Proposal: a way to express template arguments when returning callables|return inferred type

[danog]

Currently, there is no way in psalm to express a function that returns a callable, if the callable has standalone template arguments.

For example: https://psalm.dev/r/7be6130bcb

I can call the templated function identity, and expect it to return exactly the provided type.
However, the makeIdentity function will not work as expected, unless I manually specify the templated type of the returned callable using a @var annotation.

Of course, I can't use the returned callable and expect it to work as the identity function, precisely because I set a specific template type when creating the function (if I hadn't provided any type, psalm would've defaulted to empty).

What I think would be really neat to have is a way to express this inside of psalm, either as:

• a @template-return annotation that is related to a returned callable
• a @psalm-infer-return to allow returning exactly the inferred type (aka exactly the templated function declared earlier; this is also extremely useful in some other scenarios).

[psalm-github-bot]

I found these snippets:

https://psalm.dev/r/7be6130bcb

<?php
/**
 * @template A
 * @param A $a
 * @return A
 */
function identity($a) { return $a; }

/**
 * @template A
 * @return (callable(A): A)
 */
function makeIdentity(): callable { return 'identity'; }

/** @psalm-trace $a */
$a = identity('a');

/** @psalm-trace $identity */
$identity = makeIdentity();

/** @psalm-trace $b */
$b = $identity('b');

Psalm output (using commit 3ce41d7):

INFO: Trace - 16:1 - $a: "a"

INFO: Trace - 19:1 - $identity: callable(empty):empty

ERROR: InvalidScalarArgument - 22:16 - Argument 1 expects empty, "b" provided

INFO: Trace - 22:1 - $b: empty

INFO: UnusedVariable - 16:1 - $a is never referenced or the value is not used

INFO: UnusedVariable - 22:1 - $b is never referenced or the value is not used

[weirdan]

Or get it into the actual type, e.g. callable<T of mixed>(T):T

[klimick]

Related to #5581 #4589 #4550