func_num_args() not usable in mutation-free/pure functions

[colinodell]

func_num_args() is detected as an impure function which prevents it from being used within a mutation-free or pure function.

In this example, I have a function which needs to differentiate between an optional parameter being explicitly set to null or not being provided at all: https://psalm.dev/r/2c0112de69

Psalm is technically correct that func_num_args() is impure since its output is not a function of its input. However, I believe that my outer getValue() function is pure since its output only relies on the parameters passed in.

Should Psalm therefore make an exception to allow the impure func_num_args() function to be called within other pure functions?

[psalm-github-bot]

I found these snippets:

https://psalm.dev/r/2c0112de69

<?php

/**
 * @param array<string, mixed> $arr     Data to pull the value from
 * @param string               $key     The key where the value should exist
 * @param mixed|void           $default Optional default value to return (can be null)
 *
 * @return mixed
 *
 * @psalm-pure
 */
function getValue(array $arr, string $key, $default = null) {
    $hasDefault = \func_num_args() === 3;
    
    if (\array_key_exists($key, $arr)) {
        return $arr[$key];
    }
    
    if ($hasDefault) {
        return $default;
    }
    
    throw new RuntimeException('Key does not exist and no default value was given');
}

$data = ['foo' => 'bar'];

assert(getValue($data, 'foo') === 'bar');
assert(getValue($data, 'qwerty', 42) === 42);
assert(getValue($data, 'asdf', null) === null);

try {
    $x = getValue($data, 'asdf');
    assert(false); // Exception should be thrown before this point
} catch (RuntimeException $ex) {
    assert(true); // Yay the exception was thrown!
}

Psalm output (using commit 5db75df):

ERROR: ImpureFunctionCall - 13:19 - Cannot call an impure function from a mutation-free context

INFO: MixedAssignment - 33:5 - Unable to determine the type that $x is being assigned to

INFO: UnusedVariable - 33:5 - Variable $x is never referenced

[muglug]

Yeah, it doesn't seem impure in any context really