PHP8 Mixed Type Not Being Detected As Nullable

[DeveloperRob]

Tested & confirmed on:

• 4.0.0-alpha5
• 3.2.6
• master

Summary
With the introduction of a 'mixed' type in PHP8's own language, but the core engine not supporting nullable mixed (?mixed), it seems that (at least) PhanCoalescingNeverNull now doesn't pick up internal functions that have a mixed return type can return null.

Example Code

<?php
declare(strict_types=1);

class Example{
	/** @var array */
	private $decodedJson;

	public function decodeJson(){
		$this->decodedJson = json_decode('{"this:"is not valid json"', true) ?? [];
	}
}

Expected Output
No errors

Actual Output

PhanCoalescingNeverNull Using non-null json_decode('{"this:"is not valid json"', true) of type mixed as the left hand side of a null coalescing (??) operation. The right hand side may be unnecessary.

[TysonAndre]

This is a bug.

I'm also able to reproduce this under php 8.0 with non-literals - the intent was to make mixed include all types including null, and ?mixed to indicate a case where Phan sees null is very plausible in one or more code paths

<?php
function decode_or_default(string $json) {
    return json_decode($json) ?? 'default';
}