Generator: TSend other than mixed not supported

[caugner]

In the context of #2984, I noticed that a different value for Generators third type parameter (TSend) does not seem to support a value other than the default (mixed):
https://psalm.dev/r/b6bedecbb9

It causes the following error:

INFO: MixedReturnTypeCoercion - 5:12 - The declared return type 'Generator<int, int, int, void>' for generate is more specific than the inferred return type 'Generator<int, int, mixed, void>'

@andrew-demb pointed out the relevant stub code:

psalm/src/Psalm/Internal/Stubs/CoreGenericClasses.phpstub

Lines 190 to 198 in 474b9dc

	/**
	* @template-covariant TKey
	* @template-covariant TValue
	* @template TSend
	* @template-covariant TReturn
	*
	* @template-implements Traversable<TKey, TValue>
	*/
	class Generator implements Traversable {

[psalm-github-bot]

I found these snippets:

https://psalm.dev/r/b6bedecbb9

<?php

/**
 * @var int $int
 * @return Generator<int, int, int, void>
 */
function generate(int $int) {
    while (true) {
        $int = yield (2 * $int);
    }
}

Psalm output (using commit 474b9dc):

INFO: MixedReturnTypeCoercion - 5:12 - The declared return type 'Generator<int, int, int, void>' for generate is more specific than the inferred return type 'Generator<int, int, mixed, void>'