Defention of internal function mb_convert_encoding() is wrong

[jsiefer]

See https://psalm.dev/r/bb126f54c3

The return type of mb_convert_encoding(string|string[] $input, string $to, string|string[] $from): string | string[] depends on its input.

if the input is a string, its return values should be a string and same for string[]

[psalm-github-bot]

I found these snippets:

https://psalm.dev/r/bb126f54c3

<?php

function test(string $str): string {
    return mb_convert_encoding($str, 'UTF-8', 'UTF-8');
}

/**
 * @param array<int, string> $str
 * @return array<int, string>
 */
function test2(array $str): array {
    return mb_convert_encoding($str, 'UTF-8', 'UTF-8');
}


test('Hello');
test2('Hello');

Psalm output (using commit 321b81e):

ERROR: InvalidReturnStatement - 4:12 - The inferred type 'array<int, string>|string' does not match the declared return type 'string' for test

ERROR: InvalidReturnType - 3:29 - The declared return type 'string' for test is incorrect, got 'array<int, string>|string'

ERROR: InvalidReturnStatement - 12:12 - The inferred type 'array<int, string>|string' does not match the declared return type 'array<int, string>' for test2

ERROR: InvalidReturnType - 9:12 - The declared return type 'array<int, string>' for test2 is incorrect, got 'array<int, string>|string'

ERROR: InvalidArgument - 17:7 - Argument 1 of test2 expects array<int, string>, string(Hello) provided