Unused variable false positive

[iluuu1994]

This was a tough one to reproduce.

https://psalm.dev/r/8220067b98

<?php

interface ClientInterface
{
    public function getUrl(): string;
}

class Foo
{
    private static function generateAWSDocumentUrl(string $documentKey): string
    {
        if ($documentKey == null) {
            return '';
        }

        $requestUrl = '';

        try {
            /** @var ClientInterface */
            $client = null;
            $requestUrl = $client->getUrl();
            echo $requestUrl; // <-- This line
        } catch (\Exception $e) {
        }

        return $requestUrl;
    }
}

INFO: UnusedVariable - 16:9 - Variable $requestUrl is never referenced

Interestingly, removing the echo $requestUrl; removes the error. So does removing the whole if ($documentKey == null) statement at the top. It looks like multiple factors are at play here.

[muglug]

Simplified slightly to

function maybeThrows() : string {
  if (rand(0, 1)) throw new \Exception('bad');
  return "hello";
}

function b(bool $a): void {
  if (!$a) {
    return;
  }

  $b = '';

  try {
    $b = maybeThrows();
    echo $b;
  } catch (\Exception $e) {
  }

  echo $b;
}

[iluuu1994]

Thank you @muglug!