Narrowing a class-string with class_exists results in mixed instead of expected string

[asgrim]

https://psalm.dev/r/c32c4d2cd8

We are narrowing a parameter of type class-string to determine if the class_exists or not; but if the class does NOT exist, them Psalm is telling me the type is mixed; however I would expect the type to be string. Not sure why this is happening?

[psalm-github-bot]

I found these snippets:

https://psalm.dev/r/c32c4d2cd8

<?php

/** @param class-string $className */
function foo(string $className): void {
    /** @psalm-trace $className */
   if ( ! class_exists($className) ) {
       /** @psalm-trace $className */
       throw new \RuntimeException($className);
   }
}

Psalm output (using commit 7428e49):

ERROR: DocblockTypeContradiction - 6:11 - Docblock-defined type class-string for $className is always class-string

INFO: MixedArgument - 8:36 - Argument 1 of RuntimeException::__construct cannot be mixed, expecting string

INFO: Trace - 8:8 - $className: mixed

INFO: Trace - 6:4 - $className: class-string