Undefined variables in string interpolation are not being caught

[Ocramius]

Taken from the convo @ https://twitter.com/mattbrowndev/status/1367965245084491783 (sorry, I just happen to review too much code :| )

<?php

(function () : void {
    echo (fn(string $b) => "$b - $undefined")("hello");
})();

https://psalm.dev/r/d5662fa596

It should report $undefined as undefined, but it doesn't.

See https://3v4l.org/D8tJR for runtime verification.

[psalm-github-bot]

I found these snippets:

https://psalm.dev/r/d5662fa596

<?php

(function () : void {
    echo (fn(string $b) => "$b - $undefined")("hello");
})();

Psalm output (using commit 96e0743):

No issues!

[weirdan]

Simplified and added one more variation: https://psalm.dev/r/9520a78b52

[psalm-github-bot]

I found these snippets:

https://psalm.dev/r/9520a78b52

<?php
function a(): void {
    fn(): string => (string) $undefined;
}

function b(): void {
    fn(): string => "$undefined";   
}

Psalm output (using commit 96e0743):

No issues!

[Ocramius]

Thanks, @muglug!

[orklah]

And thanks @weirdan for the PR! :)