Fatal Compiler Bug: Unexpected pattern about gleam

[giacomocavalieri]

I've done some looking around and narrowed this down a bit if it can be of any help.
So a test that is producing the same crash is this one:

#[test]
fn this_crashes() {
    assert_warning!(
        r#"
pub type Returned(a) {
  Returned(List(a))
}

fn foo(user: Returned(#())) -> Int {
  let Returned([#()]) = user
  1
}
"#
    );
}

You don't get a crash if Returned doesn't have a generic (for example if you say its constructor is a fixed Returned(List(#())))

The problem is that, when doing the exhaustiveness checking, it ends up running into a type variable (the Returned one) and branch_mode decides that is Infinite and the you run into the tuple pattern and get the crash.

Funnily enough, if you remove the list and just do:

pub type Returned(a) {
  Returned(a)
}

fn foo(user: Returned(#())) -> Int {
  let Returned(#()) = user
  1
}

You don't get the crash anymore, so I'm guessing it might have to do with how list patterns are handled. I'm not really familiar with the algorithm so I couldn't actually fix the bug but maybe this can be of help

(Huge thanks to @DanielSherlock for helping me out!)

[lpil]

Thank you

[lpil]

Oops! Found another bug