Type checker panic on type alias with type variable

[rtfeldman]

As of 91483db this can be reproduced by adding the following to any .roc file and running roc check on it:

Foo a : [ Foo a ]

foo = \arg ->
    x : Foo *
    x = arg

    {}

It panics with:

Rigids [116]
Failing [116]
'assertion failed: {\n let failing: Vec<_> =\n rigid_vars.iter().filter(|&var|\n {\n !subs.redundant(*var) &&\n subs.get_rank(*var) != Rank::NONE\n }).collect();\n if !failing.is_empty() {\n println!(\"Rigids {:?}\", & rigid_vars);\n println!(\"Failing {:?}\", failing);\n }\n failing.is_empty()\n}',
compiler/solve/src/solve.rs:572:21

Things that make it no longer crash:

• Removing the x : Foo * annotation
• Removing the type variable from the Foo annotation
• Removing the \arg -> lambda (so it's just foo = and then the x def)
• Assigning x = to something other than arg

Variations that still reproduce the crash:

• Foo a : List a
• Foo a : { field : a }

So the relevant part here seems to be the type variable in the alias!

[ayazhafiz]

My guess is that this would work if you instead made the type annotation “Foo b” or “Foo _”. My guess is generalization of “*” is treated specially. This will be another fun (or “fun”) bug ;)

@caiquefigueiredo would you be interested in working through this one?

[rtfeldman]

Just tried it and the panic still happens with both Foo _ and Foo b - the plot thickens! 😃

[ayazhafiz]

noooo

[rtfeldman]

[caiquefigueiredo]

@ayazhafiz sure! I'm not sure I can go at it alone, but it seems fun :)

[ayazhafiz]

This is one place where the original checker code did not diverge from Elm at all. For whatever reason Elm does not have a problem with this: https://ellie-app.com/hd3JrwdkSD3a1

But I think this is a bug in Elm. Looking further.

[ayazhafiz]

I think this is fine and the panic should just be removed. Posted in Zulip: https://roc.zulipchat.com/#narrow/stream/231635-compiler-development/topic/Can.20rigids.20flow.20out.3F

[ayazhafiz]

Elm has the same problem, but Elle doesn't show it. I'm not sure why. But I hit it in a local build of elm/compiler