Bug: Invalid_argument Type.lookup_val_field

[matthewhammer]

Fatal error: exception (Invalid_argument Type.lookup_val_field)
Raised at Mo_types__Type.invalid in file "mo_types/type.ml", line 365, characters 16-54
Called from Codegen__Compile.Object.is_mut_field in file "codegen/compile.ml", line 2635, characters 16-48

Some details here: https://github.com/dfinity/cancan/pull/70#issuecomment-774483364

[nomeata]

Here is a small reproducer:

actor {
  public func baz(b : Bool) : () {
      ignore({b = b} : {})
  };
}

It only happens if it is a shared function, so likely related to await/async translation.

I think the backend expects the exp.note.Note.typ of a NewObjE Object to be “tight”, and include every field (to find out whether it is mutable or not). But Check_ir doesn’t enforce it, and the async/await translation breaks this.

Possible fixes:

• So we could make the compiler be more liberal
• We could make Check_ir stricter (and adjust the passes)

• We could change NewObjE to include the mutability of fields in the source ast, so that the compiler doesn’t have to look at the type annotation (which, due to subtyping, is always a bit dodgy)

  Given that NewObjE behaves quite differently for immutable fiels (copy value) and mutable fiels (alias mutable values), this option seem to be the most robust.

@crusso what do you think?

[nomeata]

Actually, there is an easier fix: Look at the type note on the field, not the object. Then not even subtyping can get into the way.

[nomeata]

Fix in https://github.com/dfinity/motoko/pull/2318

[nomeata]

That branch has both fixes, one of them reverted. Claudio, do you have a hunch which one is preferable?

[crusso]

Taking a look