experiment: partial fix for for subtyping loop due to polymorphic recursion

[crusso]

This is a known issue, but capturing the one @rvanasa found in the wild (that might get in the way of fancy class based libraries)

module {

  public class Iter<T>({next = n : () -> ?T}) = this {

    public let next = n;

    public func map<R>(f : T -> R) : Iter<R> { // NOTE polymorphic recursion on <R> (not <T>)
       loop {}
    };
  }

}

[nix-shell:~/motoko/test/fail]$ moc polyrec.mo 
OOPS! You've triggered a compiler bug.
Please report this at https://github.com/dfinity/motoko/issues/new with the following details:

Motoko (source 0.13.4-30-gcedc663ea-dirty)

Fatal error: exception Stack overflow
Raised by primitive operation at Mo_types__Type.rel_typ in file "mo_types/type.ml", line 980, characters 4-11
Called from Stdlib__list.for_all2 in file "list.ml", line 173, characters 24-31
Called from Mo_types__Type.rel_list in file "mo_types/type.ml", line 911, characters 6-38
Called from Mo_types__Type.rel_typ in file "mo_types/type.ml", line 983, characters 6-81
Called from Mo_types__Type.rel_fields in file "mo_types/type.ml", line 1004, characters 6-36
Called from Stdlib__list.for_all2 in file "list.ml", line 173, characters 24-31
Called from Mo_types__Type.rel_list in file "mo_types/type.ml", line 911, characters 6-38
Called from Mo_types__Type.rel_fields in file "mo_types/type.ml", line 1004, characters 6-36
Called from Stdlib__list.for_all2 in file "list.ml", line 173, characters 24-31
Called from Mo_types__Type.rel_list in file "mo_types/type.ml", line 911, characters 6-38
Called from Mo_types__Type.rel_fields in file "mo_types/type.ml", line 1004, characters 6-36
Called from Stdlib__list.for_all2 in file "list.ml", line 173, characters 24-31
Called from Mo_types__Type.rel_list in file "mo_types/type.ml", line 911, characters 6-38
Called from Mo_types__Type.rel_fields in file "mo_types/type.ml", line 1004, characters 6-36
Called from Stdlib__list.for_all2 in file "list.ml", line 173, characters 24-31
Called from Mo_types__Type.rel_list in file "mo_types/type.ml", line 911, characters 6-38
Called from Mo_types__Type.rel_fields in file "mo_types/type.ml", line 1004, characters 6-36
Called from Stdlib__list.for_all2 in file "list.ml", line 173, characters 24-31
Called from Mo_types__Type.rel_list in file "mo_types/type.ml", line 911, characters 6-38

• Related issue: Segmentation fault: 11 when trying to define a method which returns a new “instance” of a class #3057
• Type checker hack previously considered here too: WIP/Dog fooding Joins patterns as an AS library (DON'T MERGE) #158. Note, IIRC Andreas did not approve of the hack because it doesn't respect structural equivalence of type definitions

[ggreif]

this should be normalised, I guess

[crusso]

Ok, added counterexample polyrec1.mo that shows why this isn't a good idea. Isomorphic types will still lead to non-termination.