It doesn't understand that the type in canister is the same

[vporton]

The bug is triggered by https://github.com/vporton/bug-canister-type (84b7e50):

$ dfx deploy 
...
/home/porton/Projects/bug/mytest/src/mytest_backend/main.mo:9.14-9.18: type error [M0096], expression of type
  actor {type X = Null; f : shared X__1 -> async ()}
cannot produce expected type
  actor {f : shared X -> async ()}

f has the same type in both and should compile without an error.

For completeness, here is the code:

lib.mo:

module {
    public type X = ();
}

call.mo:

import lib "lib";

actor {
    public shared func f(x: lib.X): async () {
        ();
    };
}

main.mo:

import lib "lib";
import call "canister:call";

actor {
  type T = actor {
    f(x: lib.X): async ();
  };

  let z: T = call;
};

dfx.json:

{
  "canisters": {
    "mytest_backend": {
      "main": "src/mytest_backend/main.mo",
      "type": "motoko",
      "dependencies": ["call"]
    },
    "call": {
      "main": "src/mytest_backend/call.mo",
      "type": "motoko"
    }
  },
  "defaults": {
    "build": {
      "args": "",
      "packtool": ""
    }
  },
  "output_env_file": ".env",
  "version": 1
}

[vporton]

$ moc --version
Motoko compiler 0.11.2+ (source 3w73qd3r-5pqc5q4p-crdjd8hk-1q2s4w4s)

[ggreif]

I can reproduce the error. If I have

module {
    public type X = Int;
}

in lib.mo then everything deploys just fine. My hunch is that the Candid conversion of () to Null is the problem. I also tried type X = Null and it is fine too.

[ggreif]

Please see the comments in the above repro. The gist of the problem is that you define a shared unary function (that has nullary result), and the Candid type mapping is not bijective. I.e. it maps the unary (unit) argument to a unary null Candid type. The reverse mapping from Candid to Motoko then makes the choice to map null to Null. While the two type systems are sufficiently similar for most purposes, here the round-tripping fails unfortunately. Are you really dependent on type X = (), or is this just a sketch of a problem you encounter in the wild?

[vporton]

Are you really dependent on type X = (), or is this just a sketch of a problem you encounter in the wild?

I don't remember the exact location of the problem where I met it. So, I can't answer your question. Right now the repository from which, if I remember correctly, I got the bug compiles well. But that may be another repository. Sorry.