No Error Explanation in User-Defined Pack & Unpack

[nordlow]

Classes with errorneous user-defined msgpacking, such as

/** Directory Kind.
 */
class DirKind {
    this(string fn,
         string kn) {
        this.fileName = fn;
        this.kindName = kn;
    }

    version (msgpack) {
        this(Unpacker)(ref Unpacker unpacker) {
            fromMsgpack(msgpack.Unpacker(unpacker));
        }
        void toMsgpack(Packer)(ref Packer packer) const {
            packer.beginArray(this.tupleof.length);
            packer.pack(this.tupleof);
            errror_here;
        }
        void fromMsgpack(Unpacker)(auto ref Unpacker unpacker) {
            unpacker.beginArray();
            unpacker.unpack(this.tupleof);
        }
    }

    string fileName;
    string kindName;
}
version (msgpack) unittest {
    auto k = tuple("", "");
    auto data = pack(k);
    Tuple!(string, string) k_; data.unpack(k_);
    assert(k == k_);
}

makes the compiler error without any usable explanations:

/home/per/Work/justd/msgpack.d(807): Error: static assert  "Failed to invoke 'toMsgpack' on type 'Dir'"
/home/per/Work/justd/msgpack.d(4349):        instantiated from here: pack!(Dir)
/home/per/Work/justd/fs.d(850):        instantiated from here: pack!(false, Dir)
/home/per/Work/justd/fs.d(2056):        instantiated from here: saveDirTree!(Terminal)
/home/per/Work/justd/fs.d(2076):        instantiated from here: fs!(Terminal)

Compilation exited abnormally with code 1 at Fri Nov 22 19:43:51

[repeatedly]

"Failed to invoke 'toMsgpack' on type 'Dir'" shows Dir's toMsgpack can't pass a compilation check.

Hmm, do you have a suggestion?

[nordlow]

The problem is your use of __traits(compiles....).

You overuse them in the error handling in msgpack.d.

For example

        static if (__traits(compiles, { T t; t.toMsgpack(this, withFieldName_); })) {
            object.toMsgpack(this, withFieldName_);
        } else static if (__traits(compiles, { T t; t.toMsgpack(this); })) { // backward compatible
            object.toMsgpack(this);
        } else {
            object.toMsgpack(this);
            static assert(0, "Failed to invoke 'toMsgpack' on type '" ~ T.stringof ~ "'");
        }

should be replaced with

        static if (__traits(compiles, { T t; t.toMsgpack(this, withFieldName_); })) {
            object.toMsgpack(this, withFieldName_);
        } else {
            object.toMsgpack(this);
        }

Do you want me to fix and provide and pull request or do you want to do it yourself?

[ghost]

I did suggest this sort of thing before: #2 (comment)

[nordlow]

I don't understand...how can something be more important than having the possibility to know what error I have done in my pack or unpack function?