Subclass actions don't typecheck until ivytocpp

[dijkstracula]

Hi Ken,

I have the following message type with a bunch of subclasses thereof:

 16     class msg_t = {
 17         action handle(cid: node, ^msg:msg_t)
 18     }
...
 38     subclass commit_msg_t of msg_t = {
 39         field src: node
 40         field txn: txn_t
 41         field execute_at: timestamp
 42
 43         field deps: vector[txn_t]
 44
 45         action handle(self: node, ^msg:commit_msg_t)
 46     }
 47
 48     subclass accept_msg_t of msg_t = {
 49         field src: node
 50         field txn: txn_t
 51         field execute_at: timestamp
 52
 53         field deps: vector[txn_t]
 54
 55         action handle(self: node, ^msg:commit_msg_t)
 56     }

Note the copy and paste error on line 55: clearly, that should be accept_msg_t. However, ivy appears to accept this, only to die while trying to compile the extracted C++.

(venv) ➜  accord-ivy git:(main) ✗ make
cd src; ivyc target=test protocol.ivy
g++ -Wno-parentheses-equality  -std=c++11  -I /Users/ntaylor/code/ivy/ivy/include -L /Users/ntaylor/code/ivy/ivy/lib -Xlinker -rpath -Xlinker /Users/ntaylor/code/ivy/ivy/lib -I /Users/ntaylor/code/ivy/submodules/z3/src/api/python/z3/include -L /Users/ntaylor/code/ivy/submodules/z3/src/api/python/z3/lib -Xlinker -rpath -Xlinker /Users/ntaylor/code/ivy/submodules/z3/src/api/python/z3/lib -I /usr/local/opt/openssl/include -L /usr/local/opt/openssl/lib -Xlinker -rpath -Xlinker /usr/local/opt/openssl/lib -g -o protocol protocol.cpp -lz3 -pthread
protocol.cpp:3382:47: error: no viable conversion from 'protocol::accept_msg_t' to 'const protocol::commit_msg_t'
                accept_msg_t__handle(prm__V0, self__COLON__accept_msg_t);
                                              ^~~~~~~~~~~~~~~~~~~~~~~~~
./protocol.h:829:12: note: candidate constructor (the implicit copy constructor) not viable: no known conversion from 'protocol::accept_msg_t' to 'const protocol::commit_msg_t &' for 1st argument
    struct commit_msg_t {
           ^
./protocol.h:829:12: note: candidate constructor (the implicit move constructor) not viable: no known conversion from 'protocol::accept_msg_t' to 'protocol::commit_msg_t &&' for 1st argument
protocol.cpp:3090:67: note: passing argument to parameter 'msg' here
void protocol::accept_msg_t__handle(int self, const commit_msg_t& msg){
                                                                  ^
1 error generated.
make: *** [srcprotocol] Error 1
(venv) ➜  accord-ivy git:(main) ✗

Seems like either the parsing or extraction code should complain; or, C++ codegen should compile. (I suspect the former is the thing we want; not sure exactly what the execution semantics of the latter would be.)