with tuple causes compiler to abort

[anacrolix]

The following code causes the compiler to abort:

actor Main
  new create(env: Env) =>
    with (a,b) = (1,2) do
      None
    end

I think it's the (a,b) on the LHS. The output:

$ ~/src/ponyc/build/debug/ponyc 
Building builtin -> /Users/anacrolix/src/ponyc/packages/builtin
Building . -> /Users/anacrolix/src/mylittle/with-abort
/Users/anacrolix/src/ponyc/src/libponyc/pass/sugar.c:542: build_with_dispose: Assertion `ast_id(idseq) == TK_TUPLE` failed.

Backtrace functionality not available.
Abort trap: 6

[mfelsche]

This is clearly a bug. Though, strictly speaking, (a, b) needs to have a dispose function or behaviour for this to fully compile, and a tuple doesnt have such a function. On the other hand it is clear that tuple destructuring should work inside a with-expression, so a and b should have a dispose method or behaviour.

Also there is also no test coverage yet for tuple destructuring in with expressions.

The bug is in https://github.com/ponylang/ponyc/blob/master/src/libponyc/pass/sugar.c#L542 when the left side of the with-expression is a tuple, it calls itself recursively with the AST-child of that tuple, expecting a let or another tuple. But the child of the given tuple AST is a TK_SEQ (a list of things) of let expressions (for a and b in the case above).