guarded: expected Tuple(Dyn,Dyn) got [<...>,<...>]

[bennn]

The guarded semantics doesn't allow certain values as Dyn (inside a list).

from retic import List

class A:
  def f(a:A, b:A)->List(A):
    return [a, b]

class B:

    vals = [A(), A()]

    def g(self: B):
        [a1, a2] = (self.vals[0]).f(self.vals[1])

b = B()
b.g()

Traceback

Traceback (most recent call last):
  File "/usr/local/bin/retic", line 5, in <module>
    retic.main()
  File "/home/ben/code/gradual/reticulated/retic/retic.py", line 149, in main
    reticulate(program, prog_args=args.args.split(), flag_sets=args)
  File "/home/ben/code/gradual/reticulated/retic/retic.py", line 98, in reticulate
    utils.handle_runtime_error(exit=True)
  File "/home/ben/code/gradual/reticulated/retic/retic.py", line 96, in reticulate
    _exec(code, __main__.__dict__)
  File "/home/ben/code/gradual/reticulated/retic/exec3/__init__.py", line 2, in _exec
    exec (obj, globs, locs)
  File "bug-expected-tuple.py", line 15, in <module>
    b.g()
  File "/home/ben/code/gradual/reticulated/retic/rproxy.py", line 326, in __call__
    return self.__call__(*args, **kwds)
  File "/home/ben/code/gradual/reticulated/retic/guarded.py", line 208, in wrapper
    else: ret = val(*cargs, **ckwds)
  File "bug-expected-tuple.py", line 12, in g
    [a1, a2] = (self.vals[0]).f(self.vals[1])
  File "/home/ben/code/gradual/reticulated/retic/guarded.py", line 60, in retic_cast
    return retic_cast(val, rtypes.Record({}), trg.structure(), msg, line)
  File "/home/ben/code/gradual/reticulated/retic/guarded.py", line 75, in retic_cast
    retic_assert(hasattr(val, m), val,msg, exc=ObjectTypeAttributeCastError)
  File "/home/ben/code/gradual/reticulated/retic/guarded.py", line 35, in retic_assert
    raise exc(msg % val)
retic.guarded.ObjectTypeAttributeCastError: 
bug-expected-tuple.py:12:8: Right hand side of assignment was expected to be of type Tuple(Dyn,Dyn), but value [<__main__.A object at 0x7fa2b3baa128>, <__main__.A object at 0x7fa2b4323940>] provided instead. (code SINGLE_ASSIGN_ERROR)

[mvitousek]

Thanks. This actually looks like a bug in how multiple assignment works; it casts the RHS to a tuple type to make sure it has the correct length but there's a bug in how I do list => tuple casts that leads to this error.
(FYI, Reticulated's implementations of guarded and monotonic are somewhat on the back burner right now compared to transient, so I might not fix these as quickly as I otherwise would. But I'll get to them as soon as possible and I appreciate the reports!)