Crash on multiple assignment to tuple #1420

a, b = c, d = 0, 1

Traceback (most recent call last):
  File "/Users/guido/v3/bin/mypy", line 6, in <module>
    main(__file__)
  File "/Users/guido/v3/lib/python3.5/site-packages/mypy/main.py", line 52, in main
    res = type_check_only(sources, bin_dir, options)
  File "/Users/guido/v3/lib/python3.5/site-packages/mypy/main.py", line 100, in type_check_only
    python_path=options.python_path)
  File "/Users/guido/v3/lib/python3.5/site-packages/mypy/build.py", line 208, in build
    dispatch(sources, manager)
  File "/Users/guido/v3/lib/python3.5/site-packages/mypy/build.py", line 1251, in dispatch
    process_graph(graph, manager)
  File "/Users/guido/v3/lib/python3.5/site-packages/mypy/build.py", line 1382, in process_graph
    process_stale_scc(graph, scc)
  File "/Users/guido/v3/lib/python3.5/site-packages/mypy/build.py", line 1412, in process_stale_scc
    graph[id].type_check()
  File "/Users/guido/v3/lib/python3.5/site-packages/mypy/build.py", line 1231, in type_check
    manager.type_checker.visit_file(self.tree, self.xpath)
  File "/Users/guido/v3/lib/python3.5/site-packages/mypy/checker.py", line 419, in visit_file
    self.accept(d)
  File "/Users/guido/v3/lib/python3.5/site-packages/mypy/checker.py", line 460, in accept
    typ = node.accept(self)
  File "/Users/guido/v3/lib/python3.5/site-packages/mypy/nodes.py", line 753, in accept
    return visitor.visit_assignment_stmt(self)
  File "/Users/guido/v3/lib/python3.5/site-packages/mypy/checker.py", line 1147, in visit_assignment_stmt
    rvalue = self.temp_node(self.type_map[s.rvalue], s)
KeyError: <mypy.nodes.TupleExpr object at 0x105d158d0>