Crash when class defined in method used for instance variable's type

[gvanrossum]

While thinking about #2535 I realized there's a similar case not involving NamedTuple (nor TypedDict):

• main.py:

import crash
blah  # anything that triggers an error

• crash.py:

class C:
    def f(self) -> None:
        class A:
            pass
        self.a = A()

This triggers a very similar crash in fixup.py when running mypy -i main.py TWICE (from a cold cache):

$ rm -rf .mypy_cache/
$ mypy -i main.py 
main.py:1: error: Name 'blah' is not defined
$ mypy -i main.py 
Traceback (most recent call last):
  File "/usr/local/lib/python3.6/runpy.py", line 193, in _run_module_as_main
    "__main__", mod_spec)
  File "/usr/local/lib/python3.6/runpy.py", line 85, in _run_code
    exec(code, run_globals)
  File "/Users/guido/src/mypy/mypy/__main__.py", line 5, in <module>
    main(None)
  File "/Users/guido/src/mypy/mypy/main.py", line 41, in main
    res = type_check_only(sources, bin_dir, options)
  File "/Users/guido/src/mypy/mypy/main.py", line 86, in type_check_only
    options=options)
  File "/Users/guido/src/mypy/mypy/build.py", line 183, in build
    dispatch(sources, manager)
  File "/Users/guido/src/mypy/mypy/build.py", line 1524, in dispatch
    process_graph(graph, manager)
  File "/Users/guido/src/mypy/mypy/build.py", line 1753, in process_graph
    process_fresh_scc(graph, prev_scc)
  File "/Users/guido/src/mypy/mypy/build.py", line 1822, in process_fresh_scc
    graph[id].fix_cross_refs()
  File "/Users/guido/src/mypy/mypy/build.py", line 1307, in fix_cross_refs
    fixup_module_pass_one(self.tree, self.manager.modules)
  File "/Users/guido/src/mypy/mypy/fixup.py", line 21, in fixup_module_pass_one
    node_fixer.visit_symbol_table(tree.names)
  File "/Users/guido/src/mypy/mypy/fixup.py", line 86, in visit_symbol_table
    self.visit_type_info(value.node)
  File "/Users/guido/src/mypy/mypy/fixup.py", line 55, in visit_type_info
    self.visit_symbol_table(info.names)
  File "/Users/guido/src/mypy/mypy/fixup.py", line 88, in visit_symbol_table
    value.node.accept(self)
  File "/Users/guido/src/mypy/mypy/nodes.py", line 658, in accept
    return visitor.visit_var(self)
  File "/Users/guido/src/mypy/mypy/fixup.py", line 131, in visit_var
    v.type.accept(self.type_fixer)
  File "/Users/guido/src/mypy/mypy/types.py", line 433, in accept
    return visitor.visit_instance(self)
  File "/Users/guido/src/mypy/mypy/fixup.py", line 144, in visit_instance
    node = lookup_qualified(self.modules, type_ref)
  File "/Users/guido/src/mypy/mypy/fixup.py", line 234, in lookup_qualified
    stnode = lookup_qualified_stnode(modules, name)
  File "/Users/guido/src/mypy/mypy/fixup.py", line 245, in lookup_qualified_stnode
    assert '.' in head, "Cannot find %s" % (name,)
AssertionError: Cannot find A