# Buggy code
def visit_instance(self, inst: Instance) -> None:
    # TODO: Combine Instances that are exactly the same?
    type_ref = inst.type_ref
    if type_ref is None:
        return  # We've already been here.
    del inst.type_ref
    node = lookup_qualified(self.modules, type_ref, self.quick_and_dirty)
    if isinstance(node, TypeInfo):
        inst.type = node
        # TODO: Is this needed or redundant?
        # Also fix up the bases, just in case.
        for base in inst.type.bases:
            if base.type is NOT_READY:
                base.accept(self)
    for a in inst.args:
        a.accept(self)


# Traceback (AttributeError)
$ mypy --quick --show-traceback a.py
/Users/jukka/src/mypy/b.py:3: error: INTERNAL ERROR -- please report a bug at https://github.com/python/mypy/issues version: 0.510-dev-c856a53fe06c0ad9193d86ab35fb86693940f273-dirty
Traceback (most recent call last):
File "/Users/jukka/src/mypy/scripts/mypy", line 6, in <module>
main(__file__)
File "/Users/jukka/src/mypy/mypy/main.py", line 46, in main
res = type_check_only(sources, bin_dir, options)
File "/Users/jukka/src/mypy/mypy/main.py", line 93, in type_check_only
options=options)
File "/Users/jukka/src/mypy/mypy/build.py", line 188, in build
graph = dispatch(sources, manager)
File "/Users/jukka/src/mypy/mypy/build.py", line 1570, in dispatch
process_graph(graph, manager)
File "/Users/jukka/src/mypy/mypy/build.py", line 1813, in process_graph
process_stale_scc(graph, scc, manager)
File "/Users/jukka/src/mypy/mypy/build.py", line 1912, in process_stale_scc
graph[id].type_check_first_pass()
File "/Users/jukka/src/mypy/mypy/build.py", line 1485, in type_check_first_pass
self.type_checker.check_first_pass()
File "/Users/jukka/src/mypy/mypy/checker.py", line 177, in check_first_pass
self.accept(d)
File "/Users/jukka/src/mypy/mypy/checker.py", line 262, in accept
stmt.accept(self)
File "/Users/jukka/src/mypy/mypy/nodes.py", line 823, in accept
return visitor.visit_expression_stmt(self)
File "/Users/jukka/src/mypy/mypy/checker.py", line 1817, in visit_expression_stmt
self.expr_checker.accept(s.expr, allow_none_return=True)
File "/Users/jukka/src/mypy/mypy/checkexpr.py", line 2056, in accept
typ = node.accept(self)
File "/Users/jukka/src/mypy/mypy/nodes.py", line 1254, in accept
return visitor.visit_member_expr(self)
File "/Users/jukka/src/mypy/mypy/checkexpr.py", line 1010, in visit_member_expr
result = self.analyze_ordinary_member_access(e, False)
File "/Users/jukka/src/mypy/mypy/checkexpr.py", line 1025, in analyze_ordinary_member_access
original_type=original_type, chk=self.chk)
File "/Users/jukka/src/mypy/mypy/checkmember.py", line 74, in analyze_member_access
method = info.get_method(name)
AttributeError: 'NoneType' object has no attribute 'get_method'
/Users/jukka/src/mypy/b.py:3: note: use --pdb to drop into pdb

# Fixed code
def visit_instance(self, inst: Instance) -> None:
    # TODO: Combine Instances that are exactly the same?
    type_ref = inst.type_ref
    if type_ref is None:
        return  # We've already been here.
    del inst.type_ref
    node = lookup_qualified(self.modules, type_ref, self.quick_and_dirty)
    if isinstance(node, TypeInfo):
        inst.type = node
        # TODO: Is this needed or redundant?
        # Also fix up the bases, just in case.
        for base in inst.type.bases:
            if base.type is NOT_READY:
                base.accept(self)
    else:
        # Looks like a missing TypeInfo in quick mode, put something there
        assert self.quick_and_dirty, "Should never get here in normal mode"
        inst.type = stale_info()
    for a in inst.args:
        a.accept(self)