# Buggy code
def map_instance_to_supertype(instance: Instance, superclass: TypeInfo) -> Instance:
    """Produce a supertype of `instance` that is an Instance
    of `superclass`, mapping type arguments up the chain of bases.

    `superclass` is required to be a superclass of `instance.type`.
    """
    if instance.type == superclass:
        # Fast path: `instance` already belongs to `superclass`.
        return instance

    if not superclass.type_vars:
        # Fast path: `superclass` has no type variables to map to.
        return Instance(superclass, [])

    return map_instance_to_supertypes(instance, superclass)[0]


# Traceback (AssertionError)
mypy --py2 --show-traceback test.py

test.py:6: error: INTERNAL ERROR -- please report a bug at https://github.com/python/mypy/issues version: 0.521-858f7512cf3b4e39c0f4e8de5a13eee0e1e138fb-dirty
Traceback (most recent call last):
File "/usr/local/Cellar/mypy/0.521/libexec/bin/mypy", line 11, in <module>
load_entry_point('mypy===0.521-858f7512cf3b4e39c0f4e8de5a13eee0e1e138fb-dirty', 'console_scripts', 'mypy')()
File "/usr/local/Cellar/mypy/0.521/libexec/lib/python3.6/site-packages/mypy/__main__.py", line 7, in console_entry
main(None)
File "/usr/local/Cellar/mypy/0.521/libexec/lib/python3.6/site-packages/mypy/main.py", line 50, in main
res = type_check_only(sources, bin_dir, options)
File "/usr/local/Cellar/mypy/0.521/libexec/lib/python3.6/site-packages/mypy/main.py", line 97, in type_check_only
options=options)
File "/usr/local/Cellar/mypy/0.521/libexec/lib/python3.6/site-packages/mypy/build.py", line 196, in build
graph = dispatch(sources, manager)
File "/usr/local/Cellar/mypy/0.521/libexec/lib/python3.6/site-packages/mypy/build.py", line 1801, in dispatch
process_graph(graph, manager)
File "/usr/local/Cellar/mypy/0.521/libexec/lib/python3.6/site-packages/mypy/build.py", line 2044, in process_graph
process_stale_scc(graph, scc, manager)
File "/usr/local/Cellar/mypy/0.521/libexec/lib/python3.6/site-packages/mypy/build.py", line 2147, in process_stale_scc
graph[id].type_check_first_pass()
File "/usr/local/Cellar/mypy/0.521/libexec/lib/python3.6/site-packages/mypy/build.py", line 1716, in type_check_first_pass
self.type_checker.check_first_pass()
File "/usr/local/Cellar/mypy/0.521/libexec/lib/python3.6/site-packages/mypy/checker.py", line 185, in check_first_pass
self.accept(d)
File "/usr/local/Cellar/mypy/0.521/libexec/lib/python3.6/site-packages/mypy/checker.py", line 273, in accept
stmt.accept(self)
File "/usr/local/Cellar/mypy/0.521/libexec/lib/python3.6/site-packages/mypy/nodes.py", line 565, in accept
return visitor.visit_func_def(self)
File "/usr/local/Cellar/mypy/0.521/libexec/lib/python3.6/site-packages/mypy/checker.py", line 519, in visit_func_def
self.check_func_item(defn, name=defn.name())
File "/usr/local/Cellar/mypy/0.521/libexec/lib/python3.6/site-packages/mypy/checker.py", line 578, in check_func_item
self.check_func_def(defn, typ, name)
File "/usr/local/Cellar/mypy/0.521/libexec/lib/python3.6/site-packages/mypy/checker.py", line 738, in check_func_def
self.accept(item.body)
File "/usr/local/Cellar/mypy/0.521/libexec/lib/python3.6/site-packages/mypy/checker.py", line 273, in accept
stmt.accept(self)
File "/usr/local/Cellar/mypy/0.521/libexec/lib/python3.6/site-packages/mypy/nodes.py", line 815, in accept
return visitor.visit_block(self)
File "/usr/local/Cellar/mypy/0.521/libexec/lib/python3.6/site-packages/mypy/checker.py", line 1202, in visit_block
self.accept(s)
File "/usr/local/Cellar/mypy/0.521/libexec/lib/python3.6/site-packages/mypy/checker.py", line 273, in accept
stmt.accept(self)
File "/usr/local/Cellar/mypy/0.521/libexec/lib/python3.6/site-packages/mypy/nodes.py", line 859, in accept
return visitor.visit_assignment_stmt(self)
File "/usr/local/Cellar/mypy/0.521/libexec/lib/python3.6/site-packages/mypy/checker.py", line 1209, in visit_assignment_stmt
self.check_assignment(s.lvalues[-1], s.rvalue, s.type is None, s.new_syntax)
File "/usr/local/Cellar/mypy/0.521/libexec/lib/python3.6/site-packages/mypy/checker.py", line 1236, in check_assignment
infer_lvalue_type)
File "/usr/local/Cellar/mypy/0.521/libexec/lib/python3.6/site-packages/mypy/checker.py", line 1483, in check_assignment_to_multiple_lvalues
self.check_multi_assignment(lvalues, rvalue, context, infer_lvalue_type)
File "/usr/local/Cellar/mypy/0.521/libexec/lib/python3.6/site-packages/mypy/checker.py", line 1522, in check_multi_assignment
context, undefined_rvalue, infer_lvalue_type)
File "/usr/local/Cellar/mypy/0.521/libexec/lib/python3.6/site-packages/mypy/checker.py", line 1550, in check_multi_assignment_from_tuple
assert isinstance(reinferred_rvalue_type, TupleType)
AssertionError:
test.py:6: note: use --pdb to drop into pdb

# Fixed code
def map_instance_to_supertype(instance: Instance, superclass: TypeInfo) -> Instance:
    """Produce a supertype of `instance` that is an Instance
    of `superclass`, mapping type arguments up the chain of bases.

    If `superclass` is not a nominal superclass of `instance.type`,
    then all type arguments are mapped to 'Any'.
    """
    if instance.type == superclass:
        # Fast path: `instance` already belongs to `superclass`.
        return instance

    if not superclass.type_vars:
        # Fast path: `superclass` has no type variables to map to.
        return Instance(superclass, [])

    return map_instance_to_supertypes(instance, superclass)[0]