# Buggy code
def join_instances(t: Instance, s: Instance) -> ProperType:
    """Calculate the join of two instance types."""
    if t.type == s.type:
        # Simplest case: join two types with the same base type (but
        # potentially different arguments).
        if is_subtype(t, s) or is_subtype(s, t):
            # Compatible; combine type arguments.
            args = []  # type: List[Type]
            for i in range(len(t.args)):
                args.append(join_types(t.args[i], s.args[i]))
            return Instance(t.type, args)
        else:
            # Incompatible; return trivial result object.
            return object_from_instance(t)
    elif t.type.bases and is_subtype_ignoring_tvars(t, s):
        return join_instances_via_supertype(t, s)
    else:
        # Now t is not a subtype of s, and t != s. Now s could be a subtype
        # of t; alternatively, we need to find a common supertype. This works
        # in of the both cases.
        return join_instances_via_supertype(s, t)


# Traceback (IndexError)
$ mypy --quick --show-traceback a.py
b.py:7: error: Revealed type is 'def () -> b.C'
/Users/jukka/src/mypy/b.py:11: 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 853, in accept
return visitor.visit_assignment_stmt(self)
File "/Users/jukka/src/mypy/mypy/checker.py", line 1177, in visit_assignment_stmt
self.check_assignment(s.lvalues[-1], s.rvalue, s.type is None, s.new_syntax)
File "/Users/jukka/src/mypy/mypy/checker.py", line 1239, in check_assignment
rvalue_type = self.check_simple_assignment(lvalue_type, rvalue, lvalue)
File "/Users/jukka/src/mypy/mypy/checker.py", line 1698, in check_simple_assignment
rvalue_type = self.expr_checker.accept(rvalue, lvalue_type)
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 1213, in accept
return visitor.visit_name_expr(self)
File "/Users/jukka/src/mypy/mypy/checkexpr.py", line 123, in visit_name_expr
return self.narrow_type_from_binder(e, result)
File "/Users/jukka/src/mypy/mypy/checkexpr.py", line 2271, in narrow_type_from_binder
ans = narrow_declared_type(known_type, restriction)
File "/Users/jukka/src/mypy/mypy/meet.py", line 46, in narrow_declared_type
return meet_types(declared, narrowed)
File "/Users/jukka/src/mypy/mypy/meet.py", line 25, in meet_types
return t.accept(TypeMeetVisitor(s))
File "/Users/jukka/src/mypy/mypy/types.py", line 381, in accept
return visitor.visit_instance(self)
File "/Users/jukka/src/mypy/mypy/meet.py", line 198, in visit_instance
args.append(self.meet(t.args[i], si.args[i]))
IndexError: list index out of range
/Users/jukka/src/mypy/b.py:11: note: use --pdb to drop into pdb

# Fixed code
def join_instances(t: Instance, s: Instance) -> ProperType:
    """Calculate the join of two instance types."""
    if t.type == s.type:
        # Simplest case: join two types with the same base type (but
        # potentially different arguments).
        if is_subtype(t, s) or is_subtype(s, t):
            # Compatible; combine type arguments.
            args = []  # type: List[Type]
            # N.B: We use zip instead of indexing because the lengths might have
            # mismatches during daemon reprocessing.
            for ta, sa in zip(t.args, s.args):
                args.append(join_types(ta, sa))
            return Instance(t.type, args)
        else:
            # Incompatible; return trivial result object.
            return object_from_instance(t)
    elif t.type.bases and is_subtype_ignoring_tvars(t, s):
        return join_instances_via_supertype(t, s)
    else:
        # Now t is not a subtype of s, and t != s. Now s could be a subtype
        # of t; alternatively, we need to find a common supertype. This works
        # in of the both cases.
        return join_instances_via_supertype(s, t)