crash on @overload together with @singledispatch

[shaunc]

Crash Report

(Tell us what happened.)

I'm trying to tell typing the relation between argument and return types on a singledispatch function:

from functools import singledispatch
from typing import overload


class F:
    ...


class G:
    ...


class F1(F):
    ...


class G1(G):
    ...


class F2(F):
    ...


class G2(G):
    ...


@overload
def foo(f: F1) -> G1:
    ...


@overload
def foo(f: F2) -> G2:
    ...


@singledispatch
def foo(f: F) -> G:
    ...


@foo.register
def foo_1(f: F1) -> G1:
    ...


@foo.register
def foo_2(f: F2) -> G2:
    ...

Traceback

% mypy src/emm/test_sd.py --follow-imports=silent --strict --show-traceback
src/emm/test_sd.py:29: error: INTERNAL ERROR -- Please try using mypy master on Github:
https://mypy.readthedocs.io/en/stable/common_issues.html#using-a-development-mypy-build
Please report a bug at https://github.com/python/mypy/issues
version: 0.820+dev.f00bb49066810589b5bb122425381ef833fe471b
Traceback (most recent call last):
  File "/home/shauncutts/.cache/pypoetry/virtualenvs/emm-5s1w60K6-py3.8/bin/mypy", line 8, in <module>
    sys.exit(console_entry())
  File "/home/shauncutts/.cache/pypoetry/virtualenvs/emm-5s1w60K6-py3.8/lib/python3.8/site-packages/mypy/__main__.py", line 11, in console_entry
    main(None, sys.stdout, sys.stderr)
  File "/home/shauncutts/.cache/pypoetry/virtualenvs/emm-5s1w60K6-py3.8/lib/python3.8/site-packages/mypy/main.py", line 98, in main
    res = build.build(sources, options, None, flush_errors, fscache, stdout, stderr)
  File "/home/shauncutts/.cache/pypoetry/virtualenvs/emm-5s1w60K6-py3.8/lib/python3.8/site-packages/mypy/build.py", line 179, in build
    result = _build(
  File "/home/shauncutts/.cache/pypoetry/virtualenvs/emm-5s1w60K6-py3.8/lib/python3.8/site-packages/mypy/build.py", line 253, in _build
    graph = dispatch(sources, manager, stdout)
  File "/home/shauncutts/.cache/pypoetry/virtualenvs/emm-5s1w60K6-py3.8/lib/python3.8/site-packages/mypy/build.py", line 2688, in dispatch
    process_graph(graph, manager)
  File "/home/shauncutts/.cache/pypoetry/virtualenvs/emm-5s1w60K6-py3.8/lib/python3.8/site-packages/mypy/build.py", line 3012, in process_graph
    process_stale_scc(graph, scc, manager)
  File "/home/shauncutts/.cache/pypoetry/virtualenvs/emm-5s1w60K6-py3.8/lib/python3.8/site-packages/mypy/build.py", line 3110, in process_stale_scc
    graph[id].type_check_first_pass()
  File "/home/shauncutts/.cache/pypoetry/virtualenvs/emm-5s1w60K6-py3.8/lib/python3.8/site-packages/mypy/build.py", line 2164, in type_check_first_pass
    self.type_checker().check_first_pass()
  File "/home/shauncutts/.cache/pypoetry/virtualenvs/emm-5s1w60K6-py3.8/lib/python3.8/site-packages/mypy/checker.py", line 294, in check_first_pass
    self.accept(d)
  File "/home/shauncutts/.cache/pypoetry/virtualenvs/emm-5s1w60K6-py3.8/lib/python3.8/site-packages/mypy/checker.py", line 401, in accept
    stmt.accept(self)
  File "/home/shauncutts/.cache/pypoetry/virtualenvs/emm-5s1w60K6-py3.8/lib/python3.8/site-packages/mypy/nodes.py", line 526, in accept
    return visitor.visit_overloaded_func_def(self)
  File "/home/shauncutts/.cache/pypoetry/virtualenvs/emm-5s1w60K6-py3.8/lib/python3.8/site-packages/mypy/checker.py", line 434, in visit_overloaded_func_def
    self._visit_overloaded_func_def(defn)
  File "/home/shauncutts/.cache/pypoetry/virtualenvs/emm-5s1w60K6-py3.8/lib/python3.8/site-packages/mypy/checker.py", line 461, in _visit_overloaded_func_def
    self.check_overlapping_overloads(defn)
  File "/home/shauncutts/.cache/pypoetry/virtualenvs/emm-5s1w60K6-py3.8/lib/python3.8/site-packages/mypy/checker.py", line 482, in check_overlapping_overloads
    assert isinstance(inner_type, CallableType)
AssertionError: 
src/emm/test_sd.py:29: : note: use --pdb to drop into pdb

To Reproduce

The module above suffices

Your Environment

• Mypy version used: 0.820+dev.f00bb49066810589b5bb122425381ef833fe471b (initially encountered on 0.812)
• Mypy command-line flags: --follow-imports=silent --strict --show-traceback
• Mypy configuration options from mypy.ini (and other config files):

[mypy]
allow_untyped_calls = False

• Python version used: 3.8
• Operating system and version: ubuntu 20.04

[hauntsaninja]

Dupe of #8356