Internal error analyzing generic protocol with nested class

[damianw]

Mypy crashes when analyzing a generic protocol with a nested class, up to and including the latest version from Git master (770276d70c3892f4154be85b96ec028705941184).

test.py

from typing import TypeVar, Protocol

T = TypeVar("T", covariant=True)


class MyProtocol(Protocol[T]):
    class Nested:
        pass

Command

python3.8 -m mypy --show-traceback test.py
(default configuration)

Output

test.py:6: error: INTERNAL ERROR -- Please try using mypy master on Github:
https://mypy.rtfd.io/en/latest/common_issues.html#using-a-development-mypy-build
Please report a bug at https://github.com/python/mypy/issues
version: 0.770+dev.770276d70c3892f4154be85b96ec028705941184
Traceback (most recent call last):
  File "/Library/Frameworks/Python.framework/Versions/3.8/lib/python3.8/runpy.py", line 193, in _run_module_as_main
    return _run_code(code, main_globals, None,
  File "/Library/Frameworks/Python.framework/Versions/3.8/lib/python3.8/runpy.py", line 86, in _run_code
    exec(code, run_globals)
  File "/Users/.../.virtualenvs/.../lib/python3.8/site-packages/mypy/__main__.py", line 12, in <module>
    main(None, sys.stdout, sys.stderr)
  File "/Users/.../.virtualenvs/.../lib/python3.8/site-packages/mypy/main.py", line 89, in main
    res = build.build(sources, options, None, flush_errors, fscache, stdout, stderr)
  File "/Users/.../.virtualenvs/.../lib/python3.8/site-packages/mypy/build.py", line 165, in build
    result = _build(
  File "/Users/.../.virtualenvs/.../lib/python3.8/site-packages/mypy/build.py", line 234, in _build
    graph = dispatch(sources, manager, stdout)
  File "/Users/.../.virtualenvs/.../lib/python3.8/site-packages/mypy/build.py", line 2631, in dispatch
    process_graph(graph, manager)
  File "/Users/.../.virtualenvs/.../lib/python3.8/site-packages/mypy/build.py", line 2938, in process_graph
    process_stale_scc(graph, scc, manager)
  File "/Users/.../.virtualenvs/.../lib/python3.8/site-packages/mypy/build.py", line 3037, in process_stale_scc
    graph[id].type_check_first_pass()
  File "/Users/.../.virtualenvs/.../lib/python3.8/site-packages/mypy/build.py", line 2130, in type_check_first_pass
    self.type_checker().check_first_pass()
  File "/Users/.../.virtualenvs/.../lib/python3.8/site-packages/mypy/checker.py", line 293, in check_first_pass
    self.accept(d)
  File "/Users/.../.virtualenvs/.../lib/python3.8/site-packages/mypy/checker.py", line 400, in accept
    stmt.accept(self)
  File "/Users/.../.virtualenvs/.../lib/python3.8/site-packages/mypy/nodes.py", line 939, in accept
    return visitor.visit_class_def(self)
  File "/Users/.../.virtualenvs/.../lib/python3.8/site-packages/mypy/checker.py", line 1755, in visit_class_def
    self.check_protocol_variance(defn)
  File "/Users/.../.virtualenvs/.../lib/python3.8/site-packages/mypy/checker.py", line 1823, in check_protocol_variance
    if is_subtype(down, up, ignore_declared_variance=True):
  File "/Users/.../.virtualenvs/.../lib/python3.8/site-packages/mypy/subtypes.py", line 93, in is_subtype
    return _is_subtype(left, right,
  File "/Users/.../.virtualenvs/.../lib/python3.8/site-packages/mypy/subtypes.py", line 135, in _is_subtype
    return left.accept(SubtypeVisitor(orig_right,
  File "/Users/.../.virtualenvs/.../lib/python3.8/site-packages/mypy/types.py", line 794, in accept
    return visitor.visit_instance(self)
  File "/Users/.../.virtualenvs/.../lib/python3.8/site-packages/mypy/subtypes.py", line 259, in visit_instance
    if right.type.is_protocol and is_protocol_implementation(left, right):
  File "/Users/.../.virtualenvs/.../lib/python3.8/site-packages/mypy/subtypes.py", line 530, in is_protocol_implementation
    assert supertype is not None
AssertionError:
test.py:6: : note: use --pdb to drop into pdb

[ilevkivskyi]

This is essentially a duplicate of #6393