Overriding definition such as type alias in a subclass causes a crash

[Michael0x2a]

The following program crashes on both mypy 0.620 and the latest version on master:

from typing import Any, Callable

class Parent:
    foo = Callable[..., int] 

class Child(Parent):
    def foo(self, val: int) -> int:
        return val * 2

(In this case, I meant to do foo: Callable[..., int].)

Here's the traceback:

test.py:13: error: INTERNAL ERROR -- please report a bug at https://github.com/python/mypy/
issues version: 0.630+dev-1222c9637b673bcca6218a943cd4b88b8749b0ad
Traceback (most recent call last):
  File "C:\Python3.6\lib\runpy.py", line 193, in _run_module_as_main
    "__main__", mod_spec)
  File "C:\Python3.6\lib\runpy.py", line 85, in _run_code
    exec(code, run_globals)
  File "C:\Users\Michael\Dropbox\mypy\mypy\__main__.py", line 11, in <module>
    main(None)
  File "C:\Users\Michael\Dropbox\mypy\mypy\main.py", line 91, in main
    res = type_check_only(sources, bin_dir, options, flush_errors, fscache)  # noqa
  File "C:\Users\Michael\Dropbox\mypy\mypy\main.py", line 148, in type_check_only
    fscache=fscache)
  File "C:\Users\Michael\Dropbox\mypy\mypy\build.py", line 177, in build
    flush_errors, fscache)
  File "C:\Users\Michael\Dropbox\mypy\mypy\build.py", line 350, in _build
    graph = dispatch(sources, manager)
  File "C:\Users\Michael\Dropbox\mypy\mypy\build.py", line 2572, in dispatch
    process_graph(graph, manager)
  File "C:\Users\Michael\Dropbox\mypy\mypy\build.py", line 2866, in process_graph
    process_stale_scc(graph, scc, manager)
  File "C:\Users\Michael\Dropbox\mypy\mypy\build.py", line 2989, in process_stale_scc
    graph[id].type_check_first_pass()
  File "C:\Users\Michael\Dropbox\mypy\mypy\build.py", line 2162, in type_check_first_pass
    self.type_checker().check_first_pass()
  File "C:\Users\Michael\Dropbox\mypy\mypy\checker.py", line 253, in check_first_pass
    self.accept(d)
  File "C:\Users\Michael\Dropbox\mypy\mypy\checker.py", line 352, in accept
    stmt.accept(self)
  File "C:\Users\Michael\Dropbox\mypy\mypy\nodes.py", line 808, in accept
    return visitor.visit_class_def(self)
  File "C:\Users\Michael\Dropbox\mypy\mypy\checker.py", line 1389, in visit_class_def
    self.accept(defn.defs)
  File "C:\Users\Michael\Dropbox\mypy\mypy\checker.py", line 352, in accept
    stmt.accept(self)
  File "C:\Users\Michael\Dropbox\mypy\mypy\nodes.py", line 873, in accept
    return visitor.visit_block(self)
  File "C:\Users\Michael\Dropbox\mypy\mypy\checker.py", line 1535, in visit_block
    self.accept(s)
  File "C:\Users\Michael\Dropbox\mypy\mypy\checker.py", line 352, in accept
    stmt.accept(self)
  File "C:\Users\Michael\Dropbox\mypy\mypy\nodes.py", line 591, in accept
    return visitor.visit_func_def(self)
  File "C:\Users\Michael\Dropbox\mypy\mypy\checker.py", line 663, in visit_func_def
    self._visit_func_def(defn)
  File "C:\Users\Michael\Dropbox\mypy\mypy\checker.py", line 674, in _visit_func_def
    self.check_method_override(defn)
  File "C:\Users\Michael\Dropbox\mypy\mypy\checker.py", line 1200, in check_method_override
    self.check_method_or_accessor_override_for_base(defn, base)
  File "C:\Users\Michael\Dropbox\mypy\mypy\checker.py", line 1210, in check_method_or_accessor_override_for_base
    self.check_method_override_for_base_with_name(defn, name, base)
  File "C:\Users\Michael\Dropbox\mypy\mypy\checker.py", line 1254, in check_method_override_for_base_with_name
    assert False, str(base_attr.node)
AssertionError: <mypy.nodes.TypeAlias object at 0x000002139F12C588>
test.py:13: : note: use --pdb to drop into pdb

I did a cursory skim of the existing crash issues, but maybe this is already a known issue? If so, feel free to close.

[ilevkivskyi]

I think this is a new crash caused by the recent refactoring, previously type aliases were just Vars (hence the assert), but now they have a new dedicated SymbolNode. The fix is straightforward however.

[JukkaL]

As discussed in #5662, similar crashes are possible for other kinds of overrides. We should verify that all combinations of base class / subclass node types are handled correctly, including these:

• Variable
• Method
• Type alias
• Type variable
• Class definition
• Module reference
• Other special assignments (new type, named tuple, typed dict)

[pdeblanc]

I have an example for a crash on a trivial override (i.e. not changing the type):

from typing import Generic, TypeVar

T = TypeVar('T')


class Parent(Generic[T]):
    def __init__(self, value: T) -> None:
        self.value: T = value


class Child(Parent[T]):
    def __init__(self, value: T) -> None:
        self.value: T = value

Mypy:

➜  ~ git:(mypy-bug) ✗ mypy --version
mypy 0.701
➜  ~ git:(mypy-bug) ✗ mypy foo.py --show-traceback
foo.py:13: error: INTERNAL ERROR -- please report a bug at https://github.com/python/mypy/issues version: 0.701
Traceback (most recent call last):
  File "/usr/local/bin/mypy", line 10, in <module>
    sys.exit(console_entry())
  File "mypy/checker.py", line 395, in accept
  File "mypy/nodes.py", line 1019, in accept
  File "mypy/checker.py", line 1711, in visit_assignment_stmt__StatementVisitor_glue
  File "mypy/checker.py", line 1717, in visit_assignment_stmt
  File "mypy/checker.py", line 1777, in check_assignment
  File "mypy/checker.py", line 1880, in check_compatibility_all_supers
  File "mypy/checker.py", line 1967, in lvalue_type_from_base
AssertionError: Internal error: base lookup outside class
foo.py:13: : note: use --pdb to drop into pdb

[ilevkivskyi]

@pdeblanc I think what you see is a symptom of another horrible crash #5846