Internal error: base lookup outside class

[PoggledOats]

I am getting this error with mypy 0.650 from git master branch with python3.5.2 on Ubuntu16.04 and with python3.6.5 on Ubuntu18.04

$ cat tree3.py 

from typing import List, Optional, TypeVar

T = TypeVar('T', bound='TreeBase')

class TreeBase(object):
    def __init__(self : T) -> None:
        self.parent = None # type: Optional[T]
        self.children = [] # type: List[T]

    def addChild(self : T, node : T) -> None:
        self.children.append(node)
        node.parent = self

    def extract(self : T) -> None:
        if self.parent:
            self.parent.children.remove(self)
            self.parent = None

class IdTree(TreeBase):
    def __init__(self, name : str) -> None:
        super().__init__()
        self.id = name      

    def childById(self : 'IdTree', name : str) -> Optional['IdTree']:
        for child in self.children:
            if child.id == name:
                return child
        return None

class IdTree2(TreeBase):
    def __init__(self, name : str) -> None:
        self.parent = None # type: Optional['IdTree2']
        self.children = [] # type: List['IdTree2']
        self.id = name      

    def childById(self, name : str) -> Optional['IdTree2']:
        for child in self.children:
            if child.id == name:
                return child
        return None

$ python3 -m mypy tree3.py 
tree3.py:27: error: "T" has no attribute "id"
tree3.py:28: error: Incompatible return value type (got "T", expected "Optional[IdTree]")
tree3.py:33: error: INTERNAL ERROR -- please report a bug at https://github.com/python/mypy/issues version: 0.650+dev.49726682ad93af49ae2efdbe44b55e966d99165a
tree3.py:33: : note: please use --show-traceback to print a traceback when reporting a bug

$ python3 -m mypy --show-traceback tree3.py 
tree3.py:27: error: "T" has no attribute "id"
tree3.py:28: error: Incompatible return value type (got "T", expected "Optional[IdTree]")
tree3.py:33: error: INTERNAL ERROR -- please report a bug at https://github.com/python/mypy/issues version: 0.650+dev.49726682ad93af49ae2efdbe44b55e966d99165a
Traceback (most recent call last):
  File "/usr/lib/python3.5/runpy.py", line 184, in _run_module_as_main
    "__main__", mod_spec)
  File "/usr/lib/python3.5/runpy.py", line 85, in _run_code
    exec(code, run_globals)
  File "/usr/local/lib/python3.5/dist-packages/mypy/__main__.py", line 11, in 
    main(None)
  File "/usr/local/lib/python3.5/dist-packages/mypy/main.py", line 92, in main
    res = build.build(sources, options, None, flush_errors, fscache)
  File "/usr/local/lib/python3.5/dist-packages/mypy/build.py", line 157, in build
    result = _build(sources, options, alt_lib_path, flush_errors, fscache)
  File "/usr/local/lib/python3.5/dist-packages/mypy/build.py", line 207, in _build
    graph = dispatch(sources, manager)
  File "/usr/local/lib/python3.5/dist-packages/mypy/build.py", line 2233, in dispatch
    process_graph(graph, manager)
  File "/usr/local/lib/python3.5/dist-packages/mypy/build.py", line 2533, in process_graph
    process_stale_scc(graph, scc, manager)
  File "/usr/local/lib/python3.5/dist-packages/mypy/build.py", line 2656, in process_stale_scc
    graph[id].type_check_first_pass()
  File "/usr/local/lib/python3.5/dist-packages/mypy/build.py", line 1792, in type_check_first_pass
    self.type_checker().check_first_pass()
  File "/usr/local/lib/python3.5/dist-packages/mypy/checker.py", line 282, in check_first_pass
    self.accept(d)
  File "/usr/local/lib/python3.5/dist-packages/mypy/checker.py", line 393, in accept
    stmt.accept(self)
  File "/usr/local/lib/python3.5/dist-packages/mypy/nodes.py", line 848, in accept
    return visitor.visit_class_def(self)
  File "/usr/local/lib/python3.5/dist-packages/mypy/checker.py", line 1536, in visit_class_def
    self.accept(defn.defs)
  File "/usr/local/lib/python3.5/dist-packages/mypy/checker.py", line 393, in accept
    stmt.accept(self)
  File "/usr/local/lib/python3.5/dist-packages/mypy/nodes.py", line 913, in accept
    return visitor.visit_block(self)
  File "/usr/local/lib/python3.5/dist-packages/mypy/checker.py", line 1688, in visit_block
    self.accept(s)
  File "/usr/local/lib/python3.5/dist-packages/mypy/checker.py", line 393, in accept
    stmt.accept(self)
  File "/usr/local/lib/python3.5/dist-packages/mypy/nodes.py", line 611, in accept
    return visitor.visit_func_def(self)
  File "/usr/local/lib/python3.5/dist-packages/mypy/checker.py", line 704, in visit_func_def
    self._visit_func_def(defn)
  File "/usr/local/lib/python3.5/dist-packages/mypy/checker.py", line 708, in _visit_func_def
    self.check_func_item(defn, name=defn.name())
  File "/usr/local/lib/python3.5/dist-packages/mypy/checker.py", line 770, in check_func_item
    self.check_func_def(defn, typ, name)
  File "/usr/local/lib/python3.5/dist-packages/mypy/checker.py", line 934, in check_func_def
    self.accept(item.body)
  File "/usr/local/lib/python3.5/dist-packages/mypy/checker.py", line 393, in accept
    stmt.accept(self)
  File "/usr/local/lib/python3.5/dist-packages/mypy/nodes.py", line 913, in accept
    return visitor.visit_block(self)
  File "/usr/local/lib/python3.5/dist-packages/mypy/checker.py", line 1688, in visit_block
    self.accept(s)
  File "/usr/local/lib/python3.5/dist-packages/mypy/checker.py", line 393, in accept
    stmt.accept(self)
  File "/usr/local/lib/python3.5/dist-packages/mypy/nodes.py", line 968, in accept
    return visitor.visit_assignment_stmt(self)
  File "/usr/local/lib/python3.5/dist-packages/mypy/checker.py", line 1696, in visit_assignment_stmt
    self.check_assignment(s.lvalues[-1], s.rvalue, s.type is None, s.new_syntax)
  File "/usr/local/lib/python3.5/dist-packages/mypy/checker.py", line 1751, in check_assignment
    if self.check_compatibility_all_supers(lvalue, lvalue_type, rvalue):
  File "/usr/local/lib/python3.5/dist-packages/mypy/checker.py", line 1849, in check_compatibility_all_supers
    base_type, base_node = self.lvalue_type_from_base(lvalue_node, base)
  File "/usr/local/lib/python3.5/dist-packages/mypy/checker.py", line 1936, in lvalue_type_from_base
    assert self_type is not None, "Internal error: base lookup outside class"
AssertionError: Internal error: base lookup outside class
tree3.py:33: : note: use --pdb to drop into pdb

[JelleZijlstra]

Confirmed on master. Here's a more concise repro:

from typing import Optional, TypeVar

T = TypeVar('T')

class TreeBase(object):
    def __init__(self : T) -> None:
        self.parent = None # type: Optional[T]

class IdTree2(TreeBase):
    def __init__(self) -> None:
        self.parent = None # type: Optional['IdTree2']

If you're looking for a workaround, the error goes away when I remove the : T annotation from __init__'s self parameter. TreeBase should inherit from Generic[T] instead.

[ilevkivskyi]

I think this is a duplicate of #5846

[Levitanus]

sorry, wrote to #5846