Crash related to partial type in subclass

[JukkaL]

Mypy crashes when type checking this program:

class A:
    x: str

class B(A):
    x = None
    x = ''

Here's the traceback:

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 80, in main
    type_check_only(sources, bin_dir, options, flush_errors)
  File "/Users/jukka/src/mypy/mypy/main.py", line 129, in type_check_only
    flush_errors=flush_errors)
  File "/Users/jukka/src/mypy/mypy/build.py", line 173, in build
    result = _build(sources, options, alt_lib_path, bin_dir, saved_cache, flush_errors)
  File "/Users/jukka/src/mypy/mypy/build.py", line 259, in _build
    graph = dispatch(sources, manager)
  File "/Users/jukka/src/mypy/mypy/build.py", line 2094, in dispatch
    process_graph(graph, manager)
  File "/Users/jukka/src/mypy/mypy/build.py", line 2395, in process_graph
    process_stale_scc(graph, scc, manager)
  File "/Users/jukka/src/mypy/mypy/build.py", line 2579, in process_stale_scc
    graph[id].type_check_first_pass()
  File "/Users/jukka/src/mypy/mypy/build.py", line 1951, in type_check_first_pass
    self.type_checker().check_first_pass()
  File "/Users/jukka/src/mypy/mypy/checker.py", line 229, in check_first_pass
    self.accept(d)
  File "/Users/jukka/src/mypy/mypy/checker.py", line 324, in accept
    stmt.accept(self)
  File "/Users/jukka/src/mypy/mypy/nodes.py", line 704, in accept
    return visitor.visit_class_def(self)
  File "/Users/jukka/src/mypy/mypy/checker.py", line 1252, in visit_class_def
    self.accept(defn.defs)
  File "/Users/jukka/src/mypy/mypy/checker.py", line 324, in accept
    stmt.accept(self)
  File "/Users/jukka/src/mypy/mypy/nodes.py", line 766, in accept
    return visitor.visit_block(self)
  File "/Users/jukka/src/mypy/mypy/checker.py", line 1372, in visit_block
    self.accept(s)
  File "/Users/jukka/src/mypy/mypy/checker.py", line 324, in accept
    stmt.accept(self)
  File "/Users/jukka/src/mypy/mypy/nodes.py", line 810, in accept
    return visitor.visit_assignment_stmt(self)
  File "/Users/jukka/src/mypy/mypy/checker.py", line 1379, 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 1412, in check_assignment
    if self.check_compatibility_all_supers(lvalue, lvalue_type, rvalue):
  File "/Users/jukka/src/mypy/mypy/checker.py", line 1513, in check_compatibility_all_supers
    base_node):
  File "/Users/jukka/src/mypy/mypy/checker.py", line 1571, in check_compatibility_super
    'base class "%s" defined the type as' % base.name())
  File "/Users/jukka/src/mypy/mypy/checker.py", line 2961, in check_subtype
    if is_subtype(subtype, supertype):
  File "/Users/jukka/src/mypy/mypy/subtypes.py", line 83, in is_subtype
    ignore_declared_variance=ignore_declared_variance))
  File "/Users/jukka/src/mypy/mypy/types.py", line 1292, in accept
    return visitor.visit_partial_type(self)
  File "/Users/jukka/src/mypy/mypy/subtypes.py", line 349, in visit_partial_type
    raise RuntimeError
RuntimeError:

We can fix this issue by checking the compatibility of attributes with base classes at the end of type checking a class.

[ilevkivskyi]

Currently this crash happens only with --no-strict-optional.