Internal Error with Union Type Involving Invalid Type

[ben-e-whitney]

A crash results when mypy --strict is run on the following.

import typing

import numpy as np

#This is the type of `np.ma.masked`.
MC = np.ma.core.MaskedConstant

class A:
    def __init__(self) -> None:
        self._error: typing.Union[float, MC, None] = None

    def error(self) -> float:
        if self._error is None:
            pass
        return 0.0

There is no crash when either of the following changes are made:

1. I use the full type name instead of the alias in the annotation for self._error.
2. I remove float and/or None from the union.

mypy complains that MC is an invalid type. I assume that is related. The traceback:

minimal.py:3: error: No library stub file for module 'numpy'
minimal.py:3: note: (Stub files are from https://github.com/python/typeshed)
minimal.py:10: error: Invalid type "minimal.MC"
minimal.py:13: error: INTERNAL ERROR -- please report a bug at https://github.com/python/mypy/issues version: 0.560
Traceback (most recent call last):
  File "/usr/bin/mypy", line 11, in <module>
    load_entry_point('mypy==0.560', 'console_scripts', 'mypy')()
  File "/usr/lib/python3/dist-packages/mypy/__main__.py", line 7, in console_entry
    main(None)
  File "/usr/lib/python3/dist-packages/mypy/main.py", line 66, in main
    res = type_check_only(sources, bin_dir, options)
  File "/usr/lib/python3/dist-packages/mypy/main.py", line 119, in type_check_only
    options=options)
  File "/usr/lib/python3/dist-packages/mypy/build.py", line 218, in build
    graph = dispatch(sources, manager)
  File "/usr/lib/python3/dist-packages/mypy/build.py", line 1997, in dispatch
    process_graph(graph, manager)
  File "/usr/lib/python3/dist-packages/mypy/build.py", line 2299, in process_graph
    process_stale_scc(graph, scc, manager)
  File "/usr/lib/python3/dist-packages/mypy/build.py", line 2475, in process_stale_scc
    graph[id].type_check_first_pass()
  File "/usr/lib/python3/dist-packages/mypy/build.py", line 1876, in type_check_first_pass
    self.type_checker().check_first_pass()
  File "/usr/lib/python3/dist-packages/mypy/checker.py", line 196, in check_first_pass
    self.accept(d)
  File "/usr/lib/python3/dist-packages/mypy/checker.py", line 286, in accept
    stmt.accept(self)
  File "/usr/lib/python3/dist-packages/mypy/nodes.py", line 696, in accept
    return visitor.visit_class_def(self)
  File "/usr/lib/python3/dist-packages/mypy/checker.py", line 1210, in visit_class_def
    self.accept(defn.defs)
  File "/usr/lib/python3/dist-packages/mypy/checker.py", line 286, in accept
    stmt.accept(self)
  File "/usr/lib/python3/dist-packages/mypy/nodes.py", line 756, in accept
    return visitor.visit_block(self)
  File "/usr/lib/python3/dist-packages/mypy/checker.py", line 1330, in visit_block
    self.accept(s)
  File "/usr/lib/python3/dist-packages/mypy/checker.py", line 286, in accept
    stmt.accept(self)
  File "/usr/lib/python3/dist-packages/mypy/nodes.py", line 497, in accept
    return visitor.visit_func_def(self)
  File "/usr/lib/python3/dist-packages/mypy/checker.py", line 534, in visit_func_def
    self.check_func_item(defn, name=defn.name())
  File "/usr/lib/python3/dist-packages/mypy/checker.py", line 599, in check_func_item
    self.check_func_def(defn, typ, name)
  File "/usr/lib/python3/dist-packages/mypy/checker.py", line 762, in check_func_def
    self.accept(item.body)
  File "/usr/lib/python3/dist-packages/mypy/checker.py", line 286, in accept
    stmt.accept(self)
  File "/usr/lib/python3/dist-packages/mypy/nodes.py", line 756, in accept
    return visitor.visit_block(self)
  File "/usr/lib/python3/dist-packages/mypy/checker.py", line 1330, in visit_block
    self.accept(s)
  File "/usr/lib/python3/dist-packages/mypy/checker.py", line 286, in accept
    stmt.accept(self)
  File "/usr/lib/python3/dist-packages/mypy/nodes.py", line 921, in accept
    return visitor.visit_if_stmt(self)
  File "/usr/lib/python3/dist-packages/mypy/checker.py", line 2191, in visit_if_stmt
    if_map, else_map = self.find_isinstance_check(e)
  File "/usr/lib/python3/dist-packages/mypy/checker.py", line 2812, in find_isinstance_check
    return find_isinstance_check(n, self.type_map)
  File "/usr/lib/python3/dist-packages/mypy/checker.py", line 3140, in find_isinstance_check
    if_vars, else_vars = conditional_type_map(expr, vartype, none_typ)
  File "/usr/lib/python3/dist-packages/mypy/checker.py", line 2877, in conditional_type_map
    remaining_type = restrict_subtype_away(current_type, proposed_precise_type)
  File "/usr/lib/python3/dist-packages/mypy/subtypes.py", line 784, in restrict_subtype_away
    new_items = [item for item in t.relevant_items()
  File "/usr/lib/python3/dist-packages/mypy/subtypes.py", line 785, in <listcomp>
    if (not (is_proper_subtype(erase_type(item), erased_s) or
  File "/usr/lib/python3/dist-packages/mypy/erasetype.py", line 25, in erase_type
    return typ.accept(EraseTypeVisitor())
  File "/usr/lib/python3/dist-packages/mypy/types.py", line 194, in accept
    return visitor.visit_unbound_type(self)
  File "/usr/lib/python3/dist-packages/mypy/erasetype.py", line 31, in visit_unbound_type
    assert False, 'Not supported'
AssertionError: Not supported
minimal.py:13: : note: use --pdb to drop into pdb

Version numbers:

$ mypy --version
pymypy 0.560
$ python3 --version
Python 3.6.4+

[gvanrossum]

There are no numpy stubs, so that's presumably why MC is deemed invalid. The crash itself is probably due to #4543, but I'm not sure whether to call it a duplicate (since that issue has a forward reference, not an invalid type).

[ilevkivskyi]

Yes, this is a duplicate of #4543, so I am closing this.