Crash with invalid NewType and union

[JukkaL]

Type checking the following program causes a crash (simplified from a real-world example):

from typing import List, NewType, Union

N = NewType('N', XXX)

x: List[Union[N, int]]

Here's the traceback:

$ mypy --show-traceback t/rt.py
t/rt.py:3: error: Argument 2 to NewType(...) must be subclassable (got "Any")
t/rt.py:3: error: Name 'XXX' is not defined
t/rt.py:1: error: INTERNAL ERROR -- please report a bug at https://github.com/python/mypy/issues version: 0.560-dev-fcd91d00253449ea2571f1ffdcb5bb587ab7d8a5
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 66, in main
    res = type_check_only(sources, bin_dir, options)
  File "/Users/jukka/src/mypy/mypy/main.py", line 119, in type_check_only
    options=options)
  File "/Users/jukka/src/mypy/mypy/build.py", line 218, in build
    graph = dispatch(sources, manager)
  File "/Users/jukka/src/mypy/mypy/build.py", line 2000, in dispatch
    process_graph(graph, manager)
  File "/Users/jukka/src/mypy/mypy/build.py", line 2304, in process_graph
    process_stale_scc(graph, scc, manager)
  File "/Users/jukka/src/mypy/mypy/build.py", line 2474, in process_stale_scc
    graph[id].semantic_analysis_pass_three()
  File "/Users/jukka/src/mypy/mypy/build.py", line 1866, in semantic_analysis_pass_three
    self.options, patches)
  File "/Users/jukka/src/mypy/mypy/semanal_pass3.py", line 61, in visit_file
    self.accept(file_node)
  File "/Users/jukka/src/mypy/mypy/semanal_pass3.py", line 79, in accept
    node.accept(self)
  File "/Users/jukka/src/mypy/mypy/nodes.py", line 228, in accept
    return visitor.visit_mypy_file(self)
  File "/Users/jukka/src/mypy/mypy/traverser.py", line 30, in visit_mypy_file
    d.accept(self)
  File "/Users/jukka/src/mypy/mypy/nodes.py", line 805, in accept
    return visitor.visit_assignment_stmt(self)
  File "/Users/jukka/src/mypy/mypy/semanal_pass3.py", line 200, in visit_assignment_stmt
    self.analyze(s.type, s)
  File "/Users/jukka/src/mypy/mypy/semanal_pass3.py", line 348, in analyze
    type.accept(analyzer)
  File "/Users/jukka/src/mypy/mypy/types.py", line 475, in accept
    return visitor.visit_instance(self)
  File "/Users/jukka/src/mypy/mypy/typeanal.py", line 727, in visit_instance
    if not is_subtype(arg, bound):
  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 1230, in accept
    return visitor.visit_union_type(self)
  File "/Users/jukka/src/mypy/mypy/subtypes.py", line 312, in visit_union_type
    for item in left.items)
  File "/Users/jukka/src/mypy/mypy/subtypes.py", line 312, in <genexpr>
    for item in left.items)
  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 1415, in accept
    return visitor.visit_forwardref_type(self)
  File "/Users/jukka/src/mypy/mypy/types.py", line 1500, in visit_forwardref_type
    raise RuntimeError('Internal error: unresolved forward reference')
RuntimeError: Internal error: unresolved forward reference
t/rt.py:1: : note: use --pdb to drop into pdb

[ilevkivskyi]

This is a duplicate of #4200 (see also #3977 for underlying cause, also #4200 (comment) and #4200 (comment) for solution strategy.)