Type alias problems with typeshed's bytes

[elazarg]

Running mypy on typeshed's stdlib/2/builtins.pyi violates the assertion in semanal.py:931 - sym.node is a Var instead of a TypeInfo for builtins.bytes. This is related to the statement bytes = str and type promotion of str to bytes.

  ...
  File ".../mypy/mypy/build.py", line 1425, in semantic_analysis
    self.manager.semantic_analyzer.visit_file(self.tree, self.xpath, self.options)
  File ".../mypy/mypy/semanal.py", line 246, in visit_file
    self.accept(d)
  File ".../mypy/mypy/semanal.py", line 2719, in accept
    node.accept(self)
  File ".../mypy/mypy/nodes.py", line 709, in accept
    return visitor.visit_class_def(self)
  File ".../mypy/mypy/semanal.py", line 579, in visit_class_def
    self.setup_type_promotion(defn)
  File ".../mypy/mypy/semanal.py", line 663, in setup_type_promotion
    promote_target = self.named_type_or_none(promotions[defn.fullname])
  File ".../mypy/mypy/semanal.py", line 931, in named_type_or_none
    assert isinstance(sym.node, TypeInfo)

[gvanrossum]

For those trying to repro, the command line is mypy typeshed/stdlib/2/builtins.pyi. So that's using Python 3 mode for the Python 2 builtins stub. It may well conflict with the Python 3 builtins stub that it has already loaded. So I think this is low priority.

[gvanrossum]

Also the error claims to be on line 370 in builtins.pyi. That line says

class bytearray(MutableSequence[int]):

[ethanhs]

This seems to not cause a crash on 0.521 and master. On both I get

$ mypy typeshed/stdlib/2/builtins.pyi
typeshed\stdlib\2\builtins.pyi: error: Name '__builtins__.str' is not defined
typeshed\stdlib\2\builtins.pyi:1: error: Name '__builtin__' is not defined

This is the expected output as Python 3 has no __builtin__.