overwriting None from Tuple[AnyStr]'s only item causes PartialType-related crash

[ddfisher]

import os.path

x = None
if 0:
    _, x = os.path.split("blah")

results in:

Traceback (most recent call last):
  File "/usr/local/bin/mypy", line 6, in <module>
    main(__file__)
  File "/usr/local/lib/python3.5/site-packages/mypy/main.py", line 54, in main
    res = type_check_only(sources, bin_dir, options)
  File "/usr/local/lib/python3.5/site-packages/mypy/main.py", line 102, in type_check_only
    python_path=options.python_path)
  File "/usr/local/lib/python3.5/site-packages/mypy/build.py", line 209, in build
    dispatch(sources, manager)
  File "/usr/local/lib/python3.5/site-packages/mypy/build.py", line 1325, in dispatch
    process_graph(graph, manager)
  File "/usr/local/lib/python3.5/site-packages/mypy/build.py", line 1456, in process_graph
    process_stale_scc(graph, scc)
  File "/usr/local/lib/python3.5/site-packages/mypy/build.py", line 1486, in process_stale_scc
    graph[id].type_check()
  File "/usr/local/lib/python3.5/site-packages/mypy/build.py", line 1305, in type_check
    manager.type_checker.visit_file(self.tree, self.xpath)
  File "/usr/local/lib/python3.5/site-packages/mypy/checker.py", line 424, in visit_file
    self.accept(d)
  File "/usr/local/lib/python3.5/site-packages/mypy/checker.py", line 465, in accept
    typ = node.accept(self)
  File "/usr/local/lib/python3.5/site-packages/mypy/nodes.py", line 862, in accept
    return visitor.visit_if_stmt(self)
  File "/usr/local/lib/python3.5/site-packages/mypy/checker.py", line 1661, in visit_if_stmt
    self.accept(b)
  File "/usr/local/lib/python3.5/site-packages/mypy/checker.py", line 465, in accept
    typ = node.accept(self)
  File "/usr/local/lib/python3.5/site-packages/mypy/nodes.py", line 715, in accept
    return visitor.visit_block(self)
  File "/usr/local/lib/python3.5/site-packages/mypy/checker.py", line 1149, in visit_block
    self.accept(s)
  File "/usr/local/lib/python3.5/site-packages/mypy/checker.py", line 465, in accept
    typ = node.accept(self)
  File "/usr/local/lib/python3.5/site-packages/mypy/nodes.py", line 753, in accept
    return visitor.visit_assignment_stmt(self)
  File "/usr/local/lib/python3.5/site-packages/mypy/checker.py", line 1158, in visit_assignment_stmt
    self.check_assignment(s.lvalues[-1], s.rvalue, s.type is None)
  File "/usr/local/lib/python3.5/site-packages/mypy/checker.py", line 1175, in check_assignment
    infer_lvalue_type)
  File "/usr/local/lib/python3.5/site-packages/mypy/checker.py", line 1244, in check_assignment_to_multiple_lvalues
    self.check_multi_assignment(lvalues, rvalue, context, infer_lvalue_type)
  File "/usr/local/lib/python3.5/site-packages/mypy/checker.py", line 1277, in check_multi_assignment
    context, undefined_rvalue, infer_lvalue_type)
  File "/usr/local/lib/python3.5/site-packages/mypy/checker.py", line 1297, in check_multi_assignment_from_tuple
    rvalue_type = cast(TupleType, self.accept(rvalue, lvalue_type))
  File "/usr/local/lib/python3.5/site-packages/mypy/checker.py", line 465, in accept
    typ = node.accept(self)
  File "/usr/local/lib/python3.5/site-packages/mypy/nodes.py", line 1185, in accept
    return visitor.visit_call_expr(self)
  File "/usr/local/lib/python3.5/site-packages/mypy/checker.py", line 1997, in visit_call_expr
    return self.expr_checker.visit_call_expr(e)
  File "/usr/local/lib/python3.5/site-packages/mypy/checkexpr.py", line 141, in visit_call_expr
    return self.check_call_expr_with_callee_type(callee_type, e)
  File "/usr/local/lib/python3.5/site-packages/mypy/checkexpr.py", line 192, in check_call_expr_with_callee_type
    e.arg_names, callable_node=e.callee)[0]
  File "/usr/local/lib/python3.5/site-packages/mypy/checkexpr.py", line 230, in check_call
    callee, context)
  File "/usr/local/lib/python3.5/site-packages/mypy/checkexpr.py", line 370, in infer_function_type_arguments_using_context
    args = infer_type_arguments(callable.type_var_ids(), ret_type, erased_ctx)
  File "/usr/local/lib/python3.5/site-packages/mypy/infer.py", line 43, in infer_type_arguments
    return solve_constraints(type_var_ids, constraints)
  File "/usr/local/lib/python3.5/site-packages/mypy/solve.py", line 50, in solve_constraints
    top = meet_types(top, c.target)
  File "/usr/local/lib/python3.5/site-packages/mypy/meet.py", line 22, in meet_types
    return t.accept(TypeMeetVisitor(s))
  File "/usr/local/lib/python3.5/site-packages/mypy/types.py", line 763, in accept
    return visitor.visit_partial_type(self)
  File "/usr/local/lib/python3.5/site-packages/mypy/meet.py", line 208, in visit_partial_type
    assert False, 'Internal error'
AssertionError: Internal error

*** INTERNAL ERROR ***

test.py:5: error: Internal error -- please report a bug at https://github.com/python/mypy/issues

NOTE: you can use "mypy --pdb ..." to drop into the debugger when this happens.

We might want to fix this before releasing.

[ddfisher]

Here's a simpler repro which produces a similar error:

from typing import AnyStr, Tuple

def split() -> Tuple[int, AnyStr]: ...
x = None
_, x = split()

[gvanrossum]

What is it supposed to mean? That x becomes an optional AnyStr? Maybe it's crashing trying to report the problem?

…

--Guido (mobile)

[euresti]

I can hit this using mkstemp with an overload that returns AnyStr. It only happens if the variable is defined as None beforehand and not annotated.

import tempfile
from typing import Any

def fine_test():
    # type: () -> Any
    # Annotated
    hash_set_path = None  # type: Any
    try:
        fd, hash_set_path = tempfile.mkstemp(dir=u"blah")
    except Exception:
        print("Ack moving on")
    return hash_set_path

def fine_test2():
    # type: () -> Any
    # Not annotated but not overloaded
    hash_set_path = None
    try:
        fd, hash_set_path = tempfile.mkstemp()
    except Exception:
        print("Ack moving on")
    return hash_set_path

def fine_test3():
    # type: () -> Any
    # Variable not defined beforehand
    try:
        fd, hash_set_path = tempfile.mkstemp()
    except Exception:
        print("Ack moving on")
        return None
    return hash_set_path

def crashes_test():
    # type: () -> Any
    hash_set_path = None
    try:
        fd, hash_set_path = tempfile.mkstemp(dir=u"blah")
    except Exception:
        print("Ack moving on")
    return hash_set_path

[gvanrossum]

Simpler repro (similar to #1587 (comment) but without the confusing "output-only type var"):

from typing import AnyStr, Tuple
def f(a: AnyStr) -> Tuple[AnyStr]: pass
x = None
(x,) = f('')