Crash during type inference with union of type variable and callable

[JukkaL]

Mypy crashes when inferring the call to f below:

from typing import Union, Callable, Match, TypeVar
T = TypeVar('T')
def f(x: T, y: Union[T, Callable[[T], None]]) -> None: pass
f('', '')

The above is about the simplest signature that seems to trigger the bug. In practice, a signature for re.sub also causes the crash (see #665).

Here's the traceback:

Traceback (most recent call last):
  File "scripts/mypy", line 182, in <module>
    main()
  File "scripts/mypy", line 36, in main
    type_check_only(path, module, bin_dir, options)
  File "scripts/mypy", line 80, in type_check_only
    python_path=options.python_path)
  File "/home/jukka/project/mypy/mypy/build.py", line 164, in build
    result = manager.process(UnprocessedFile(info, program_text))
  File "/home/jukka/project/mypy/mypy/build.py", line 341, in process
    next.process()
  File "/home/jukka/project/mypy/mypy/build.py", line 814, in process
    self.type_checker().visit_file(self.tree, self.tree.path)
  File "/home/jukka/project/mypy/mypy/checker.py", line 350, in visit_file
    self.accept(d)
  File "/home/jukka/project/mypy/mypy/checker.py", line 357, in accept
    typ = node.accept(self)
  File "/home/jukka/project/mypy/mypy/nodes.py", line 519, in accept
    return visitor.visit_expression_stmt(self)
  File "/home/jukka/project/mypy/mypy/checker.py", line 1302, in visit_expression_stmt
    self.accept(s.expr)
  File "/home/jukka/project/mypy/mypy/checker.py", line 357, in accept
    typ = node.accept(self)
  File "/home/jukka/project/mypy/mypy/nodes.py", line 938, in accept
    return visitor.visit_call_expr(self)
  File "/home/jukka/project/mypy/mypy/checker.py", line 1709, in visit_call_expr
    result = self.expr_checker.visit_call_expr(e)
  File "/home/jukka/project/mypy/mypy/checkexpr.py", line 118, in visit_call_expr
    return self.check_call_expr_with_callee_type(callee_type, e)
  File "/home/jukka/project/mypy/mypy/checkexpr.py", line 128, in check_call_expr_with_callee_type
    e.arg_names, callable_node=e.callee)[0]
  File "/home/jukka/project/mypy/mypy/checkexpr.py", line 171, in check_call
    callee, args, arg_kinds, formal_to_actual, context)
  File "/home/jukka/project/mypy/mypy/checkexpr.py", line 351, in infer_function_type_arguments
    callee_type, pass1_args, arg_kinds, formal_to_actual)  # type: List[Type]
  File "/home/jukka/project/mypy/mypy/infer.py", line 33, in infer_function_type_arguments
    return solve_constraints(type_vars, constraints)
  File "/home/jukka/project/mypy/mypy/solve.py", line 41, in solve_constraints
    bottom = join_types(bottom, c.target)
  File "/home/jukka/project/mypy/mypy/join.py", line 68, in join_types
    return t.accept(TypeJoinVisitor(s))
AttributeError: 'NoneType' object has no attribute 'accept'