Crash on incompatible dictionary unpack

[JukkaL]

Type checking this code fragment causes a mypy crash:

from typing import Dict

def f() -> Dict[int, str]:
    return {1: '', **d()}

def d() -> Dict[int, int]:
    return {}

Here's the traceback:

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 88, in main
    res = type_check_only(sources, bin_dir, options, flush_errors)  # noqa
  File "/Users/jukka/src/mypy/mypy/main.py", line 143, in type_check_only
    flush_errors=flush_errors)
  File "/Users/jukka/src/mypy/mypy/build.py", line 183, in build
    flush_errors, fscache)
  File "/Users/jukka/src/mypy/mypy/build.py", line 268, in _build
    graph = dispatch(sources, manager)
  File "/Users/jukka/src/mypy/mypy/build.py", line 2253, in dispatch
    process_graph(graph, manager)
  File "/Users/jukka/src/mypy/mypy/build.py", line 2543, in process_graph
    process_stale_scc(graph, scc, manager)
  File "/Users/jukka/src/mypy/mypy/build.py", line 2659, in process_stale_scc
    graph[id].type_check_first_pass()
  File "/Users/jukka/src/mypy/mypy/build.py", line 1903, in type_check_first_pass
    self.type_checker().check_first_pass()
  File "/Users/jukka/src/mypy/mypy/checker.py", line 252, in check_first_pass
    self.accept(d)
  File "/Users/jukka/src/mypy/mypy/checker.py", line 351, in accept
    stmt.accept(self)
  File "/Users/jukka/src/mypy/mypy/nodes.py", line 539, in accept
    return visitor.visit_func_def(self)
  File "/Users/jukka/src/mypy/mypy/checker.py", line 607, in visit_func_def
    self._visit_func_def(defn)
  File "/Users/jukka/src/mypy/mypy/checker.py", line 611, in _visit_func_def
    self.check_func_item(defn, name=defn.name())
  File "/Users/jukka/src/mypy/mypy/checker.py", line 671, in check_func_item
    self.check_func_def(defn, typ, name)
  File "/Users/jukka/src/mypy/mypy/checker.py", line 834, in check_func_def
    self.accept(item.body)
  File "/Users/jukka/src/mypy/mypy/checker.py", line 351, in accept
    stmt.accept(self)
  File "/Users/jukka/src/mypy/mypy/nodes.py", line 804, in accept
    return visitor.visit_block(self)
  File "/Users/jukka/src/mypy/mypy/checker.py", line 1433, in visit_block
    self.accept(s)
  File "/Users/jukka/src/mypy/mypy/checker.py", line 351, in accept
    stmt.accept(self)
  File "/Users/jukka/src/mypy/mypy/nodes.py", line 919, in accept
    return visitor.visit_return_stmt(self)
  File "/Users/jukka/src/mypy/mypy/checker.py", line 2204, in visit_return_stmt
    self.check_return_stmt(s)
  File "/Users/jukka/src/mypy/mypy/checker.py", line 2235, in check_return_stmt
    allow_none_return=allow_none_func_call)
  File "/Users/jukka/src/mypy/mypy/checkexpr.py", line 2379, in accept
    typ = node.accept(self)
  File "/Users/jukka/src/mypy/mypy/nodes.py", line 1570, in accept
    return visitor.visit_dict_expr(self)
  File "/Users/jukka/src/mypy/mypy/checkexpr.py", line 2018, in visit_dict_expr
    self.check_call(method, [arg], [nodes.ARG_POS], arg)
  File "/Users/jukka/src/mypy/mypy/checkexpr.py", line 620, in check_call
    object_type=object_type)
  File "/Users/jukka/src/mypy/mypy/checkexpr.py", line 587, in check_call
    messages=arg_messages)
  File "/Users/jukka/src/mypy/mypy/checkexpr.py", line 1061, in check_argument_types
    actual + 1, i + 1, callee, context, messages)
  File "/Users/jukka/src/mypy/mypy/checkexpr.py", line 1097, in check_arg
    caller_kind, context)
  File "/Users/jukka/src/mypy/mypy/messages.py", line 643, in incompatible_argument
    arg_name = context.arg_names[n - 1]
IndexError: list index out of range