Internal error when building a list from starred tuples

[pdeblanc]

Using python 3.7.0, mypy 0.630.

Example file combine.py:

from typing import List, Tuple


def combine_tuples_into_list(a: Tuple[int, int], b: Tuple[int]) -> List[int]:
    return [*a, *b]

I then run mypy combine.py --show-traceback. Here's the output:

combine.py:5: error: INTERNAL ERROR -- please report a bug at https://github.com/python/mypy/issues version: 0.630
Traceback (most recent call last):
  File "/usr/local/bin/mypy", line 11, in <module>
    sys.exit(console_entry())
  File "/usr/local/lib/python3.7/site-packages/mypy/__main__.py", line 7, in console_entry
    main(None)
  File "/usr/local/lib/python3.7/site-packages/mypy/main.py", line 91, in main
    res = type_check_only(sources, bin_dir, options, flush_errors, fscache)  # noqa
  File "/usr/local/lib/python3.7/site-packages/mypy/main.py", line 148, in type_check_only
    fscache=fscache)
  File "/usr/local/lib/python3.7/site-packages/mypy/build.py", line 183, in build
    flush_errors, fscache)
  File "/usr/local/lib/python3.7/site-packages/mypy/build.py", line 356, in _build
    graph = dispatch(sources, manager)
  File "/usr/local/lib/python3.7/site-packages/mypy/build.py", line 2543, in dispatch
    process_graph(graph, manager)
  File "/usr/local/lib/python3.7/site-packages/mypy/build.py", line 2840, in process_graph
    process_stale_scc(graph, scc, manager)
  File "/usr/local/lib/python3.7/site-packages/mypy/build.py", line 2963, in process_stale_scc
    graph[id].type_check_first_pass()
  File "/usr/local/lib/python3.7/site-packages/mypy/build.py", line 2103, in type_check_first_pass
    self.type_checker().check_first_pass()
  File "/usr/local/lib/python3.7/site-packages/mypy/checker.py", line 256, in check_first_pass
    self.accept(d)
  File "/usr/local/lib/python3.7/site-packages/mypy/checker.py", line 355, in accept
    stmt.accept(self)
  File "/usr/local/lib/python3.7/site-packages/mypy/nodes.py", line 603, in accept
    return visitor.visit_func_def(self)
  File "/usr/local/lib/python3.7/site-packages/mypy/checker.py", line 666, in visit_func_def
    self._visit_func_def(defn)
  File "/usr/local/lib/python3.7/site-packages/mypy/checker.py", line 670, in _visit_func_def
    self.check_func_item(defn, name=defn.name())
  File "/usr/local/lib/python3.7/site-packages/mypy/checker.py", line 732, in check_func_item
    self.check_func_def(defn, typ, name)
  File "/usr/local/lib/python3.7/site-packages/mypy/checker.py", line 898, in check_func_def
    self.accept(item.body)
  File "/usr/local/lib/python3.7/site-packages/mypy/checker.py", line 355, in accept
    stmt.accept(self)
  File "/usr/local/lib/python3.7/site-packages/mypy/nodes.py", line 885, in accept
    return visitor.visit_block(self)
  File "/usr/local/lib/python3.7/site-packages/mypy/checker.py", line 1607, in visit_block
    self.accept(s)
  File "/usr/local/lib/python3.7/site-packages/mypy/checker.py", line 355, in accept
    stmt.accept(self)
  File "/usr/local/lib/python3.7/site-packages/mypy/nodes.py", line 1011, in accept
    return visitor.visit_return_stmt(self)
  File "/usr/local/lib/python3.7/site-packages/mypy/checker.py", line 2404, in visit_return_stmt
    self.check_return_stmt(s)
  File "/usr/local/lib/python3.7/site-packages/mypy/checker.py", line 2437, in check_return_stmt
    allow_none_return=allow_none_func_call)
  File "/usr/local/lib/python3.7/site-packages/mypy/checkexpr.py", line 2987, in accept
    typ = node.accept(self)
  File "/usr/local/lib/python3.7/site-packages/mypy/nodes.py", line 1691, in accept
    return visitor.visit_list_expr(self)
  File "/usr/local/lib/python3.7/site-packages/mypy/checkexpr.py", line 2461, in visit_list_expr
    return self.check_lst_expr(e.items, 'builtins.list', '<list>', e)
  File "/usr/local/lib/python3.7/site-packages/mypy/checkexpr.py", line 2487, in check_lst_expr
    context)[0]
  File "/usr/local/lib/python3.7/site-packages/mypy/checkexpr.py", line 627, in check_call
    callee, args, arg_kinds, formal_to_actual, context)
  File "/usr/local/lib/python3.7/site-packages/mypy/checkexpr.py", line 875, in infer_function_type_arguments
    strict=self.chk.in_checked_function())
  File "/usr/local/lib/python3.7/site-packages/mypy/infer.py", line 31, in infer_function_type_arguments
    callee_type, arg_types, arg_kinds, formal_to_actual)
  File "/usr/local/lib/python3.7/site-packages/mypy/constraints.py", line 60, in infer_constraints_for_callable
    tuple_counter)
  File "/usr/local/lib/python3.7/site-packages/mypy/constraints.py", line 88, in get_actual_type
    return arg_type.items[tuple_counter[0] - 1]
IndexError: list index out of range
combine.py:5: : note: use --pdb to drop into pdb

[emmatyping]

Confirmed on 0.630 and master with this simple repro:

from typing import Tuple
a: Tuple[int, int]
b: Tuple[int]
f = [*a, *b]

[ilevkivskyi]

There is another crash with star expressions, #3825, but it looks lie this is unrelated.

[refi64]

Was this fixed? Can't reproduce on master now.

[refi64]

This was fixed by dd1c5d0.