Report of INTERNAL ERROR with minimal reproduction

[evhub]

The following code snippet

from functools import reduce
M = Iterable
def f(m1: M, m2):
    ...
def g(ms: T[M]) -> None:
    reduce(f, ms)
T = Iterable

when run through mypy --show-traceback on version 0.560 produces the following traceback

.\break_mypy.py:22: error: INTERNAL ERROR -- please report a bug at https://github.com/python/mypy/issues version: 0.560
Traceback (most recent call last):
  File "c:\anaconda3\lib\runpy.py", line 193, in _run_module_as_main
    "__main__", mod_spec)
  File "c:\anaconda3\lib\runpy.py", line 85, in _run_code
    exec(code, run_globals)
  File "C:\Anaconda3\Scripts\mypy.exe\__main__.py", line 9, in <module>
    sys.exit(console_entry())
  File "c:\anaconda3\lib\site-packages\mypy\__main__.py", line 7, in console_entry
    main(None)
  File "c:\anaconda3\lib\site-packages\mypy\main.py", line 66, in main
    res = type_check_only(sources, bin_dir, options)
  File "c:\anaconda3\lib\site-packages\mypy\main.py", line 119, in type_check_only
    options=options)
  File "c:\anaconda3\lib\site-packages\mypy\build.py", line 218, in build
    graph = dispatch(sources, manager)
  File "c:\anaconda3\lib\site-packages\mypy\build.py", line 1997, in dispatch
    process_graph(graph, manager)
  File "c:\anaconda3\lib\site-packages\mypy\build.py", line 2299, in process_graph
    process_stale_scc(graph, scc, manager)
  File "c:\anaconda3\lib\site-packages\mypy\build.py", line 2475, in process_stale_scc
    graph[id].type_check_first_pass()
  File "c:\anaconda3\lib\site-packages\mypy\build.py", line 1876, in type_check_first_pass
    self.type_checker().check_first_pass()
  File "c:\anaconda3\lib\site-packages\mypy\checker.py", line 196, in check_first_pass
    self.accept(d)
  File "c:\anaconda3\lib\site-packages\mypy\checker.py", line 286, in accept
    stmt.accept(self)
  File "c:\anaconda3\lib\site-packages\mypy\nodes.py", line 497, in accept
    return visitor.visit_func_def(self)
  File "c:\anaconda3\lib\site-packages\mypy\checker.py", line 534, in visit_func_def
    self.check_func_item(defn, name=defn.name())
  File "c:\anaconda3\lib\site-packages\mypy\checker.py", line 599, in check_func_item
    self.check_func_def(defn, typ, name)
  File "c:\anaconda3\lib\site-packages\mypy\checker.py", line 762, in check_func_def
    self.accept(item.body)
  File "c:\anaconda3\lib\site-packages\mypy\checker.py", line 286, in accept
    stmt.accept(self)
  File "c:\anaconda3\lib\site-packages\mypy\nodes.py", line 756, in accept
    return visitor.visit_block(self)
  File "c:\anaconda3\lib\site-packages\mypy\checker.py", line 1330, in visit_block
    self.accept(s)
  File "c:\anaconda3\lib\site-packages\mypy\checker.py", line 286, in accept
    stmt.accept(self)
  File "c:\anaconda3\lib\site-packages\mypy\nodes.py", line 770, in accept
    return visitor.visit_expression_stmt(self)
  File "c:\anaconda3\lib\site-packages\mypy\checker.py", line 2097, in visit_expression_stmt
    self.expr_checker.accept(s.expr, allow_none_return=True, always_allow_any=True)
  File "c:\anaconda3\lib\site-packages\mypy\checkexpr.py", line 2361, in accept
    typ = self.visit_call_expr(node, allow_none_return=True)
  File "c:\anaconda3\lib\site-packages\mypy\checkexpr.py", line 268, in visit_call_expr
    ret_type = self.check_call_expr_with_callee_type(callee_type, e, fullname, object_type)
  File "c:\anaconda3\lib\site-packages\mypy\checkexpr.py", line 505, in check_call_expr_with_callee_type
    object_type=object_type)[0]
  File "c:\anaconda3\lib\site-packages\mypy\checkexpr.py", line 611, in check_call
    object_type=object_type)
  File "c:\anaconda3\lib\site-packages\mypy\checkexpr.py", line 568, in check_call
    callee, args, arg_kinds, formal_to_actual, context)
  File "c:\anaconda3\lib\site-packages\mypy\checkexpr.py", line 823, in infer_function_type_arguments
    inferred_args, context)
  File "c:\anaconda3\lib\site-packages\mypy\checkexpr.py", line 877, in infer_function_type_arguments_pass2
    callee_type, arg_types, arg_kinds, formal_to_actual)
  File "c:\anaconda3\lib\site-packages\mypy\infer.py", line 31, in infer_function_type_arguments
    callee_type, arg_types, arg_kinds, formal_to_actual)
  File "c:\anaconda3\lib\site-packages\mypy\constraints.py", line 63, in infer_constraints_for_callable
    SUPERTYPE_OF)
  File "c:\anaconda3\lib\site-packages\mypy\constraints.py", line 174, in infer_constraints
    return template.accept(ConstraintBuilderVisitor(actual, direction))
  File "c:\anaconda3\lib\site-packages\mypy\types.py", line 739, in accept
    return visitor.visit_callable_type(self)
  File "c:\anaconda3\lib\site-packages\mypy\constraints.py", line 415, in visit_callable_type
    res.extend(infer_constraints(t, a, neg_op(self.direction)))
  File "c:\anaconda3\lib\site-packages\mypy\constraints.py", line 174, in infer_constraints
    return template.accept(ConstraintBuilderVisitor(actual, direction))
  File "c:\anaconda3\lib\site-packages\mypy\types.py", line 475, in accept
    return visitor.visit_instance(self)
  File "c:\anaconda3\lib\site-packages\mypy\constraints.py", line 326, in visit_instance
    mapped.args[i], instance.args[i], self.direction))
IndexError: list index out of range
.\break_mypy.py:22: : note: use --pdb to drop into pdb

[gvanrossum]

Confirmed (the snippet also needs from typing import Iterable for it to be a repro).

[ilevkivskyi]

I think that checks that are done in third pass don't happen for types that are forward references: most importantly, checking the correct number of type arguments. I.e. my guess is that T[M] is expanded to Iterable[Iterable], while it should expand to Iterable[Iterable[Any]].