"INTERNAL_ERROR": generic fun w/ Callable param w/ constrained return type followed by `*args` typed as `*Tuple[int, float]` crashes mypy

Crash Report

• Create crash_example.py as follows:

from typing import *

# mypy latest from master branch crashes (mypy 2.4.0+dev.b974556f84bd4ff5280b66bce1413fa942d1261e) (and earlier prod releases as well)
def generic_fun[
   TypeVarWithConstraints: (str, bytes),
](
   a: Callable[..., TypeVarWithConstraints],
   *args: *Tuple[int, float]
): pass

• run mypy 2.3.1 or latest master on Github (2.4.0+dev) and observe a crash with "INTERNAL_ERROR":

[liveuser@localhost-live ~]$ mypy crash_example.py --show-traceback
crash_example.py:4: error: INTERNAL ERROR -- Please try using mypy master on GitHub:
https://mypy.readthedocs.io/en/stable/common_issues.html#using-a-development-mypy-build
Please report a bug at https://github.com/python/mypy/issues
version: 2.4.0+dev.b974556f84bd4ff5280b66bce1413fa942d1261e
[--show-traceback output is included further down]

---

Traceback

Traceback (most recent call last):
  File "/home/liveuser/.local/bin/mypy", line 8, in <module>
    sys.exit(console_entry())
  File "/home/liveuser/.local/lib/python3.13/site-packages/mypy/__main__.py", line 16, in console_entry
    main()
  File "/home/liveuser/.local/lib/python3.13/site-packages/mypy/main.py", line 154, in main
    res, messages, blockers = run_build(sources, options, fscache, t0, stdout, stderr)
  File "/home/liveuser/.local/lib/python3.13/site-packages/mypy/main.py", line 244, in run_build
    res = build.build(sources, options, None, flush_errors, fscache, stdout, stderr)
  File "/home/liveuser/.local/lib/python3.13/site-packages/mypy/build.py", line 422, in build
    result = build_inner(
  File "/home/liveuser/.local/lib/python3.13/site-packages/mypy/build.py", line 537, in build_inner
    graph = dispatch(sources, manager, stdout, connect_threads)
  File "/home/liveuser/.local/lib/python3.13/site-packages/mypy/build.py", line 4150, in dispatch
    process_graph(graph, manager)
  File "/home/liveuser/.local/lib/python3.13/site-packages/mypy/build.py", line 4618, in process_graph
    done, still_working, results = manager.wait_for_done(graph)
  File "/home/liveuser/.local/lib/python3.13/site-packages/mypy/build.py", line 1484, in wait_for_done
    process_stale_scc(graph, next_scc, self)
  File "/home/liveuser/.local/lib/python3.13/site-packages/mypy/build.py", line 4793, in process_stale_scc
    graph[id].type_check_first_pass()
  File "/home/liveuser/.local/lib/python3.13/site-packages/mypy/build.py", line 3410, in type_check_first_pass
    self.type_checker().check_first_pass(recurse_into_functions=recurse_into_functions)
  File "/home/liveuser/.local/lib/python3.13/site-packages/mypy/checker.py", line 632, in check_first_pass
    self.accept(d)
  File "/home/liveuser/.local/lib/python3.13/site-packages/mypy/checker.py", line 779, in accept
    stmt.accept(self)
    ~~~~~~~~~~~^^^^^^
  File "/home/liveuser/.local/lib/python3.13/site-packages/mypy/nodes.py", line 1165, in accept
    return visitor.visit_func_def(self)
           ~~~~~~~~~~~~~~~~~~~~~~^^^^^^
  File "/home/liveuser/.local/lib/python3.13/site-packages/mypy/checker.py", line 1346, in visit_func_def
    self.visit_func_def_impl(defn)
    ~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^
  File "/home/liveuser/.local/lib/python3.13/site-packages/mypy/checker.py", line 1350, in visit_func_def_impl
    self.check_func_item(defn, name=defn.name)
    ~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^
  File "/home/liveuser/.local/lib/python3.13/site-packages/mypy/checker.py", line 1383, in check_func_item
    self.check_func_def(defn, typ, name, allow_empty)
    ~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/liveuser/.local/lib/python3.13/site-packages/mypy/checker.py", line 1514, in check_func_def
    store_argument_type(item, i, typ, self.named_generic_type)
    ~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/liveuser/.local/lib/python3.13/site-packages/mypy/types_utils.py", line 183, in store_argument_type
    defn.arguments[i].variable.type = arg_type
    ~~~~~~~~~~~~~~^^^
IndexError: list index out of range

crash_example.py:4: note: use --pdb to drop into pdb

---

Context/motivation

In PEP-0646 ( https://peps.python.org/pep-0646/ ), it is shown how an unpacked Tuple can be used to specify the type of *args:

# example from PEP-0646
def foo(*args: *Tuple[int, str]) -> None: pass

foo(1, "hello")  # OK

In https://docs.python.org/3.13/reference/compound_stmts.html#type-parameter-lists
(separate from the SyntaxError in their overly_generic example which I have reported to the python team and am working with them to fix),
saying

A parenthesized tuple of expressions after the colon indicates a set of constraints (e.g. T: (str, bytes)). Each member of the tuple should be a type (again, this is not enforced at runtime). Constrained type variables can only take on one of the types in the list of constraints.
e.g.

from typing import *

def generic_fun_takes_a_callable_that_returns_either_str_or_bytes[
        TypeVarWithConstraints: (str, bytes),
](a: Callable[..., TypeVarWithConstraints]):
    pass

def some_fun_returns_str() -> str:
    return 'abc'

def some_fun_returns_bytes() -> bytes:
    return b'abc'

def some_fun_returns_int() -> int:
    return 1

generic_fun_takes_a_callable_that_returns_either_str_or_bytes(some_fun_returns_str) # ok

generic_fun_takes_a_callable_that_returns_either_str_or_bytes(some_fun_returns_bytes) # ok

# not ok:
# error: Value of type variable "TypeVarWithConstraints" of "generic_fun_takes_a_callable_that_returns_either_str_or_bytes" cannot be "int"  [type-var]
# generic_fun_takes_a_callable_that_returns_either_str_or_bytes(some_fun_returns_int)

So given

 def generic_fun_takes_a_callable_that_returns_either_str_or_bytes[
        TypeVarWithConstraints: (str, bytes),
](a: Callable[..., TypeVarWithConstraints]):
    pass

is ok and

# example from PEP-0646
def foo(*args: *Tuple[int, str]) -> None: pass

is ok,
what about a function that has the arguments of the 1st followed by the arguments of the 2nd example, which both work fine with mypy on their own? (note PEP-0646 gives examples of *args being typed by an unpacked tuple even when preceded by other arguments)

def generic_fun[
   TypeVarWithConstraints: (str, bytes),
](
   a: Callable[..., TypeVarWithConstraints],
   *args: *Tuple[int, float]
): pass

It turns out that the above crashes mypy unexpectedly in a way that causes the problem to tell me to file a bug, so I am doing so (see traceback towards top of ticket).

---

Your Environment

• Fedora Linux 42
• mypy 2.3.1 and mypy 2.4.0+dev.b974556f84bd4ff5280b66bce1413fa942d1261e
• python 3.13.2 in this case

---

Thanks very much!

Reproduce the crash with the issue's crash_example.py using mypy 2.3.1 or the development build. Start in mypy/checker.py and follow the call into mypy/types_utils.py, where the traceback shows the IndexError. Done means this valid combination of a constrained callable parameter and unpacked *args no longer crashes; add regression coverage if the project's existing tests provide a suitable location.