Crash on sequence match statement in generic class with restricted type variable bound to sequence

[rwhender]

Crash Report

mypy crashes when the following conditions are met:

1. A generic class is present.
2. The generic class refers to a typevar T that is bound to a sequence.
3. The generic class has a method that attempts to pattern match a method parameter of type T
4. One of the pattern matching cases is a sequence pattern such as _, _.

This crash is similar to but distinct from this issue: #12448

Traceback

$ mypy mypy_mre.py --show-traceback
mypy_mre.py:7: 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: 1.14.0+dev.05a9e79068a5830e57264390c9f6bca859e92053
Traceback (most recent call last):
 File "~/venv-3.11.6/bin/mypy", line 8, in <module>
 sys.exit(console_entry())
 File "~/venv-3.11.6/lib/python3.11/site-packages/mypy/__main__.py", line 15, in console_entry
 main()
 File "~/venv-3.11.6/lib/python3.11/site-packages/mypy/main.py", line 109, in main
 res, messages, blockers = run_build(sources, options, fscache, t0, stdout, stderr)
 File "~/venv-3.11.6/lib/python3.11/site-packages/mypy/main.py", line 193, in run_build
 res = build.build(sources, options, None, flush_errors, fscache, stdout, stderr)
 File "~/venv-3.11.6/lib/python3.11/site-packages/mypy/build.py", line 194, in build
 result = _build(
 File "~/venv-3.11.6/lib/python3.11/site-packages/mypy/build.py", line 269, in _build
 graph = dispatch(sources, manager, stdout)
 File "~/venv-3.11.6/lib/python3.11/site-packages/mypy/build.py", line 2937, in dispatch
 process_graph(graph, manager)
 File "~/venv-3.11.6/lib/python3.11/site-packages/mypy/build.py", line 3335, in process_graph
 process_stale_scc(graph, scc, manager)
 File "~/venv-3.11.6/lib/python3.11/site-packages/mypy/build.py", line 3436, in process_stale_scc
 graph[id].type_check_first_pass()
 File "~/venv-3.11.6/lib/python3.11/site-packages/mypy/build.py", line 2306, in type_check_first_pass
 self.type_checker().check_first_pass()
 File "~/venv-3.11.6/lib/python3.11/site-packages/mypy/checker.py", line 483, in check_first_pass
 self.accept(d)
 File "~/venv-3.11.6/lib/python3.11/site-packages/mypy/checker.py", line 592, in accept
 stmt.accept(self)
 File "~/venv-3.11.6/lib/python3.11/site-packages/mypy/nodes.py", line 1196, in accept
 return visitor.visit_class_def(self)
 File "~/venv-3.11.6/lib/python3.11/site-packages/mypy/checker.py", line 2461, in visit_class_def
 self.accept(defn.defs)
 File "~/venv-3.11.6/lib/python3.11/site-packages/mypy/checker.py", line 592, in accept
 stmt.accept(self)
 File "~/venv-3.11.6/lib/python3.11/site-packages/mypy/nodes.py", line 1277, in accept
 return visitor.visit_block(self)
 File "~/venv-3.11.6/lib/python3.11/site-packages/mypy/checker.py", line 2951, in visit_block
 self.accept(s)
 File "~/venv-3.11.6/lib/python3.11/site-packages/mypy/checker.py", line 592, in accept
 stmt.accept(self)
 File "~/venv-3.11.6/lib/python3.11/site-packages/mypy/nodes.py", line 827, in accept
 return visitor.visit_func_def(self)
 File "~/venv-3.11.6/lib/python3.11/site-packages/mypy/checker.py", line 1046, in visit_func_def
 self._visit_func_def(defn)
 File "~/venv-3.11.6/lib/python3.11/site-packages/mypy/checker.py", line 1050, in _visit_func_def
 self.check_func_item(defn, name=defn.name)
 File "~/venv-3.11.6/lib/python3.11/site-packages/mypy/checker.py", line 1084, in check_func_item
 self.check_func_def(defn, typ, name, allow_empty)
 File "~/venv-3.11.6/lib/python3.11/site-packages/mypy/checker.py", line 1360, in check_func_def
 self.accept(item.body)
 File "~/venv-3.11.6/lib/python3.11/site-packages/mypy/checker.py", line 592, in accept
 stmt.accept(self)
 File "~/venv-3.11.6/lib/python3.11/site-packages/mypy/nodes.py", line 1277, in accept
 return visitor.visit_block(self)
 File "~/venv-3.11.6/lib/python3.11/site-packages/mypy/checker.py", line 2951, in visit_block
 self.accept(s)
 File "~/venv-3.11.6/lib/python3.11/site-packages/mypy/checker.py", line 592, in accept
 stmt.accept(self)
 File "~/venv-3.11.6/lib/python3.11/site-packages/mypy/nodes.py", line 1661, in accept
 return visitor.visit_match_stmt(self)
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 File "~/venv-3.11.6/lib/python3.11/site-packages/mypy/checker.py", line 5298, in visit_match_stmt
 pattern_types = [self.pattern_checker.accept(p, subject_type) for p in s.patterns]
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 File "~/venv-3.11.6/lib/python3.11/site-packages/mypy/checker.py", line 5298, in <listcomp>
 pattern_types = [self.pattern_checker.accept(p, subject_type) for p in s.patterns]
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 File "~/venv-3.11.6/lib/python3.11/site-packages/mypy/checkpattern.py", line 129, in accept
 result = o.accept(self)
 ^^^^^^^^^^^^^^
 File "~/venv-3.11.6/lib/python3.11/site-packages/mypy/patterns.py", line 93, in accept
 return visitor.visit_sequence_pattern(self)
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 File "~/venv-3.11.6/lib/python3.11/site-packages/mypy/checkpattern.py", line 345, in visit_sequence_pattern
 new_type = self.construct_sequence_child(current_type, new_inner_type)
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 File "~/venv-3.11.6/lib/python3.11/site-packages/mypy/checkpattern.py", line 766, in construct_sequence_child
 assert isinstance(proper_type, Instance)
AssertionError: 
mypy_mre.py:7: : note: use --pdb to drop into pdb

To Reproduce

Here is an MRE:

from typing import Generic, TypeVar, Sequence, Any

T = TypeVar("T", bound=Sequence[Any])

class C(Generic[T]):
 def f(self, x: T) -> None:
 match x:
 case _, _:
 pass
 case _:
 pass

Your Environment

• Mypy version used: 1.14.0+dev.05a9e79068a5830e57264390c9f6bca859e92053 (current master)
• Mypy command-line flags: None
• Mypy configuration options from mypy.ini (and other config files):

check_untyped_defs = true
disallow_any_generics = true
disallow_incomplete_defs = true
disallow_subclassing_any = true
disallow_untyped_calls = true
disallow_untyped_decorators = true
disallow_untyped_defs = true
extra_checks = true
no_implicit_reexport = true
python_version = "3.11"
strict_equality = true
warn_redundant_casts = true
warn_return_any = true
warn_unused_configs = true
warn_unused_ignores = true

• Python version used: 3.11.6
• Operating system and version: AlmaLinux 9.4 under WSL 2 on Windows 10