Skip to content

Fix IndexError when unpacking empty tuple in type alias #20931

New issue

Have a question about this project? Sign up for a free GitHub account to open an issue and contact its maintainers and the community.

Sign up for GitHub

By clicking “Sign up for GitHub”, you agree to our terms of service and privacy statement. We’ll occasionally send you account related emails.

Already on GitHub? Sign in to your account

Open
bxff wants to merge 1 commit into
base: master
Choose a base branch
from
+14 −1
Open

Fix IndexError when unpacking empty tuple in type alias #20931

Show file tree
Hide file tree
Changes from all commits
Commits
Show all changes
1 commit
Select commit
File filter

Filter by extension

Filter by extension
Conversations
Failed to load comments.
Loading
Jump to
Jump to file
Failed to load files.
Loading
Diff view
Diff view
2 changes: 1 addition & 1 deletion mypy/expandtype.py
View file
Open in desktop
Original file line number Diff line number Diff line change
Expand Up @@ -223,7 +223,7 @@ def visit_instance(self, t: Instance) -> Type:
# See: https://github.com/python/mypy/issues/16649
return t.copy_modified(args=args)

if t.type.fullname == "builtins.tuple":
if t.type.fullname == "builtins.tuple" and len(args) > 0:
# Normalize Tuple[*Tuple[X, ...], ...] -> Tuple[X, ...]
arg = args[0]
if isinstance(arg, UnpackType):
Expand Down
13 changes: 13 additions & 0 deletions test-data/unit/check-python312.test
View file
Open in desktop
Original file line number Diff line number Diff line change
Expand Up @@ -2237,3 +2237,16 @@ class D[*Ts](Generic[Unpack[Us]]): # E: Generic[...] base class is redundant \
# E: Can only use one type var tuple in a class def
pass
[builtins fixtures/tuple.pyi]

[case testUnpackEmptyTupleInTypeAliasNoCrash]
# https://github.com/python/mypy/issues/20913
from typing import Unpack

class C[*Ts]:
pass

type T[T, *Ts] = C[*Ts]

x: T[bool, Unpack[tuple[()]]]
Copy link
Collaborator

@A5rocks A5rocks Mar 1, 2026

Choose a reason for hiding this comment

The reason will be displayed to describe this comment to others. Learn more.

Can you add a reveal type to make sure the type is being preserved?

[builtins fixtures/tuple.pyi]
[typing fixtures/typing-full.pyi]