From ebc2cb8befbadfc10b962af018b3fa3842d3fd87 Mon Sep 17 00:00:00 2001
From: Stanislav Terliakov <50529348+sterliakov@users.noreply.github.com>
Date: Mon, 3 Feb 2025 20:54:40 +0100
Subject: [PATCH] Prevent crash on generic NamedTuple with unresolved typevar
 bound (#18585)

Fixes #18582. Fixes #17396. Supersedes #18351.

---------

Co-authored-by: hauntsaninja <hauntsaninja@gmail.com>
---
 mypy/checker.py                       |  5 +++++
 mypy/type_visitor.py                  |  4 ++--
 test-data/unit/check-incremental.test | 19 +++++++++++++++++++
 test-data/unit/check-inference.test   |  8 ++++++++
 4 files changed, 34 insertions(+), 2 deletions(-)

diff --git a/mypy/checker.py b/mypy/checker.py
index c9495ca31842b..131a8036508f0 100644
--- a/mypy/checker.py
+++ b/mypy/checker.py
@@ -8347,6 +8347,11 @@ def visit_type_var(self, t: TypeVarType) -> bool:
         # multi-step type inference.
         return t.id.is_meta_var()
 
+    def visit_tuple_type(self, t: TupleType, /) -> bool:
+        # Exclude fallback to avoid bogus "need type annotation" errors
+        # TODO: Maybe erase plain tuples used as fallback in TupleType constructor?
+        return self.query_types(t.items)
+
 
 class SetNothingToAny(TypeTranslator):
     """Replace all ambiguous Uninhabited types with Any (to avoid spurious extra errors)."""
diff --git a/mypy/type_visitor.py b/mypy/type_visitor.py
index f62d67bc26ccb..d935b9a47a51a 100644
--- a/mypy/type_visitor.py
+++ b/mypy/type_visitor.py
@@ -410,7 +410,7 @@ def visit_callable_type(self, t: CallableType, /) -> T:
         return self.query_types(t.arg_types + [t.ret_type])
 
     def visit_tuple_type(self, t: TupleType, /) -> T:
-        return self.query_types(t.items)
+        return self.query_types([t.partial_fallback] + t.items)
 
     def visit_typeddict_type(self, t: TypedDictType, /) -> T:
         return self.query_types(t.items.values())
@@ -550,7 +550,7 @@ def visit_callable_type(self, t: CallableType, /) -> bool:
             return args and ret
 
     def visit_tuple_type(self, t: TupleType, /) -> bool:
-        return self.query_types(t.items)
+        return self.query_types([t.partial_fallback] + t.items)
 
     def visit_typeddict_type(self, t: TypedDictType, /) -> bool:
         return self.query_types(list(t.items.values()))
diff --git a/test-data/unit/check-incremental.test b/test-data/unit/check-incremental.test
index 77170280ecae9..5a9f5d3b3174d 100644
--- a/test-data/unit/check-incremental.test
+++ b/test-data/unit/check-incremental.test
@@ -6745,3 +6745,22 @@ from typing_extensions import TypeAlias
 IntOrStr: TypeAlias = int | str
 assert isinstance(1, IntOrStr)
 [builtins fixtures/type.pyi]
+
+[case testSerializeDeferredGenericNamedTuple]
+import pkg
+[file pkg/__init__.py]
+from .lib import NT
+[file pkg/lib.py]
+from typing import Generic, NamedTuple, TypeVar
+from pkg import does_not_exist  # type: ignore
+from pkg.missing import also_missing  # type: ignore
+
+T = TypeVar("T", bound=does_not_exist)
+class NT(NamedTuple, Generic[T]):
+    values: also_missing[T]
+[file pkg/__init__.py.2]
+# touch
+from .lib import NT
+[builtins fixtures/tuple.pyi]
+[out]
+[out2]
diff --git a/test-data/unit/check-inference.test b/test-data/unit/check-inference.test
index 0da1c092efe89..bdd0ac305904b 100644
--- a/test-data/unit/check-inference.test
+++ b/test-data/unit/check-inference.test
@@ -3886,3 +3886,11 @@ def a4(x: List[str], y: List[Never]) -> None:
     reveal_type(z2)  # N: Revealed type is "builtins.list[builtins.object]"
     z1[1].append("asdf")  # E: "object" has no attribute "append"
 [builtins fixtures/dict.pyi]
+
+[case testTupleJoinFallbackInference]
+foo = [
+    (1, ("a", "b")),
+    (2, []),
+]
+reveal_type(foo)  # N: Revealed type is "builtins.list[Tuple[builtins.int, typing.Sequence[builtins.str]]]"
+[builtins fixtures/tuple.pyi]