From 86d93a26bd6807b3c58e793e6c53d8a9d70473b6 Mon Sep 17 00:00:00 2001
From: TheMuffinMan1320 <76186189+TheMuffinMan1320@users.noreply.github.com>
Date: Thu, 13 Aug 2026 14:48:29 -0500
Subject: [PATCH] Fix crash on dataclass field assigned a functional
 namedtuple() call (#21583)

When a dataclass field's default value is a `namedtuple('Name', ...)`
call, the semantic analyzer treats the assignment as a NamedTuple class
definition and binds the field's name to a TypeInfo instead of a Var,
regardless of any type annotation on the left-hand side. The dataclass
plugin's collect_attributes() assumed every non-alias, non-decorator
symbol table node was a Var and asserted so, crashing when it
encountered this TypeInfo.

Skip such fields the same way TypeAlias/Decorator nodes are already
skipped, since they aren't valid dataclass fields.

Fixes #21583
---
 mypy/plugins/dataclasses.py           |  4 ++++
 test-data/unit/check-dataclasses.test | 13 +++++++++++++
 2 files changed, 17 insertions(+)

diff --git a/mypy/plugins/dataclasses.py b/mypy/plugins/dataclasses.py
index a511e714ac6b4..09341bc453c23 100644
--- a/mypy/plugins/dataclasses.py
+++ b/mypy/plugins/dataclasses.py
@@ -599,6 +599,10 @@ def collect_attributes(self) -> list[DataclassAttribute] | None:
                 # This might be a property / field name clash.
                 # We will issue an error later.
                 continue
+            if isinstance(node, TypeInfo):
+                # The declared type is shadowed by a class created from a call like
+                # `x: Foo = namedtuple('Foo', [...])`, so there is no dataclass field here.
+                continue
 
             assert isinstance(node, Var), node
 
diff --git a/test-data/unit/check-dataclasses.test b/test-data/unit/check-dataclasses.test
index f43ac255373e6..1a81750e3e01f 100644
--- a/test-data/unit/check-dataclasses.test
+++ b/test-data/unit/check-dataclasses.test
@@ -2746,6 +2746,19 @@ class B2(B1):  # E: A NamedTuple cannot be a dataclass
 
 [builtins fixtures/tuple.pyi]
 
+[case testNoCrashForDataclassFieldAssignedFunctionalNamedTuple]
+from collections import namedtuple
+from dataclasses import dataclass
+
+@dataclass
+class C:
+    p: "Point" = namedtuple("Point", ["x", "y"])  # E: First argument to namedtuple() should be "p", not "Point"
+
+@dataclass
+class D:
+    Point: "Point" = namedtuple("Point", ["x", "y"])
+[builtins fixtures/tuple.pyi]
+
 [case testDataclassesTypeGuard]
 import dataclasses