Report that NamedTuple and dataclass are incompatile instead of crashing. (#18633)

Fixes #18527

The fix is pretty simple. I could not find a situation where combining
`NamedTuple` and `dataclass` makes sense, so emitting an error and just
not applying the dataclass transformations seems sensible.

---------

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>
Co-authored-by: sobolevn <[email protected]>

Author: 3 people

mypy/plugins/dataclasses.py

@@ -965,6 +965,9 @@ def dataclass_tag_callback(ctx: ClassDefContext) -> None:
 
 def dataclass_class_maker_callback(ctx: ClassDefContext) -> bool:
     """Hooks into the class typechecking process to add support for dataclasses."""
+    if any(i.is_named_tuple for i in ctx.cls.info.mro):
+        ctx.api.fail("A NamedTuple cannot be a dataclass", ctx=ctx.cls.info)
+        return True
     transformer = DataclassTransformer(
         ctx.cls, ctx.reason, _get_transform_spec(ctx.reason), ctx.api
     )

test-data/unit/check-dataclasses.test

@@ -2576,3 +2576,20 @@ reveal_type(m.b)  # N: Revealed type is "builtins.int"
 m.a = 1  # E: Cannot assign to final attribute "a"
 m.b = 2  # E: Cannot assign to final attribute "b"
 [builtins fixtures/tuple.pyi]
+
+[case testNoCrashForDataclassNamedTupleCombination]
+# flags: --python-version 3.13
+from dataclasses import dataclass
+from typing import NamedTuple
+
+@dataclass
+class A(NamedTuple):  # E: A NamedTuple cannot be a dataclass
+    i: int
+
+class B1(NamedTuple):
+    i: int
+@dataclass
+class B2(B1):  # E: A NamedTuple cannot be a dataclass
+    pass
+
+[builtins fixtures/tuple.pyi]