Fix DuplicateBasesError crash in dataclass transform (#2970)

ChangeLog

diff --git a/ChangeLog b/ChangeLog
index 7d15bf0..b6621fa 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -13,6 +13,13 @@
 ============================
 Release date: TBA
 
+* Fix ``DuplicateBasesError`` crash in dataclass transform when a class has
+  duplicate bases in its MRO (e.g., ``Protocol`` appearing both directly and
+  indirectly). Catch ``MroError`` at ``.mro()`` call sites in
+  ``brain_dataclasses.py``, consistent with the existing pattern elsewhere.
+
+  Closes #2628
+
 * Catch ``MemoryError`` when inferring f-strings with extremely large format
   widths (e.g. ``f'{0:11111111111}'``) so that inference yields ``Uninferable``
   instead of crashing.

astroid/brain/brain_dataclasses.py

diff --git a/astroid/brain/brain_dataclasses.py b/astroid/brain/brain_dataclasses.py
index 244665e..b6b1956 100644
--- a/astroid/brain/brain_dataclasses.py
+++ b/astroid/brain/brain_dataclasses.py
@@ -21,7 +21,12 @@
 from astroid.brain.helpers import is_class_var
 from astroid.builder import parse
 from astroid.const import PY313_PLUS
-from astroid.exceptions import AstroidSyntaxError, InferenceError, UseInferenceDefault
+from astroid.exceptions import (
+    AstroidSyntaxError,
+    InferenceError,
+    MroError,
+    UseInferenceDefault,
+)
 from astroid.inference_tip import inference_tip
 from astroid.manager import AstroidManager
 from astroid.typing import InferenceResult
@@ -174,7 +179,12 @@
     # See TODO down below
     # all_have_defaults = True
 
-    for base in reversed(node.mro()):
+    try:
+        mro = node.mro()
+    except MroError:
+        return pos_only_store, kw_only_store
+
+    for base in reversed(mro):
         if not base.is_dataclass:
             continue
         try:
@@ -224,7 +234,12 @@
 
 def _get_previous_field_default(node: nodes.ClassDef, name: str) -> nodes.NodeNG | None:
     """Get the default value of a previously defined field."""
-    for base in reversed(node.mro()):
+    try:
+        mro = node.mro()
+    except MroError:
+        return None
+
+    for base in reversed(mro):
         if not base.is_dataclass:
             continue
         if name in base.locals:

tests/brain/test_dataclasses.py

diff --git a/tests/brain/test_dataclasses.py b/tests/brain/test_dataclasses.py
index d6ab13e..4795327 100644
--- a/tests/brain/test_dataclasses.py
+++ b/tests/brain/test_dataclasses.py
@@ -1237,3 +1237,73 @@
     fourth_init: bases.UnboundMethod = next(fourth.infer())
     assert [a.name for a in fourth_init.args.args] == ["self", "other_attr", "attr"]
     assert [a.name for a in fourth_init.args.defaults] == ["Uninferable"]
+
+
+def test_dataclass_with_duplicate_bases_no_crash():
+    """Regression test for https://github.com/pylint-dev/astroid/issues/2628.
+
+    A dataclass inheriting from a class with duplicate bases in MRO
+    (e.g., Protocol appearing both directly and indirectly) should not
+    crash with DuplicateBasesError during AST transformation.
+    """
+    code = """
+    import dataclasses
+    from typing import TypeVar, Protocol
+
+    BaseT = TypeVar("BaseT")
+    T = TypeVar("T", bound=BaseT)
+
+    class ConfigBase(Protocol[BaseT]):
+        ...
+
+    class Config(ConfigBase[T], Protocol[T]):
+        ...
+
+    @dataclasses.dataclass
+    class DatasetConfig(Config[T]):
+        name: str = "default"
+
+    DatasetConfig.__init__  #@
+    """
+    node = astroid.extract_node(code)
+    # Should not raise DuplicateBasesError — graceful degradation instead
+    inferred = next(node.infer())
+    assert inferred is not None
+
+
+def test_dataclass_with_duplicate_bases_field_default():
+    """Regression test for _get_previous_field_default with broken MRO.
+
+    When a parent dataclass defines a field with a default and a child (with
+    duplicate bases in its MRO) re-annotates that field without a value,
+    _get_previous_field_default should not crash with DuplicateBasesError.
+
+    See https://github.com/pylint-dev/astroid/issues/2628.
+    """
+    code = """
+    import dataclasses
+    from typing import TypeVar, Protocol
+
+    BaseT = TypeVar("BaseT")
+    T = TypeVar("T", bound=BaseT)
+
+    class ConfigBase(Protocol[BaseT]):
+        ...
+
+    class Config(ConfigBase[T], Protocol[T]):
+        ...
+
+    @dataclasses.dataclass
+    class BaseConfig(Config[T]):
+        name: str = "default"
+
+    @dataclasses.dataclass
+    class ChildConfig(BaseConfig[T]):
+        name: str
+
+    ChildConfig.__init__  #@
+    """
+    node = astroid.extract_node(code)
+    # Should not raise DuplicateBasesError in _get_previous_field_default
+    inferred = next(node.infer())
+    assert inferred is not None