Prevent dataclass parsing from triggering DuplicateBasesError

ChangeLog

@@ -27,6 +27,7 @@ What's New in astroid 3.3.6?
 Release date: TBA
 
 * Fix precedence of `path` arg in `modpath_from_file_with_callback` to be higher than `sys.path`
+* Prevent dataclass parsing from triggering `DuplicateBasesError`.
 
 
 What's New in astroid 3.3.5?

astroid/brain/brain_dataclasses.py

@@ -171,7 +171,7 @@ def _find_arguments_from_base_classes(
     # See TODO down below
     # all_have_defaults = True
 
-    for base in reversed(node.mro()):
+    for base in reversed(node.mro(ignore_duplicates=True)):
         if not base.is_dataclass:
             continue
         try:
@@ -221,7 +221,7 @@ def _parse_arguments_into_strings(
 
 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()):
+    for base in reversed(node.mro(ignore_duplicates=True)):
         if not base.is_dataclass:
             continue
         if name in base.locals:

astroid/nodes/scoped_nodes/scoped_nodes.py

@@ -144,12 +144,13 @@ def clean_duplicates_mro(
     sequences: list[list[ClassDef]],
     cls: ClassDef,
     context: InferenceContext | None,
+    ignore_duplicates: bool,
 ) -> list[list[ClassDef]]:
     for sequence in sequences:
         seen = set()
         for node in sequence:
             lineno_and_qname = (node.lineno, node.qname())
-            if lineno_and_qname in seen:
+            if lineno_and_qname in seen and not ignore_duplicates:
                 raise DuplicateBasesError(
                     message="Duplicates found in MROs {mros} for {cls!r}.",
                     mros=sequences,
@@ -2834,7 +2835,9 @@ def _inferred_bases(self, context: InferenceContext | None = None):
             else:
                 yield from baseobj.bases
 
-    def _compute_mro(self, context: InferenceContext | None = None):
+    def _compute_mro(
+        self, context: InferenceContext | None = None, ignore_duplicates: bool = False
+    ):
         if self.qname() == "builtins.object":
             return [self]
 
@@ -2844,23 +2847,31 @@ def _compute_mro(self, context: InferenceContext | None = None):
             if base is self:
                 continue
 
-            mro = base._compute_mro(context=context)
+            mro = base._compute_mro(
+                context=context, ignore_duplicates=ignore_duplicates
+            )
             bases_mro.append(mro)
 
         unmerged_mro: list[list[ClassDef]] = [[self], *bases_mro, inferred_bases]
-        unmerged_mro = clean_duplicates_mro(unmerged_mro, self, context)
+        unmerged_mro = clean_duplicates_mro(
+            unmerged_mro, self, context, ignore_duplicates=ignore_duplicates
+        )
         clean_typing_generic_mro(unmerged_mro)
         return _c3_merge(unmerged_mro, self, context)
 
-    def mro(self, context: InferenceContext | None = None) -> list[ClassDef]:
+    def mro(
+        self, context: InferenceContext | None = None, ignore_duplicates: bool = False
+    ) -> list[ClassDef]:
         """Get the method resolution order, using C3 linearization.
 
+        :param ignore_duplicates: Don't raise DuplicateBasesError on duplicate bases
+            of the same base class.
         :returns: The list of ancestors, sorted by the mro.
         :rtype: list(NodeNG)
         :raises DuplicateBasesError: Duplicate bases in the same class base
         :raises InconsistentMroError: A class' MRO is inconsistent
         """
-        return self._compute_mro(context=context)
+        return self._compute_mro(context=context, ignore_duplicates=ignore_duplicates)
 
     def bool_value(self, context: InferenceContext | None = None) -> Literal[True]:
         """Determine the boolean value of this node.

tests/brain/test_dataclasses.py

@@ -1350,3 +1350,33 @@ def attr(self, value: int) -> None:
     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"]
+
+
+@parametrize_module
+def test_dataclass_inherited_from_multiple_protocol_bases(module: str):
+    code = astroid.extract_node(
+        f"""
+    from {module} import dataclass
+    from typing import TypeVar, Protocol
+
+    BaseT = TypeVar("BaseT")
+    T = TypeVar("T", bound=BaseT)
+
+
+    class A(Protocol[BaseT]):
+        pass
+
+
+    class B(A[T], Protocol[T]):
+        pass
+
+
+    @dataclass
+    class Dataclass(B[T]):
+        pass
+    """
+    )
+    inferred = code.inferred()
+    assert len(inferred) == 1
+    assert isinstance(inferred[0], nodes.ClassDef)
+    assert inferred[0].is_dataclass

tests/test_scoped_nodes.py

@@ -1938,6 +1938,7 @@ class A(Generic[T1], Generic[T2]): ...
         assert isinstance(cls, nodes.ClassDef)
         with self.assertRaises(DuplicateBasesError):
             cls.mro()
+        assert len(cls.mro(ignore_duplicates=True)) == 3
 
     def test_mro_generic_error_2(self):
         cls = builder.extract_node(
@@ -1951,6 +1952,8 @@ class B(A[T], A[T]): ...
         assert isinstance(cls, nodes.ClassDef)
         with self.assertRaises(DuplicateBasesError):
             cls.mro()
+        with self.assertRaises(InconsistentMroError):
+            cls.mro(ignore_duplicates=True)
 
     def test_mro_typing_extensions(self):
         """Regression test for mro() inference on typing_extensions.