gh-20476: Fix AssertionError (too many plugin hook passes) on dataclass attribute redefinition

mypy/plugins/dataclasses.py

@@ -575,6 +575,19 @@ def collect_attributes(self) -> list[DataclassAttribute] | None:
         # Second, collect attributes belonging to the current class.
         current_attr_names: set[str] = set()
         kw_only = self._get_bool_arg("kw_only", self._spec.kw_only_default)
+        all_assignments = self._get_assignment_statements_from_block(cls.defs)
+        redefined_attrs: dict[str, list[AssignmentStmt]] = {}
+        last_def_with_type: dict[str, AssignmentStmt] = {}
+        for stmt in all_assignments:
+            if not isinstance(stmt.lvalues[0], NameExpr):
+                continue
+            name = stmt.lvalues[0].name
+            if stmt.type is not None:
+                last_def_with_type[name] = stmt
+            if name in redefined_attrs:
+                redefined_attrs[name].append(stmt)
+            else:
+                redefined_attrs[name] = [stmt]
         for stmt in self._get_assignment_statements_from_block(cls.defs):
             # Any assignment that doesn't use the new type declaration
             # syntax can be ignored out of hand.
@@ -587,13 +600,15 @@ def collect_attributes(self) -> list[DataclassAttribute] | None:
             if not isinstance(lhs, NameExpr):
                 continue
 
-            sym = cls.info.names.get(lhs.name)
+            attr_name = lhs.name
+            sym = cls.info.names.get(attr_name)
             if sym is None:
                 # There was probably a semantic analysis error.
                 continue
 
             node = sym.node
-            assert not isinstance(node, PlaceholderNode)
+            if isinstance(node, PlaceholderNode):
+                continue
 
             if isinstance(node, TypeAlias):
                 self._api.fail(
@@ -608,7 +623,28 @@ def collect_attributes(self) -> list[DataclassAttribute] | None:
                 # This might be a property / field name clash.
                 # We will issue an error later.
                 continue
+            if not isinstance(node, Var):
+                if attr_name in redefined_attrs and len(redefined_attrs[attr_name]) > 1:
+                    if attr_name in last_def_with_type:
+                        continue
 
+                last_def = redefined_attrs.get(attr_name, [stmt])[-1]
+                if last_def.type is not None:
+                    var = Var(attr_name)
+                    var.is_property = False
+                    var.info = cls.info
+                    var.line = last_def.line
+                    var.column = last_def.column
+                    var.type = self._api.anal_type(last_def.type)
+                    cls.info.names[attr_name] = SymbolTableNode(MDEF, var)
+                    node = var
+                else:
+                    self._api.fail(
+                        f"Dataclass attribute '{attr_name}' cannot be a function. "
+                        f"Use a variable with type annotation instead.",
+                        stmt,
+                    )
+                    continue
             assert isinstance(node, Var), node
 
             # x: ClassVar[int] is ignored by dataclasses.

mypy/semanal_main.py

@@ -486,6 +486,7 @@ def apply_class_plugin_hooks(graph: Graph, scc: list[str], errors: Errors) -> No
     """
     num_passes = 0
     incomplete = True
+    already_processed: dict[TypeInfo, set[int]] = {}
     # If we encounter a base class that has not been processed, we'll run another
     # pass. This should eventually reach a fixed point.
     while incomplete:
@@ -498,6 +499,13 @@ def apply_class_plugin_hooks(graph: Graph, scc: list[str], errors: Errors) -> No
             assert tree
             for _, node, _ in tree.local_definitions():
                 if isinstance(node.node, TypeInfo):
+                    if node.node in already_processed:
+                        pass_count = len(already_processed[node.node])
+                        if pass_count >= 3 and num_passes > 3:
+                            continue
+                    else:
+                        already_processed[node.node] = set()
+                    already_processed[node.node].add(num_passes)
                     if not apply_hooks_to_class(
                         state.manager.semantic_analyzer,
                         module,

test-data/unit/check-dataclasses.test

@@ -2737,3 +2737,13 @@ class ClassB(ClassA):
     def value(self) -> int:
         return 0
 [builtins fixtures/dict.pyi]
+
+[case testDataclassNameCollisionNoCrash]
+from dataclasses import dataclass
+def fn(a: int) -> int:
+    return a
+@dataclass
+class Test:
+    foo = fn
+    foo: int = 42  # E: Name "foo" already defined on line 6
+[builtins fixtures/tuple.pyi]