@@ -692,9 +692,33 @@ def get_class_decorator_hook(self, fullname: str
 
         The plugin can modify a TypeInfo _in place_ (for example add some generated
         methods to the symbol table). This hook is called after the class body was
-        semantically analyzed.
+        semantically analyzed, but *there may still be placeholders* (typically
+        caused by forward references).
 
-        The hook is called with full names of all class decorators, for example
+        NOTE: Usually get_class_decorator_hook_2 is the better option, since it
+              guarantees that there are no placeholders.
+
+        The hook is called with full names of all class decorators.
+
+        The hook can be called multiple times per class, so it must be
+        idempotent.
+        """
+        return None
+
+    def get_class_decorator_hook_2(self, fullname: str
+                                   ) -> Optional[Callable[[ClassDefContext], bool]]:
+        """Update class definition for given class decorators.
+
+        Similar to get_class_decorator_hook, but this runs in a later pass when
+        placeholders have been resolved.
+
+        The hook can return False if some base class hasn't been
+        processed yet using class hooks. It causes all class hooks
+        (that are run in this same pass) to be invoked another time for
+        the file(s) currently being processed.
+
+        The hook can be called multiple times per class, so it must be
+        idempotent.
         """
         return None
 
@@ -815,6 +839,10 @@ def get_class_decorator_hook(self, fullname: str
                                  ) -> Optional[Callable[[ClassDefContext], None]]:
         return self._find_hook(lambda plugin: plugin.get_class_decorator_hook(fullname))
 
+    def get_class_decorator_hook_2(self, fullname: str
+                                   ) -> Optional[Callable[[ClassDefContext], bool]]:
+        return self._find_hook(lambda plugin: plugin.get_class_decorator_hook_2(fullname))
+
     def get_metaclass_hook(self, fullname: str
                            ) -> Optional[Callable[[ClassDefContext], None]]:
         return self._find_hook(lambda plugin: plugin.get_metaclass_hook(fullname))
 
@@ -107,10 +107,19 @@ def expand_typevar_from_subtype(self, sub_type: TypeInfo) -> None:
 
 
 class DataclassTransformer:
+    """Implement the behavior of @dataclass.
+
+    Note that this may be executed multiple times on the same class, so
+    everything here must be idempotent.
+
+    This runs after the main semantic analysis pass, so you can assume that
+    there are no placeholders.
+    """
+
     def __init__(self, ctx: ClassDefContext) -> None:
         self._ctx = ctx
 
-    def transform(self) -> None:
+    def transform(self) -> bool:
         """Apply all the necessary transformations to the underlying
         dataclass so as to ensure it is fully type checked according
         to the rules in PEP 557.
@@ -119,12 +128,11 @@ def transform(self) -> None:
         info = self._ctx.cls.info
         attributes = self.collect_attributes()
         if attributes is None:
-            # Some definitions are not ready, defer() should be already called.
-            return
+            # Some definitions are not ready. We need another pass.
+            return False
         for attr in attributes:
             if attr.type is None:
-                ctx.api.defer()
-                return
+                return False
         decorator_arguments = {
             'init': _get_decorator_bool_argument(self._ctx, 'init', True),
             'eq': _get_decorator_bool_argument(self._ctx, 'eq', True),
@@ -236,6 +244,8 @@ def transform(self) -> None:
             'frozen': decorator_arguments['frozen'],
         }
 
+        return True
+
     def add_slots(self,
                   info: TypeInfo,
                   attributes: List[DataclassAttribute],
@@ -294,6 +304,9 @@ def collect_attributes(self) -> Optional[List[DataclassAttribute]]:
           b: SomeOtherType = ...
 
         are collected.
+
+        Return None if some dataclass base class hasn't been processed
+        yet and thus we'll need to ask for another pass.
         """
         # First, collect attributes belonging to the current class.
         ctx = self._ctx
@@ -315,14 +328,11 @@ def collect_attributes(self) -> Optional[List[DataclassAttribute]]:
 
             sym = cls.info.names.get(lhs.name)
             if sym is None:
-                # This name is likely blocked by a star import. We don't need to defer because
-                # defer() is already called by mark_incomplete().
+                # There was probably a semantic analysis error.
                 continue
 
             node = sym.node
-            if isinstance(node, PlaceholderNode):
-                # This node is not ready yet.
-                return None
+            assert not isinstance(node, PlaceholderNode)
             assert isinstance(node, Var)
 
             # x: ClassVar[int] is ignored by dataclasses.
@@ -390,6 +400,9 @@ def collect_attributes(self) -> Optional[List[DataclassAttribute]]:
         # we'll have unmodified attrs laying around.
         all_attrs = attrs.copy()
         for info in cls.info.mro[1:-1]:
+            if 'dataclass_tag' in info.metadata and 'dataclass' not in info.metadata:
+                # We haven't processed the base class yet. Need another pass.
+                return None
             if 'dataclass' not in info.metadata:
                 continue
 
@@ -517,11 +530,21 @@ def _add_dataclass_fields_magic_attribute(self) -> None:
         )
 
 
-def dataclass_class_maker_callback(ctx: ClassDefContext) -> None:
+def dataclass_tag_callback(ctx: ClassDefContext) -> None:
+    """Record that we have a dataclass in the main semantic analysis pass.
+
+    The later pass implemented by DataclassTransformer will use this
+    to detect dataclasses in base classes.
+    """
+    # The value is ignored, only the existence matters.
+    ctx.cls.info.metadata['dataclass_tag'] = {}
+
+
+def dataclass_class_maker_callback(ctx: ClassDefContext) -> bool:
     """Hooks into the class typechecking process to add support for dataclasses.
     """
     transformer = DataclassTransformer(ctx)
-    transformer.transform()
+    return transformer.transform()
 
 
 def _collect_field_args(expr: Expression,
 
@@ -117,12 +117,21 @@ def get_class_decorator_hook(self, fullname: str
                 auto_attribs_default=None,
             )
         elif fullname in dataclasses.dataclass_makers:
-            return dataclasses.dataclass_class_maker_callback
+            return dataclasses.dataclass_tag_callback
         elif fullname in functools.functools_total_ordering_makers:
             return functools.functools_total_ordering_maker_callback
 
         return None
 
+    def get_class_decorator_hook_2(self, fullname: str
+                                   ) -> Optional[Callable[[ClassDefContext], bool]]:
+        from mypy.plugins import dataclasses
+
+        if fullname in dataclasses.dataclass_makers:
+            return dataclasses.dataclass_class_maker_callback
+
+        return None
+
 
 def contextmanager_callback(ctx: FunctionContext) -> Type:
     """Infer a better return type for 'contextlib.contextmanager'."""
 
@@ -1234,43 +1234,44 @@ def analyze_namedtuple_classdef(self, defn: ClassDef) -> bool:
 
     def apply_class_plugin_hooks(self, defn: ClassDef) -> None:
         """Apply a plugin hook that may infer a more precise definition for a class."""
-        def get_fullname(expr: Expression) -> Optional[str]:
-            if isinstance(expr, CallExpr):
-                return get_fullname(expr.callee)
-            elif isinstance(expr, IndexExpr):
-                return get_fullname(expr.base)
-            elif isinstance(expr, RefExpr):
-                if expr.fullname:
-                    return expr.fullname
-                # If we don't have a fullname look it up. This happens because base classes are
-                # analyzed in a different manner (see exprtotype.py) and therefore those AST
-                # nodes will not have full names.
-                sym = self.lookup_type_node(expr)
-                if sym:
-                    return sym.fullname
-            return None
 
         for decorator in defn.decorators:
-            decorator_name = get_fullname(decorator)
+            decorator_name = self.get_fullname_for_hook(decorator)
             if decorator_name:
                 hook = self.plugin.get_class_decorator_hook(decorator_name)
                 if hook:
                     hook(ClassDefContext(defn, decorator, self))
 
         if defn.metaclass:
-            metaclass_name = get_fullname(defn.metaclass)
+            metaclass_name = self.get_fullname_for_hook(defn.metaclass)
             if metaclass_name:
                 hook = self.plugin.get_metaclass_hook(metaclass_name)
                 if hook:
                     hook(ClassDefContext(defn, defn.metaclass, self))
 
         for base_expr in defn.base_type_exprs:
-            base_name = get_fullname(base_expr)
+            base_name = self.get_fullname_for_hook(base_expr)
             if base_name:
                 hook = self.plugin.get_base_class_hook(base_name)
                 if hook:
                     hook(ClassDefContext(defn, base_expr, self))
 
+    def get_fullname_for_hook(self, expr: Expression) -> Optional[str]:
+        if isinstance(expr, CallExpr):
+            return self.get_fullname_for_hook(expr.callee)
+        elif isinstance(expr, IndexExpr):
+            return self.get_fullname_for_hook(expr.base)
+        elif isinstance(expr, RefExpr):
+            if expr.fullname:
+                return expr.fullname
+            # If we don't have a fullname look it up. This happens because base classes are
+            # analyzed in a different manner (see exprtotype.py) and therefore those AST
+            # nodes will not have full names.
+            sym = self.lookup_type_node(expr)
+            if sym:
+                return sym.fullname
+        return None
+
     def analyze_class_keywords(self, defn: ClassDef) -> None:
         for value in defn.keywords.values():
             value.accept(self)
 
@@ -45,6 +45,8 @@
 from mypy.checker import FineGrainedDeferredNode
 from mypy.server.aststrip import SavedAttributes
 from mypy.util import is_typeshed_file
+from mypy.options import Options
+from mypy.plugin import ClassDefContext
 import mypy.build
 
 if TYPE_CHECKING:
@@ -82,6 +84,8 @@ def semantic_analysis_for_scc(graph: 'Graph', scc: List[str], errors: Errors) ->
     apply_semantic_analyzer_patches(patches)
     # This pass might need fallbacks calculated above.
     check_type_arguments(graph, scc, errors)
+    # Run class decorator hooks (they requite complete MROs and no placeholders).
+    apply_class_plugin_hooks(graph, scc, errors)
     calculate_class_properties(graph, scc, errors)
     check_blockers(graph, scc)
     # Clean-up builtins, so that TypeVar etc. are not accessible without importing.
@@ -132,6 +136,7 @@ def semantic_analysis_for_targets(
 
     check_type_arguments_in_targets(nodes, state, state.manager.errors)
     calculate_class_properties(graph, [state.id], state.manager.errors)
+    apply_class_plugin_hooks(graph, [state.id], state.manager.errors)
 
 
 def restore_saved_attrs(saved_attrs: SavedAttributes) -> None:
@@ -382,14 +387,62 @@ def check_type_arguments_in_targets(targets: List[FineGrainedDeferredNode], stat
                     target.node.accept(analyzer)
 
 
+def apply_class_plugin_hooks(graph: 'Graph', scc: List[str], errors: Errors) -> None:
+    """Apply class plugin hooks within a SCC.
+
+    We run these after to the main semantic analysis so that the hooks
+    don't need to deal with incomplete definitions such as placeholder
+    types.
+
+    Note that some hooks incorrectly run during the main semantic
+    analysis pass, for historical reasons.
+    """
+    num_passes = 0
+    incomplete = True
+    # 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:
+        assert num_passes < 10, "Internal error: too many class plugin hook passes"
+        num_passes += 1
+        incomplete = False
+        for module in scc:
+            state = graph[module]
+            tree = state.tree
+            assert tree
+            for _, node, _ in tree.local_definitions():
+                if isinstance(node.node, TypeInfo):
+                    if not apply_hooks_to_class(state.manager.semantic_analyzer,
+                                                module, node.node, state.options, tree, errors):
+                        incomplete = True
+
+
+def apply_hooks_to_class(self: SemanticAnalyzer,
+                         module: str,
+                         info: TypeInfo,
+                         options: Options,
+                         file_node: MypyFile,
+                         errors: Errors) -> bool:
+    # TODO: Move more class-related hooks here?
+    defn = info.defn
+    ok = True
+    for decorator in defn.decorators:
+        with self.file_context(file_node, options, info):
+            decorator_name = self.get_fullname_for_hook(decorator)
+            if decorator_name:
+                hook = self.plugin.get_class_decorator_hook_2(decorator_name)
+                if hook:
+                    ok = ok and hook(ClassDefContext(defn, decorator, self))
+    return ok
+
+
 def calculate_class_properties(graph: 'Graph', scc: List[str], errors: Errors) -> None:
     for module in scc:
-        tree = graph[module].tree
+        state = graph[module]
+        tree = state.tree
         assert tree
         for _, node, _ in tree.local_definitions():
             if isinstance(node.node, TypeInfo):
-                saved = (module, node.node, None)  # module, class, function
-                with errors.scope.saved_scope(saved) if errors.scope else nullcontext():
+                with state.manager.semantic_analyzer.file_context(tree, state.options, node.node):
                     calculate_class_abstract_status(node.node, tree.is_stub, errors)
                     check_protocol_status(node.node, errors)
                     calculate_class_vars(node.node)