Check Enum definition for invalid base classes (#12026)

Closes #11948

mypy/checker.py

@@ -1837,6 +1837,7 @@ def visit_class_def(self, defn: ClassDef) -> None:
             for base in defn.info.mro[1:-1]:  # we don't need self and `object`
                 if base.is_enum and base.fullname not in ENUM_BASES:
                     self.check_final_enum(defn, base)
+            self.check_enum_bases(defn)
 
     def check_final_deletable(self, typ: TypeInfo) -> None:
         # These checks are only for mypyc. Only perform some checks that are easier
@@ -1931,6 +1932,38 @@ def is_final_enum_value(self, sym: SymbolTableNode) -> bool:
             return True
         return False
 
+    def check_enum_bases(self, defn: ClassDef) -> None:
+        enum_base: Optional[Instance] = None
+        data_base: Optional[Instance] = None
+        for base in defn.info.bases:
+            if enum_base is None and base.type.fullname in ENUM_BASES:
+                enum_base = base
+                continue
+            elif enum_base is not None:
+                self.fail(
+                    'No base classes are allowed after "{}"'.format(enum_base),
+                    defn,
+                )
+                break
+
+            # This might not be 100% correct, because runtime `__new__`
+            # and `__new__` from `typeshed` are sometimes different,
+            # but it is good enough.
+            new_method = base.type.get('__new__')
+            if (data_base is None
+                    and enum_base is None  # data type is always before `Enum`
+                    and new_method
+                    and new_method.node
+                    and new_method.node.fullname != 'builtins.object.__new__'):
+                data_base = base
+                continue
+            elif data_base is not None:
+                self.fail(
+                    'Only a single data type mixin is allowed for Enum subtypes, '
+                    'found extra "{}"'.format(base),
+                    defn,
+                )
+
     def check_protocol_variance(self, defn: ClassDef) -> None:
         """Check that protocol definition is compatible with declared
         variances of type variables.

test-data/unit/check-enum.test

@@ -1883,6 +1883,71 @@ class SubWithOverload(WithOverload):  # Should pass
     pass
 [builtins fixtures/tuple.pyi]
 
+[case testEnumBaseClassesOrder]
+import enum
+
+# Base types:
+
+class First:
+    def __new__(cls, val):
+        pass
+
+class Second:
+    def __new__(cls, val):
+        pass
+
+class Third:
+    def __new__(cls, val):
+        pass
+
+class Mixin:
+    pass
+
+# Correct Enums:
+
+class Correct1(Mixin, First, enum.Enum):
+    pass
+
+class Correct2(First, enum.Enum):
+    pass
+
+class Correct3(Mixin, enum.Enum):
+    pass
+
+class RegularClass(Mixin, First, Second):
+    pass
+
+# Wrong Enums:
+
+class MixinAfterEnum1(enum.Enum, Mixin):  # E: No base classes are allowed after "enum.Enum"
+    pass
+
+class MixinAfterEnum2(First, enum.Enum, Mixin):  # E: No base classes are allowed after "enum.Enum"
+    pass
+
+class TwoDataTypes(First, Second, enum.Enum):  # E: Only a single data type mixin is allowed for Enum subtypes, found extra "__main__.Second"
+    pass
+
+class TwoDataTypesAndIntEnumMixin(First, Second, enum.IntEnum, Mixin):  # E: Only a single data type mixin is allowed for Enum subtypes, found extra "__main__.Second" \
+                                                                        # E: No base classes are allowed after "enum.IntEnum"
+    pass
+
+class ThreeDataTypes(First, Second, Third, enum.Enum):  # E: Only a single data type mixin is allowed for Enum subtypes, found extra "__main__.Second" \
+                                                        # E: # E: Only a single data type mixin is allowed for Enum subtypes, found extra "__main__.Third"
+    pass
+
+class ThreeDataTypesAndMixin(First, Second, Third, enum.Enum, Mixin): # E: Only a single data type mixin is allowed for Enum subtypes, found extra "__main__.Second" \
+                                                        # E: # E: Only a single data type mixin is allowed for Enum subtypes, found extra "__main__.Third" \
+                                                        # E: No base classes are allowed after "enum.Enum"
+    pass
+
+class FromEnumAndOther1(Correct2, Second, enum.Enum):  # E: Only a single data type mixin is allowed for Enum subtypes, found extra "__main__.Second"
+    pass
+
+class FromEnumAndOther2(Correct2, Second):  # E: Only a single data type mixin is allowed for Enum subtypes, found extra "__main__.Second"
+    pass
+[builtins fixtures/tuple.pyi]
+
 [case testEnumtValueUnionSimplification]
 from enum import IntEnum
 from typing import Any