add support for two nested classes with the same name in multiple inheritance, better error message

Author: mkurnikov

mypy/checker.py

@@ -1605,6 +1605,16 @@ def check_multiple_inheritance(self, typ: TypeInfo) -> None:
                     if name in base2.names and base2 not in base.mro:
                         self.check_compatibility(name, base, base2, typ)
 
+    def _determine_type_of_class_member(self, sym: SymbolTableNode) -> Optional[Type]:
+        if sym.type is not None:
+            return sym.type
+        if isinstance(sym.node, FuncBase):
+            return self.function_type(sym.node)
+        if isinstance(sym.node, TypeInfo):
+            # nested class
+            return type_object_type(sym.node, self.named_type)
+        return None
+
     def check_compatibility(self, name: str, base1: TypeInfo,
                             base2: TypeInfo, ctx: Context) -> None:
         """Check if attribute name in base1 is compatible with base2 in multiple inheritance.
@@ -1618,12 +1628,12 @@ def check_compatibility(self, name: str, base1: TypeInfo,
             return
         first = base1[name]
         second = base2[name]
-        first_type = first.type
-        if first_type is None and isinstance(first.node, FuncBase):
-            first_type = self.function_type(first.node)
-        second_type = second.type
-        if second_type is None and isinstance(second.node, FuncBase):
-            second_type = self.function_type(second.node)
+        if isinstance(first.node, TypeInfo) and isinstance(second.node, TypeInfo):
+            # allow nested classes with the same name
+            return
+        first_type = self._determine_type_of_class_member(first)
+        second_type = self._determine_type_of_class_member(second)
+
         # TODO: What if some classes are generic?
         if (isinstance(first_type, FunctionLike) and
                 isinstance(second_type, FunctionLike)):

test-data/unit/check-multiple-inheritance.test

@@ -240,3 +240,37 @@ def dec(f: Callable[..., T]) -> Callable[..., T]:
 [out]
 main:3: error: Cannot determine type of 'f' in base class 'B'
 main:3: error: Cannot determine type of 'f' in base class 'C'
+
+[case testMultipleInheritanceNestedClassesWithSameNameAllowed]
+class Mixin1:
+    class Meta:
+        pass
+class Mixin2:
+    class Meta:
+        pass
+class A(Mixin1, Mixin2):
+    pass
+[out]
+
+[case testMultipleInheritanceFailIfNestedClassAndAttrHaveSameName]
+class Mixin1:
+    class Nested1:
+        pass
+class Mixin2:
+    Nested1: str
+class A(Mixin1, Mixin2):
+    pass
+[out]
+main:6: error: Definition of "Nested1" in base class "Mixin1" is incompatible with definition in base class "Mixin2"
+
+[case testMultipleInheritanceFailIfNestedClassAndFunctionHaveSameName]
+class Mixin1:
+    class Nested1:
+        pass
+class Mixin2:
+    def Nested1(self) -> str:
+        pass
+class A(Mixin1, Mixin2):
+    pass
+[out]
+main:7: error: Definition of "Nested1" in base class "Mixin1" is incompatible with definition in base class "Mixin2"