Skip error if attribute or method is private to class

mypy/checker.py

@@ -1504,6 +1504,9 @@ def check_override(self, override: FunctionLike, original: FunctionLike,
             if original_class_or_static and not override_class_or_static:
                 fail = True
 
+        if is_private(name):
+            fail = False
+
         if fail:
             emitted_msg = False
             if (isinstance(override, CallableType) and
@@ -1940,6 +1943,9 @@ def check_compatibility_all_supers(self, lvalue: RefExpr, lvalue_type: Optional[
                 if lvalue_node.name() == "__slots__" and base.fullname() != "builtins.object":
                     continue
 
+                if is_private(lvalue_node.name()):
+                    continue
+
                 base_type, base_node = self.lvalue_type_from_base(lvalue_node, base)
 
                 if base_type:
@@ -4396,3 +4402,8 @@ def is_subtype_no_promote(left: Type, right: Type) -> bool:
 
 def is_overlapping_types_no_promote(left: Type, right: Type) -> bool:
     return is_overlapping_types(left, right, ignore_promotions=True)
+
+
+def is_private(node_name: str) -> bool:
+    """Check if node is private to class definition."""
+    return node_name.startswith('__') and not node_name.endswith('__')

test-data/unit/check-classes.test

@@ -5899,3 +5899,25 @@ class A:
 class B:
     @dec  # E: Unsupported decorated constructor type
     def __new__(cls, x: int) -> B: ...
+
+[case testIgnorePrivateAttributesTypeCheck]
+class B:
+    __foo_: int
+class C(B):
+    __foo_: str
+[out]
+
+[case testIgnorePrivateMethodsTypeCheck]
+class B:
+    def __foo_(self) -> int: ...
+class C(B):
+    def __foo_(self) -> str: ...
+[out]
+
+[case testCheckForPrivateMethodsWhenPublicCheck]
+class B:
+    __foo__: int
+class C(B):
+    __foo__: str
+[out]
+main:4: error: Incompatible types in assignment (expression has type "str", base class "B" defined the type as "int")