Validate more about overrides on untyped methods

This commit fixes #9618 by making MyPy always complain if a method
overrides a base class method marked as `@final`.

In the process, it also adds a few additional validations:
- Always verify the `@override` decorator, which ought to be pretty
  backward-compatible for most projects assuming that strict override
  checks aren't enabled by default (and it appears to me that
  `--enable-error-code explicit-override` is off by default)
- Verify that the method signature is compatible (which in practice
  means only arity and argument name checks) *if* the
  `--check-untyped-defs` flag is set; it seems unlikely that a user
  would want mypy to validate the bodies of untyped functions but
  wouldn't want to be alerted about incompatible overrides.

Note: I did also explore enabling the signature compatibility check
for all code, which in principle makes sense. But the mypy_primer
results indicated that there would be backward compability issues
because too many libraries rely on us not validating this:
#17274

mypy/checker.py

@@ -1004,7 +1004,7 @@ def _visit_func_def(self, defn: FuncDef) -> None:
         """Type check a function definition."""
         self.check_func_item(defn, name=defn.name)
         if defn.info:
-            if not defn.is_dynamic() and not defn.is_overload and not defn.is_decorated:
+            if not defn.is_overload and not defn.is_decorated:
                 # If the definition is the implementation for an
                 # overload, the legality of the override has already
                 # been typechecked, and decorated methods will be
@@ -1913,9 +1913,17 @@ def check_method_override(
         Return a list of base classes which contain an attribute with the method name.
         """
         # Check against definitions in base classes.
+        check_override_compatibility = defn.name not in (
+            "__init__",
+            "__new__",
+            "__init_subclass__",
+            "__post_init__",
+        ) and (self.options.check_untyped_defs or not defn.is_dynamic())
         found_method_base_classes: list[TypeInfo] = []
         for base in defn.info.mro[1:]:
-            result = self.check_method_or_accessor_override_for_base(defn, base)
+            result = self.check_method_or_accessor_override_for_base(
+                defn, base, check_override_compatibility
+            )
             if result is None:
                 # Node was deferred, we will have another attempt later.
                 return None
@@ -1924,7 +1932,10 @@ def check_method_override(
         return found_method_base_classes
 
     def check_method_or_accessor_override_for_base(
-        self, defn: FuncDef | OverloadedFuncDef | Decorator, base: TypeInfo
+        self,
+        defn: FuncDef | OverloadedFuncDef | Decorator,
+        base: TypeInfo,
+        check_override_compatibility: bool,
     ) -> bool | None:
         """Check if method definition is compatible with a base class.
 
@@ -1945,10 +1956,8 @@ def check_method_or_accessor_override_for_base(
                 if defn.is_final:
                     self.check_if_final_var_override_writable(name, base_attr.node, defn)
                 found_base_method = True
-
-            # Check the type of override.
-            if name not in ("__init__", "__new__", "__init_subclass__", "__post_init__"):
-                # Check method override
+            if check_override_compatibility:
+                # Check compatibility of the override signature
                 # (__init__, __new__, __init_subclass__ are special).
                 if self.check_method_override_for_base_with_name(defn, name, base):
                     return None

mypy/nodes.py

@@ -615,6 +615,9 @@ def deserialize(cls, data: JsonDict) -> OverloadedFuncDef:
         # NOTE: res.info will be set in the fixup phase.
         return res
 
+    def is_dynamic(self) -> bool:
+        return all(item.is_dynamic() for item in self.items)
+
 
 class Argument(Node):
     """A single argument in a FuncItem."""
@@ -937,6 +940,9 @@ def deserialize(cls, data: JsonDict) -> Decorator:
         dec.is_overload = data["is_overload"]
         return dec
 
+    def is_dynamic(self) -> bool:
+        return self.func.is_dynamic()
+
 
 VAR_FLAGS: Final = [
     "is_self",

test-data/unit/check-dynamic-typing.test

@@ -756,6 +756,21 @@ main:5: note:          def f(self, x: A) -> None
 main:5: note:      Subclass:
 main:5: note:          def f(self, x: Any, y: Any) -> None
 
+[case testInvalidOverrideArgumentCountWithImplicitSignature4]
+# flags: --check-untyped-defs
+import typing
+class B:
+    def f(self, x: A) -> None: pass
+class A(B):
+    def f(self, x, y): # dynamic function not type checked
+        x()
+[out]
+main:6: error: Signature of "f" incompatible with supertype "B"
+main:6: note:      Superclass:
+main:6: note:          def f(self, x: A) -> None
+main:6: note:      Subclass:
+main:6: note:          def f(self, x: Any, y: Any) -> Any
+
 [case testInvalidOverrideWithImplicitSignatureAndClassMethod1]
 class B:
     @classmethod

test-data/unit/check-functions.test

@@ -3228,3 +3228,15 @@ class A:
 reveal_type(A.f)  # N: Revealed type is "__main__.something_callable"
 reveal_type(A().f)  # N: Revealed type is "builtins.str"
 [builtins fixtures/property.pyi]
+
+[case testFinalOverrideOnUntypedDef]
+from typing import final
+
+class Base:
+    @final
+    def foo(self):
+        pass
+
+class Derived(Base):
+    def foo(self):  # E: Cannot override final attribute "foo" (previously declared in base class "Base")
+        pass