Add error when making abstractmethod final (#12743)

Fixes #12164

Author: nakatomotoi

mypy/semanal.py

@@ -1103,6 +1103,8 @@ def visit_decorator(self, dec: Decorator) -> None:
             dec.func.accept(self)
         if dec.decorators and dec.var.is_property:
             self.fail('Decorated property not supported', dec)
+        if dec.func.is_abstract and dec.func.is_final:
+            self.fail(f"Method {dec.func.name} is both abstract and final", dec)
 
     def check_decorated_function_is_method(self, decorator: str,
                                            context: Context) -> None:

test-data/unit/check-final.test

@@ -1091,3 +1091,21 @@ class A:
     b: ClassVar[Final[int]]  # E: Final can be only used as an outermost qualifier in a variable annotation
     c: ClassVar[Final] = 1  # E: Final can be only used as an outermost qualifier in a variable annotation
 [out]
+
+[case testFinalClassWithAbstractMethod]
+from typing import final
+from abc import ABC, abstractmethod
+
+@final
+class A(ABC): # E: Final class __main__.A has abstract attributes "B"
+    @abstractmethod
+    def B(self) -> None: ...
+
+[case testFinalDefiningFuncWithAbstractMethod]
+from typing import final
+from abc import ABC, abstractmethod
+
+class A(ABC):
+    @final # E: Method B is both abstract and final
+    @abstractmethod
+    def B(self) -> None: ...