Generates error when staticmethod and classmethod decorators are both…

… used (#15118)

Fixes #14506

mypy/message_registry.py

@@ -268,6 +268,7 @@ def with_additional_msg(self, info: str) -> ErrorMessage:
 )
 CLASS_PATTERN_DUPLICATE_KEYWORD_PATTERN: Final = 'Duplicate keyword pattern "{}"'
 CLASS_PATTERN_UNKNOWN_KEYWORD: Final = 'Class "{}" has no attribute "{}"'
+CLASS_PATTERN_CLASS_OR_STATIC_METHOD: Final = "Cannot have both classmethod and staticmethod"
 MULTIPLE_ASSIGNMENTS_IN_PATTERN: Final = 'Multiple assignments to name "{}" in pattern'
 CANNOT_MODIFY_MATCH_ARGS: Final = 'Cannot assign to "__match_args__"'
 

mypy/semanal.py

@@ -1548,6 +1548,8 @@ def visit_decorator(self, dec: Decorator) -> None:
             self.fail("Only instance methods can be decorated with @property", dec)
         if dec.func.abstract_status == IS_ABSTRACT and dec.func.is_final:
             self.fail(f"Method {dec.func.name} is both abstract and final", dec)
+        if dec.func.is_static and dec.func.is_class:
+            self.fail(message_registry.CLASS_PATTERN_CLASS_OR_STATIC_METHOD, dec)
 
     def check_decorated_function_is_method(self, decorator: str, context: Context) -> None:
         if not self.type or self.is_func_scope():

test-data/unit/check-classes.test

@@ -1395,6 +1395,13 @@ main:8: note:          def f(cls) -> None
 main:8: note:      Subclass:
 main:8: note:          def f(self) -> None
 
+[case testClassMethodAndStaticMethod]
+class C:
+  @classmethod  # E: Cannot have both classmethod and staticmethod
+  @staticmethod
+  def foo(cls) -> None: pass
+[builtins fixtures/classmethod.pyi]
+
 -- Properties
 -- ----------