Check typeguard validity for staticmethods (#14953)

I was checking through my previous contributions and noticed I messed up
the logic here. I added a test case that currently is wrong.

Followup to #14238

Author: A5rocks

mypy/semanal.py

@@ -867,11 +867,8 @@ def analyze_func_def(self, defn: FuncDef) -> None:
                 assert isinstance(result, ProperType)
                 if isinstance(result, CallableType):
                     # type guards need to have a positional argument, to spec
-                    if (
-                        result.type_guard
-                        and ARG_POS not in result.arg_kinds[self.is_class_scope() :]
-                        and not defn.is_static
-                    ):
+                    skip_self = self.is_class_scope() and not defn.is_static
+                    if result.type_guard and ARG_POS not in result.arg_kinds[skip_self:]:
                         self.fail(
                             "TypeGuard functions must have a positional argument",
                             result,

test-data/unit/check-typeguard.test

@@ -602,12 +602,30 @@ def func(names: Tuple[str, ...]):
 from typing_extensions import TypeGuard
 
 class Z:
-    def typeguard(self, *, x: object) -> TypeGuard[int]:  # E: TypeGuard functions must have a positional argument
+    def typeguard1(self, *, x: object) -> TypeGuard[int]:  # line 4
+        ...
+    
+    @staticmethod
+    def typeguard2(x: object) -> TypeGuard[int]:
+        ...
+    
+    @staticmethod  # line 11
+    def typeguard3(*, x: object) -> TypeGuard[int]:
         ...
 
-def bad_typeguard(*, x: object) -> TypeGuard[int]:  # E: TypeGuard functions must have a positional argument
+def bad_typeguard(*, x: object) -> TypeGuard[int]: # line 15
     ...
-[builtins fixtures/tuple.pyi]
+
+# In Python 3.8 the line number associated with FunctionDef nodes changed
+[builtins fixtures/classmethod.pyi]
+[out]
+main:4: error: TypeGuard functions must have a positional argument
+main:11: error: TypeGuard functions must have a positional argument
+main:15: error: TypeGuard functions must have a positional argument
+[out version>=3.8]
+main:4: error: TypeGuard functions must have a positional argument
+main:12: error: TypeGuard functions must have a positional argument
+main:15: error: TypeGuard functions must have a positional argument
 
 [case testTypeGuardWithKeywordArg]
 from typing_extensions import TypeGuard
@@ -640,7 +658,6 @@ if Y().typeguard(x):
     reveal_type(x)  # N: Revealed type is "builtins.int"
 if Y.typeguard(x):
     reveal_type(x)  # N: Revealed type is "builtins.int"
-[builtins fixtures/tuple.pyi]
 [builtins fixtures/classmethod.pyi]
 
 [case testTypeGuardKwargFollowingThroughOverloaded]