[ty] Don't add incorrect subdiagnostic for unresolved reference (#18487)

Co-authored-by: Alex Waygood <Alex.Waygood@Gmail.com>
Co-authored-by: Carl Meyer <carl@astral.sh>

crates/ty_python_semantic/resources/mdtest/attributes.md

@@ -946,9 +946,9 @@ def _(flag1: bool, flag2: bool):
 
 <!-- snapshot-diagnostics -->
 
-If a non-declared variable is used and an attribute with the same name is defined and accessible,
-then we emit a subdiagnostic suggesting the use of `self.`.
-(`An attribute with the same name as 'x' is defined, consider using 'self.x'` in these cases)
+If an undefined variable is used in a method, and an attribute with the same name is defined and
+accessible, then we emit a subdiagnostic suggesting the use of `self.`. (These don't appear inline
+here; see the diagnostic snapshots.)
 
 ```py
 class Foo:
@@ -978,6 +978,107 @@ class Foo:
         y = x
 ```
 
+In a staticmethod, we don't suggest that it might be an attribute.
+
+```py
+class Foo:
+    def __init__(self):
+        self.x = 42
+
+    @staticmethod
+    def static_method():
+        # error: [unresolved-reference] "Name `x` used when not defined"
+        y = x
+```
+
+In a classmethod, if the name matches a class attribute, we suggest `cls.`.
+
+```py
+from typing import ClassVar
+
+class Foo:
+    x: ClassVar[int] = 42
+
+    @classmethod
+    def class_method(cls):
+        # error: [unresolved-reference] "Name `x` used when not defined"
+        y = x
+```
+
+In a classmethod, if the name matches an instance-only attribute, we don't suggest anything.
+
+```py
+class Foo:
+    def __init__(self):
+        self.x = 42
+
+    @classmethod
+    def class_method(cls):
+        # error: [unresolved-reference] "Name `x` used when not defined"
+        y = x
+```
+
+We also don't suggest anything if the method is (invalidly) decorated with both `@classmethod` and
+`@staticmethod`:
+
+```py
+class Foo:
+    x: ClassVar[int]
+
+    @classmethod
+    @staticmethod
+    def class_method(cls):
+        # error: [unresolved-reference] "Name `x` used when not defined"
+        y = x
+```
+
+In an instance method that uses some other parameter name in place of `self`, we use that parameter
+name in the sub-diagnostic.
+
+```py
+class Foo:
+    def __init__(self):
+        self.x = 42
+
+    def method(other):
+        # error: [unresolved-reference] "Name `x` used when not defined"
+        y = x
+```
+
+In a classmethod that uses some other parameter name in place of `cls`, we use that parameter name
+in the sub-diagnostic.
+
+```py
+from typing import ClassVar
+
+class Foo:
+    x: ClassVar[int] = 42
+
+    @classmethod
+    def class_method(c_other):
+        # error: [unresolved-reference] "Name `x` used when not defined"
+        y = x
+```
+
+We don't suggest anything if an instance method or a classmethod only has variadic arguments, or if
+the first parameter is keyword-only:
+
+```py
+from typing import ClassVar
+
+class Foo:
+    x: ClassVar[int] = 42
+
+    def instance_method(*args, **kwargs):
+        # error: [unresolved-reference] "Name `x` used when not defined"
+        print(x)
+
+    @classmethod
+    def class_method(*, cls):
+        # error: [unresolved-reference] "Name `x` used when not defined"
+        y = x
+```
+
 ## Unions of attributes
 
 If the (meta)class is a union type or if the attribute on the (meta) class has a union type, we