Fix Signature::bind_self

Author: sharkdp

crates/ty_python_semantic/resources/mdtest/call/callables_as_descriptors.md

@@ -117,18 +117,32 @@ from typing import Callable
 
 # TODO: this could use a generic signature, but we don't support
 # `ParamSpec` and solving of typevars inside `Callable` types.
-def memoize(f: Callable[[C, int], str]) -> Callable[[C, int], str]:
+def memoize(f: Callable[[C1, int], str]) -> Callable[[C1, int], str]:
     raise NotImplementedError
 
-class C:
+class C1:
     def method(self, x: int) -> str:
         return str(x)
 
     @memoize
     def method_decorated(self, x: int) -> str:
         return str(x)
 
-C().method(1)
+C1().method(1)
+
+C1().method_decorated(1)
+```
+
+This also works with an argumentless `Callable` annotation:
+
+```py
+def memoize2(f: Callable) -> Callable:
+    raise NotImplementedError
+
+class C2:
+    @memoize2
+    def method_decorated(self, x: int) -> str:
+        return str(x)
 
-C().method_decorated(1)
+C2().method_decorated(1)
 ```

crates/ty_python_semantic/src/types/signatures.rs

@@ -579,7 +579,13 @@ impl<'db> Signature<'db> {
     }
 
     pub(crate) fn bind_self(&self, db: &'db dyn Db, self_type: Option<Type<'db>>) -> Self {
-        let mut parameters = Parameters::new(self.parameters().iter().skip(1).cloned());
+        let mut parameters = self.parameters.iter().cloned().peekable();
+
+        if parameters.peek().is_some_and(Parameter::is_positional) {
+            parameters.next();
+        }
+
+        let mut parameters = Parameters::new(parameters);
         let mut return_ty = self.return_ty;
         if let Some(self_type) = self_type {
             parameters = parameters.apply_type_mapping_impl(