[ty] Implement the legacy PEP-484 convention for indicating positional-only parameters (#20248)

Co-authored-by: Carl Meyer <carl@astral.sh>

crates/ty_python_semantic/src/ast_node_ref.rs

@@ -49,6 +49,12 @@ pub struct AstNodeRef<T> {
     _node: PhantomData<T>,
 }
 
+impl<T> AstNodeRef<T> {
+    pub(crate) fn index(&self) -> NodeIndex {
+        self.index
+    }
+}
+
 impl<T> AstNodeRef<T>
 where
     T: HasNodeIndex + Ranged + PartialEq + Debug,

crates/ty_python_semantic/src/node_key.rs

@@ -1,5 +1,7 @@
 use ruff_python_ast::{HasNodeIndex, NodeIndex};
 
+use crate::ast_node_ref::AstNodeRef;
+
 /// Compact key for a node for use in a hash map.
 #[derive(Copy, Clone, Debug, Eq, PartialEq, Hash, get_size2::GetSize)]
 pub(super) struct NodeKey(NodeIndex);
@@ -11,4 +13,8 @@ impl NodeKey {
     {
         NodeKey(node.node_index().load())
     }
+
+    pub(super) fn from_node_ref<T>(node_ref: &AstNodeRef<T>) -> Self {
+        NodeKey(node_ref.index())
+    }
 }

crates/ty_python_semantic/src/semantic_index.rs

@@ -159,7 +159,6 @@ pub(crate) fn attribute_scopes<'db, 's>(
     class_body_scope: ScopeId<'db>,
 ) -> impl Iterator<Item = FileScopeId> + use<'s, 'db> {
     let file = class_body_scope.file(db);
-    let module = parsed_module(db, file).load(db);
     let index = semantic_index(db, file);
     let class_scope_id = class_body_scope.file_scope_id(db);
 
@@ -175,7 +174,7 @@ pub(crate) fn attribute_scopes<'db, 's>(
                 (child_scope_id, scope)
             };
 
-        function_scope.node().as_function(&module)?;
+        function_scope.node().as_function()?;
         Some(function_scope_id)
     })
 }
@@ -332,6 +331,39 @@ impl<'db> SemanticIndex<'db> {
         Some(&self.scopes[self.parent_scope_id(scope_id)?])
     }
 
+    /// Return the [`Definition`] of the class enclosing this method, given the
+    /// method's body scope, or `None` if it is not a method.
+    pub(crate) fn class_definition_of_method(
+        &self,
+        function_body_scope: FileScopeId,
+    ) -> Option<Definition<'db>> {
+        let current_scope = self.scope(function_body_scope);
+        if current_scope.kind() != ScopeKind::Function {
+            return None;
+        }
+        let parent_scope_id = current_scope.parent()?;
+        let parent_scope = self.scope(parent_scope_id);
+
+        let class_scope = match parent_scope.kind() {
+            ScopeKind::Class => parent_scope,
+            ScopeKind::TypeParams => {
+                let class_scope_id = parent_scope.parent()?;
+                let potentially_class_scope = self.scope(class_scope_id);
+
+                match potentially_class_scope.kind() {
+                    ScopeKind::Class => potentially_class_scope,
+                    _ => return None,
+                }
+            }
+            _ => return None,
+        };
+
+        class_scope
+            .node()
+            .as_class()
+            .map(|node_ref| self.expect_single_definition(node_ref))
+    }
+
     fn is_scope_reachable(&self, db: &'db dyn Db, scope_id: FileScopeId) -> bool {
         self.parent_scope_id(scope_id)
             .is_none_or(|parent_scope_id| {

crates/ty_python_semantic/src/semantic_index/builder.rs

@@ -2644,7 +2644,7 @@ impl SemanticSyntaxContext for SemanticIndexBuilder<'_, '_> {
             match scope.kind() {
                 ScopeKind::Class | ScopeKind::Lambda => return false,
                 ScopeKind::Function => {
-                    return scope.node().expect_function(self.module).is_async;
+                    return scope.node().expect_function().node(self.module).is_async;
                 }
                 ScopeKind::Comprehension
                 | ScopeKind::Module

crates/ty_python_semantic/src/semantic_index/definition.rs

@@ -1227,3 +1227,12 @@ impl From<&ast::TypeParamTypeVarTuple> for DefinitionNodeKey {
         Self(NodeKey::from_node(value))
     }
 }
+
+impl<T> From<&AstNodeRef<T>> for DefinitionNodeKey
+where
+    for<'a> &'a T: Into<DefinitionNodeKey>,
+{
+    fn from(value: &AstNodeRef<T>) -> Self {
+        Self(NodeKey::from_node_ref(value))
+    }
+}

crates/ty_python_semantic/src/semantic_index/scope.rs

@@ -397,52 +397,38 @@ impl NodeWithScopeKind {
         }
     }
 
-    pub(crate) fn expect_class<'ast>(
-        &self,
-        module: &'ast ParsedModuleRef,
-    ) -> &'ast ast::StmtClassDef {
+    pub(crate) fn as_class(&self) -> Option<&AstNodeRef<ast::StmtClassDef>> {
         match self {
-            Self::Class(class) => class.node(module),
-            _ => panic!("expected class"),
-        }
-    }
-
-    pub(crate) fn as_class<'ast>(
-        &self,
-        module: &'ast ParsedModuleRef,
-    ) -> Option<&'ast ast::StmtClassDef> {
-        match self {
-            Self::Class(class) => Some(class.node(module)),
+            Self::Class(class) => Some(class),
             _ => None,
         }
     }
 
-    pub(crate) fn expect_function<'ast>(
-        &self,
-        module: &'ast ParsedModuleRef,
-    ) -> &'ast ast::StmtFunctionDef {
-        self.as_function(module).expect("expected function")
+    pub(crate) fn expect_class(&self) -> &AstNodeRef<ast::StmtClassDef> {
+        self.as_class().expect("expected class")
     }
 
-    pub(crate) fn expect_type_alias<'ast>(
-        &self,
-        module: &'ast ParsedModuleRef,
-    ) -> &'ast ast::StmtTypeAlias {
+    pub(crate) fn as_function(&self) -> Option<&AstNodeRef<ast::StmtFunctionDef>> {
         match self {
-            Self::TypeAlias(type_alias) => type_alias.node(module),
-            _ => panic!("expected type alias"),
-        }
-    }
-
-    pub(crate) fn as_function<'ast>(
-        &self,
-        module: &'ast ParsedModuleRef,
-    ) -> Option<&'ast ast::StmtFunctionDef> {
-        match self {
-            Self::Function(function) => Some(function.node(module)),
+            Self::Function(function) => Some(function),
             _ => None,
         }
     }
+
+    pub(crate) fn expect_function(&self) -> &AstNodeRef<ast::StmtFunctionDef> {
+        self.as_function().expect("expected function")
+    }
+
+    pub(crate) fn as_type_alias(&self) -> Option<&AstNodeRef<ast::StmtTypeAlias>> {
+        match self {
+            Self::TypeAlias(type_alias) => Some(type_alias),
+            _ => None,
+        }
+    }
+
+    pub(crate) fn expect_type_alias(&self) -> &AstNodeRef<ast::StmtTypeAlias> {
+        self.as_type_alias().expect("expected type alias")
+    }
 }
 
 #[derive(Copy, Clone, Debug, Eq, PartialEq, Hash, get_size2::GetSize)]

crates/ty_python_semantic/src/types.rs

@@ -5649,7 +5649,7 @@ impl<'db> Type<'db> {
                 SpecialFormType::TypingSelf => {
                     let module = parsed_module(db, scope_id.file(db)).load(db);
                     let index = semantic_index(db, scope_id.file(db));
-                    let Some(class) = nearest_enclosing_class(db, index, scope_id, &module) else {
+                    let Some(class) = nearest_enclosing_class(db, index, scope_id) else {
                         return Err(InvalidTypeExpressionError {
                             fallback_type: Type::unknown(),
                             invalid_expressions: smallvec::smallvec_inline![
@@ -9364,9 +9364,7 @@ fn walk_pep_695_type_alias<'db, V: visitor::TypeVisitor<'db> + ?Sized>(
 impl<'db> PEP695TypeAliasType<'db> {
     pub(crate) fn definition(self, db: &'db dyn Db) -> Definition<'db> {
         let scope = self.rhs_scope(db);
-        let module = parsed_module(db, scope.file(db)).load(db);
-        let type_alias_stmt_node = scope.node(db).expect_type_alias(&module);
-
+        let type_alias_stmt_node = scope.node(db).expect_type_alias();
         semantic_index(db, scope.file(db)).expect_single_definition(type_alias_stmt_node)
     }
 
@@ -9374,9 +9372,9 @@ impl<'db> PEP695TypeAliasType<'db> {
     pub(crate) fn value_type(self, db: &'db dyn Db) -> Type<'db> {
         let scope = self.rhs_scope(db);
         let module = parsed_module(db, scope.file(db)).load(db);
-        let type_alias_stmt_node = scope.node(db).expect_type_alias(&module);
+        let type_alias_stmt_node = scope.node(db).expect_type_alias();
         let definition = self.definition(db);
-        definition_expression_type(db, definition, &type_alias_stmt_node.value)
+        definition_expression_type(db, definition, &type_alias_stmt_node.node(&module).value)
     }
 
     fn normalized_impl(self, _db: &'db dyn Db, _visitor: &NormalizedVisitor<'db>) -> Self {

crates/ty_python_semantic/src/types/class.rs

@@ -1403,7 +1403,7 @@ impl<'db> ClassLiteral<'db> {
         let scope = self.body_scope(db);
         let file = scope.file(db);
         let parsed = parsed_module(db, file).load(db);
-        let class_def_node = scope.node(db).expect_class(&parsed);
+        let class_def_node = scope.node(db).expect_class().node(&parsed);
         class_def_node.type_params.as_ref().map(|type_params| {
             let index = semantic_index(db, scope.file(db));
             let definition = index.expect_single_definition(class_def_node);
@@ -1445,14 +1445,13 @@ impl<'db> ClassLiteral<'db> {
     /// query depends on the AST of another file (bad!).
     fn node<'ast>(self, db: &'db dyn Db, module: &'ast ParsedModuleRef) -> &'ast ast::StmtClassDef {
         let scope = self.body_scope(db);
-        scope.node(db).expect_class(module)
+        scope.node(db).expect_class().node(module)
     }
 
     pub(crate) fn definition(self, db: &'db dyn Db) -> Definition<'db> {
         let body_scope = self.body_scope(db);
-        let module = parsed_module(db, body_scope.file(db)).load(db);
         let index = semantic_index(db, body_scope.file(db));
-        index.expect_single_definition(body_scope.node(db).expect_class(&module))
+        index.expect_single_definition(body_scope.node(db).expect_class())
     }
 
     pub(crate) fn apply_specialization(
@@ -2870,8 +2869,8 @@ impl<'db> ClassLiteral<'db> {
         let class_table = place_table(db, class_body_scope);
 
         let is_valid_scope = |method_scope: ScopeId<'db>| {
-            if let Some(method_def) = method_scope.node(db).as_function(&module) {
-                let method_name = method_def.name.as_str();
+            if let Some(method_def) = method_scope.node(db).as_function() {
+                let method_name = method_def.node(&module).name.as_str();
                 if let Place::Type(Type::FunctionLiteral(method_type), _) =
                     class_symbol(db, class_body_scope, method_name).place
                 {
@@ -2946,20 +2945,22 @@ impl<'db> ClassLiteral<'db> {
             }
 
             // The attribute assignment inherits the reachability of the method which contains it
-            let is_method_reachable =
-                if let Some(method_def) = method_scope.node(db).as_function(&module) {
-                    let method = index.expect_single_definition(method_def);
-                    let method_place = class_table.symbol_id(&method_def.name).unwrap();
-                    class_map
-                        .all_reachable_symbol_bindings(method_place)
-                        .find_map(|bind| {
-                            (bind.binding.is_defined_and(|def| def == method))
-                                .then(|| class_map.binding_reachability(db, &bind))
-                        })
-                        .unwrap_or(Truthiness::AlwaysFalse)
-                } else {
-                    Truthiness::AlwaysFalse
-                };
+            let is_method_reachable = if let Some(method_def) = method_scope.node(db).as_function()
+            {
+                let method = index.expect_single_definition(method_def);
+                let method_place = class_table
+                    .symbol_id(&method_def.node(&module).name)
+                    .unwrap();
+                class_map
+                    .all_reachable_symbol_bindings(method_place)
+                    .find_map(|bind| {
+                        (bind.binding.is_defined_and(|def| def == method))
+                            .then(|| class_map.binding_reachability(db, &bind))
+                    })
+                    .unwrap_or(Truthiness::AlwaysFalse)
+            } else {
+                Truthiness::AlwaysFalse
+            };
             if is_method_reachable.is_always_false() {
                 continue;
             }
@@ -3323,7 +3324,7 @@ impl<'db> ClassLiteral<'db> {
     pub(super) fn header_range(self, db: &'db dyn Db) -> TextRange {
         let class_scope = self.body_scope(db);
         let module = parsed_module(db, class_scope.file(db)).load(db);
-        let class_node = class_scope.node(db).expect_class(&module);
+        let class_node = class_scope.node(db).expect_class().node(&module);
         let class_name = &class_node.name;
         TextRange::new(
             class_name.start(),
@@ -4784,8 +4785,7 @@ impl KnownClass {
                 //   2. The first parameter of the current function (typically `self` or `cls`)
                 match overload.parameter_types() {
                     [] => {
-                        let Some(enclosing_class) =
-                            nearest_enclosing_class(db, index, scope, module)
+                        let Some(enclosing_class) = nearest_enclosing_class(db, index, scope)
                         else {
                             BoundSuperError::UnavailableImplicitArguments
                                 .report_diagnostic(context, call_expression.into());

crates/ty_python_semantic/src/types/context.rs

@@ -172,7 +172,7 @@ impl<'db, 'ast> InferContext<'db, 'ast> {
                 // Inspect all ancestor function scopes by walking bottom up and infer the function's type.
                 let mut function_scope_tys = index
                     .ancestor_scopes(scope_id)
-                    .filter_map(|(_, scope)| scope.node().as_function(self.module()))
+                    .filter_map(|(_, scope)| scope.node().as_function())
                     .map(|node| binding_type(self.db, index.expect_single_definition(node)))
                     .filter_map(Type::into_function_literal);
 

crates/ty_python_semantic/src/types/display.rs

@@ -200,15 +200,15 @@ impl ClassDisplay<'_> {
 
             match ancestor_scope.kind() {
                 ScopeKind::Class => {
-                    if let Some(class_def) = node.as_class(&module_ast) {
-                        name_parts.push(class_def.name.as_str().to_string());
+                    if let Some(class_def) = node.as_class() {
+                        name_parts.push(class_def.node(&module_ast).name.as_str().to_string());
                     }
                 }
                 ScopeKind::Function => {
-                    if let Some(function_def) = node.as_function(&module_ast) {
+                    if let Some(function_def) = node.as_function() {
                         name_parts.push(format!(
                             "<locals of function '{}'>",
-                            function_def.name.as_str()
+                            function_def.node(&module_ast).name.as_str()
                         ));
                     }
                 }

crates/ty_python_semantic/src/types/function.rs

@@ -55,7 +55,7 @@ use bitflags::bitflags;
 use ruff_db::diagnostic::{Annotation, DiagnosticId, Severity, Span};
 use ruff_db::files::{File, FileRange};
 use ruff_db::parsed::{ParsedModuleRef, parsed_module};
-use ruff_python_ast as ast;
+use ruff_python_ast::{self as ast, ParameterWithDefault};
 use ruff_text_size::Ranged;
 
 use crate::module_resolver::{KnownModule, file_to_module};
@@ -63,7 +63,7 @@ use crate::place::{Boundness, Place, place_from_bindings};
 use crate::semantic_index::ast_ids::HasScopedUseId;
 use crate::semantic_index::definition::Definition;
 use crate::semantic_index::scope::ScopeId;
-use crate::semantic_index::semantic_index;
+use crate::semantic_index::{FileScopeId, SemanticIndex, semantic_index};
 use crate::types::call::{Binding, CallArguments};
 use crate::types::constraints::{ConstraintSet, Constraints};
 use crate::types::context::InferContext;
@@ -80,7 +80,7 @@ use crate::types::{
     BoundMethodType, BoundTypeVarInstance, CallableType, ClassBase, ClassLiteral, ClassType,
     DeprecatedInstance, DynamicType, FindLegacyTypeVarsVisitor, HasRelationToVisitor,
     IsEquivalentVisitor, KnownClass, NormalizedVisitor, SpecialFormType, Truthiness, Type,
-    TypeMapping, TypeRelation, UnionBuilder, all_members, walk_type_mapping,
+    TypeMapping, TypeRelation, UnionBuilder, all_members, binding_type, walk_type_mapping,
 };
 use crate::{Db, FxOrderSet, ModuleName, resolve_module};
 
@@ -236,6 +236,22 @@ impl<'db> OverloadLiteral<'db> {
         self.has_known_decorator(db, FunctionDecorators::OVERLOAD)
     }
 
+    /// Returns true if this overload is decorated with `@staticmethod`, or if it is implicitly a
+    /// staticmethod.
+    pub(crate) fn is_staticmethod(self, db: &dyn Db) -> bool {
+        self.has_known_decorator(db, FunctionDecorators::STATICMETHOD) || self.name(db) == "__new__"
+    }
+
+    /// Returns true if this overload is decorated with `@classmethod`, or if it is implicitly a
+    /// classmethod.
+    pub(crate) fn is_classmethod(self, db: &dyn Db) -> bool {
+        self.has_known_decorator(db, FunctionDecorators::CLASSMETHOD)
+            || matches!(
+                self.name(db).as_str(),
+                "__init_subclass__" | "__class_getitem__"
+            )
+    }
+
     fn node<'ast>(
         self,
         db: &dyn Db,
@@ -249,7 +265,7 @@ impl<'db> OverloadLiteral<'db> {
             the function is defined."
         );
 
-        self.body_scope(db).node(db).expect_function(module)
+        self.body_scope(db).node(db).expect_function().node(module)
     }
 
     /// Returns the [`FileRange`] of the function's name.
@@ -258,7 +274,8 @@ impl<'db> OverloadLiteral<'db> {
             self.file(db),
             self.body_scope(db)
                 .node(db)
-                .expect_function(module)
+                .expect_function()
+                .node(module)
                 .name
                 .range,
         )
@@ -274,9 +291,8 @@ impl<'db> OverloadLiteral<'db> {
     /// over-invalidation.
     fn definition(self, db: &'db dyn Db) -> Definition<'db> {
         let body_scope = self.body_scope(db);
-        let module = parsed_module(db, self.file(db)).load(db);
         let index = semantic_index(db, body_scope.file(db));
-        index.expect_single_definition(body_scope.node(db).expect_function(&module))
+        index.expect_single_definition(body_scope.node(db).expect_function())
     }
 
     /// Returns the overload immediately before this one in the AST. Returns `None` if there is no
@@ -290,7 +306,8 @@ impl<'db> OverloadLiteral<'db> {
         let use_id = self
             .body_scope(db)
             .node(db)
-            .expect_function(&module)
+            .expect_function()
+            .node(&module)
             .name
             .scoped_use_id(db, scope);
 
@@ -325,17 +342,79 @@ impl<'db> OverloadLiteral<'db> {
         db: &'db dyn Db,
         inherited_generic_context: Option<GenericContext<'db>>,
     ) -> Signature<'db> {
+        /// `self` or `cls` can be implicitly positional-only if:
+        /// - It is a method AND
+        /// - No parameters in the method use PEP-570 syntax AND
+        /// - It is not a `@staticmethod` AND
+        /// - `self`/`cls` is not explicitly positional-only using the PEP-484 convention AND
+        /// - Either the next parameter after `self`/`cls` uses the PEP-484 convention,
+        ///   or the enclosing class is a `Protocol` class
+        fn has_implicitly_positional_only_first_param<'db>(
+            db: &'db dyn Db,
+            literal: OverloadLiteral<'db>,
+            node: &ast::StmtFunctionDef,
+            scope: FileScopeId,
+            index: &SemanticIndex,
+        ) -> bool {
+            let parameters = &node.parameters;
+
+            if !parameters.posonlyargs.is_empty() {
+                return false;
+            }
+
+            let Some(first_param) = parameters.args.first() else {
+                return false;
+            };
+
+            if first_param.uses_pep_484_positional_only_convention() {
+                return false;
+            }
+
+            if literal.is_staticmethod(db) {
+                return false;
+            }
+
+            let Some(class_definition) = index.class_definition_of_method(scope) else {
+                return false;
+            };
+
+            // `self` and `cls` are always positional-only if the next parameter uses the
+            // PEP-484 convention.
+            if parameters
+                .args
+                .get(1)
+                .is_some_and(ParameterWithDefault::uses_pep_484_positional_only_convention)
+            {
+                return true;
+            }
+
+            // If there isn't any parameter other than `self`/`cls`,
+            // or there is but it isn't using the PEP-484 convention,
+            // then `self`/`cls` are only implicitly positional-only if
+            // it is a protocol class.
+            let class_type = binding_type(db, class_definition);
+            class_type
+                .to_class_type(db)
+                .is_some_and(|class| class.is_protocol(db))
+        }
+
         let scope = self.body_scope(db);
         let module = parsed_module(db, self.file(db)).load(db);
-        let function_stmt_node = scope.node(db).expect_function(&module);
+        let function_stmt_node = scope.node(db).expect_function().node(&module);
         let definition = self.definition(db);
+        let index = semantic_index(db, scope.file(db));
         let generic_context = function_stmt_node.type_params.as_ref().map(|type_params| {
-            let index = semantic_index(db, scope.file(db));
             GenericContext::from_type_params(db, index, definition, type_params)
         });
-
-        let index = semantic_index(db, scope.file(db));
-        let is_generator = scope.file_scope_id(db).is_generator_function(index);
+        let file_scope_id = scope.file_scope_id(db);
+        let is_generator = file_scope_id.is_generator_function(index);
+        let has_implicitly_positional_first_parameter = has_implicitly_positional_only_first_param(
+            db,
+            self,
+            function_stmt_node,
+            file_scope_id,
+            index,
+        );
 
         Signature::from_function(
             db,
@@ -344,6 +423,7 @@ impl<'db> OverloadLiteral<'db> {
             definition,
             function_stmt_node,
             is_generator,
+            has_implicitly_positional_first_parameter,
         )
     }
 
@@ -356,7 +436,7 @@ impl<'db> OverloadLiteral<'db> {
         let span = Span::from(function_scope.file(db));
         let node = function_scope.node(db);
         let module = parsed_module(db, self.file(db)).load(db);
-        let func_def = node.as_function(&module)?;
+        let func_def = node.as_function()?.node(&module);
         let range = parameter_index
             .and_then(|parameter_index| {
                 func_def
@@ -376,7 +456,7 @@ impl<'db> OverloadLiteral<'db> {
         let span = Span::from(function_scope.file(db));
         let node = function_scope.node(db);
         let module = parsed_module(db, self.file(db)).load(db);
-        let func_def = node.as_function(&module)?;
+        let func_def = node.as_function()?.node(&module);
         let return_type_range = func_def.returns.as_ref().map(|returns| returns.range());
         let mut signature = func_def.name.range.cover(func_def.parameters.range);
         if let Some(return_type_range) = return_type_range {
@@ -713,17 +793,15 @@ impl<'db> FunctionType<'db> {
     /// Returns true if this method is decorated with `@classmethod`, or if it is implicitly a
     /// classmethod.
     pub(crate) fn is_classmethod(self, db: &'db dyn Db) -> bool {
-        self.has_known_decorator(db, FunctionDecorators::CLASSMETHOD)
-            || matches!(
-                self.name(db).as_str(),
-                "__init_subclass__" | "__class_getitem__"
-            )
+        self.iter_overloads_and_implementation(db)
+            .any(|overload| overload.is_classmethod(db))
     }
 
     /// Returns true if this method is decorated with `@staticmethod`, or if it is implicitly a
     /// static method.
     pub(crate) fn is_staticmethod(self, db: &'db dyn Db) -> bool {
-        self.has_known_decorator(db, FunctionDecorators::STATICMETHOD) || self.name(db) == "__new__"
+        self.iter_overloads_and_implementation(db)
+            .any(|overload| overload.is_staticmethod(db))
     }
 
     /// If the implementation of this function is deprecated, returns the `@warnings.deprecated`.

crates/ty_python_semantic/src/types/infer.rs

@@ -422,12 +422,11 @@ pub(crate) fn nearest_enclosing_class<'db>(
     db: &'db dyn Db,
     semantic: &SemanticIndex<'db>,
     scope: ScopeId,
-    parsed: &ParsedModuleRef,
 ) -> Option<ClassLiteral<'db>> {
     semantic
         .ancestor_scopes(scope.file_scope_id(db))
         .find_map(|(_, ancestor_scope)| {
-            let class = ancestor_scope.node().as_class(parsed)?;
+            let class = ancestor_scope.node().as_class()?;
             let definition = semantic.expect_single_definition(class);
             infer_definition_types(db, definition)
                 .declaration_type(definition)
@@ -2418,29 +2417,7 @@ impl<'db, 'ast> TypeInferenceBuilder<'db, 'ast> {
     /// behaviour to the [`nearest_enclosing_class`] function.
     fn class_context_of_current_method(&self) -> Option<ClassType<'db>> {
         let current_scope_id = self.scope().file_scope_id(self.db());
-        let current_scope = self.index.scope(current_scope_id);
-        if current_scope.kind() != ScopeKind::Function {
-            return None;
-        }
-        let parent_scope_id = current_scope.parent()?;
-        let parent_scope = self.index.scope(parent_scope_id);
-
-        let class_scope = match parent_scope.kind() {
-            ScopeKind::Class => parent_scope,
-            ScopeKind::TypeParams => {
-                let class_scope_id = parent_scope.parent()?;
-                let potentially_class_scope = self.index.scope(class_scope_id);
-
-                match potentially_class_scope.kind() {
-                    ScopeKind::Class => potentially_class_scope,
-                    _ => return None,
-                }
-            }
-            _ => return None,
-        };
-
-        let class_stmt = class_scope.node().as_class(self.module())?;
-        let class_definition = self.index.expect_single_definition(class_stmt);
+        let class_definition = self.index.class_definition_of_method(current_scope_id)?;
         binding_type(self.db(), class_definition).to_class_type(self.db())
     }
 
@@ -2453,7 +2430,10 @@ impl<'db, 'ast> TypeInferenceBuilder<'db, 'ast> {
         if !current_scope.kind().is_non_lambda_function() {
             return None;
         }
-        current_scope.node().as_function(self.module())
+        current_scope
+            .node()
+            .as_function()
+            .map(|node_ref| node_ref.node(self.module()))
     }
 
     fn function_decorator_types<'a>(

crates/ty_python_semantic/src/types/signatures.rs

@@ -12,7 +12,7 @@
 
 use std::{collections::HashMap, slice::Iter};
 
-use itertools::EitherOrBoth;
+use itertools::{EitherOrBoth, Itertools};
 use smallvec::{SmallVec, smallvec_inline};
 
 use super::{DynamicType, Type, TypeVarVariance, definition_expression_type};
@@ -352,9 +352,14 @@ impl<'db> Signature<'db> {
         definition: Definition<'db>,
         function_node: &ast::StmtFunctionDef,
         is_generator: bool,
+        has_implicitly_positional_first_parameter: bool,
     ) -> Self {
-        let parameters =
-            Parameters::from_parameters(db, definition, function_node.parameters.as_ref());
+        let parameters = Parameters::from_parameters(
+            db,
+            definition,
+            function_node.parameters.as_ref(),
+            has_implicitly_positional_first_parameter,
+        );
         let return_ty = function_node.returns.as_ref().map(|returns| {
             let plain_return_ty = definition_expression_type(db, definition, returns.as_ref())
                 .apply_type_mapping(
@@ -1139,6 +1144,7 @@ impl<'db> Parameters<'db> {
         db: &'db dyn Db,
         definition: Definition<'db>,
         parameters: &ast::Parameters,
+        has_implicitly_positional_first_parameter: bool,
     ) -> Self {
         let ast::Parameters {
             posonlyargs,
@@ -1149,23 +1155,46 @@ impl<'db> Parameters<'db> {
             range: _,
             node_index: _,
         } = parameters;
+
         let default_type = |param: &ast::ParameterWithDefault| {
             param
                 .default()
                 .map(|default| definition_expression_type(db, definition, default))
         };
-        let positional_only = posonlyargs.iter().map(|arg| {
+
+        let pos_only_param = |param: &ast::ParameterWithDefault| {
             Parameter::from_node_and_kind(
                 db,
                 definition,
-                &arg.parameter,
+                &param.parameter,
                 ParameterKind::PositionalOnly {
-                    name: Some(arg.parameter.name.id.clone()),
-                    default_type: default_type(arg),
+                    name: Some(param.parameter.name.id.clone()),
+                    default_type: default_type(param),
                 },
             )
-        });
-        let positional_or_keyword = args.iter().map(|arg| {
+        };
+
+        let mut positional_only: Vec<Parameter> = posonlyargs.iter().map(pos_only_param).collect();
+
+        let mut pos_or_keyword_iter = args.iter();
+
+        // If there are no PEP-570 positional-only parameters, check for the legacy PEP-484 convention
+        // for denoting positional-only parameters (parameters that start with `__` and do not end with `__`)
+        if positional_only.is_empty() {
+            let pos_or_keyword_iter = pos_or_keyword_iter.by_ref();
+
+            if has_implicitly_positional_first_parameter {
+                positional_only.extend(pos_or_keyword_iter.next().map(pos_only_param));
+            }
+
+            positional_only.extend(
+                pos_or_keyword_iter
+                    .peeking_take_while(|param| param.uses_pep_484_positional_only_convention())
+                    .map(pos_only_param),
+            );
+        }
+
+        let positional_or_keyword = pos_or_keyword_iter.map(|arg| {
             Parameter::from_node_and_kind(
                 db,
                 definition,
@@ -1176,6 +1205,7 @@ impl<'db> Parameters<'db> {
                 },
             )
         });
+
         let variadic = vararg.as_ref().map(|arg| {
             Parameter::from_node_and_kind(
                 db,
@@ -1186,6 +1216,7 @@ impl<'db> Parameters<'db> {
                 },
             )
         });
+
         let keyword_only = kwonlyargs.iter().map(|arg| {
             Parameter::from_node_and_kind(
                 db,
@@ -1197,6 +1228,7 @@ impl<'db> Parameters<'db> {
                 },
             )
         });
+
         let keywords = kwarg.as_ref().map(|arg| {
             Parameter::from_node_and_kind(
                 db,
@@ -1207,8 +1239,10 @@ impl<'db> Parameters<'db> {
                 },
             )
         });
+
         Self::new(
             positional_only
+                .into_iter()
                 .chain(positional_or_keyword)
                 .chain(variadic)
                 .chain(keyword_only)