[ty] Assume type of self is typing.Self in method calls (#18007)

Part of https://github.com/astral-sh/ty/issues/159

This PR only adjusts the signature of a method so if it has a `self`
argument then that argument will have type of `Typing.Self` even if it's
not specified. If user provides an explicit annotation then Ty will not
override that annotation.

- https://github.com/astral-sh/ty/issues/1131
- https://github.com/astral-sh/ty/issues/1157
- https://github.com/astral-sh/ty/issues/1156
- https://github.com/astral-sh/ty/issues/1173
- https://github.com/astral-sh/ruff/pull/20328
- https://github.com/astral-sh/ty/issues/1163
- https://github.com/astral-sh/ty/issues/1196

Added mdtests.
Also some tests need https://github.com/astral-sh/ruff/pull/18473 to
work completely. So I added a todo for those new cases that I added.

---------

Co-authored-by: David Peter <mail@david-peter.de>

crates/ruff_python_ast/src/nodes.rs

@@ -3030,6 +3030,12 @@ impl Parameters {
             .find(|arg| arg.parameter.name.as_str() == name)
     }
 
+    /// Returns the index of the parameter with the given name
+    pub fn index(&self, name: &str) -> Option<usize> {
+        self.iter_non_variadic_params()
+            .position(|arg| arg.parameter.name.as_str() == name)
+    }
+
     /// Returns an iterator over all parameters included in this [`Parameters`] node.
     pub fn iter(&self) -> ParametersIterator<'_> {
         ParametersIterator::new(self)

crates/ty_python_semantic/resources/mdtest/annotations/self.md

@@ -33,11 +33,6 @@ class Shape:
             reveal_type(x)  # revealed: Self@nested_func_without_enclosing_binding
         inner(self)
 
-    def implicit_self(self) -> Self:
-        # TODO: first argument in a method should be considered as "typing.Self"
-        reveal_type(self)  # revealed: Unknown
-        return self
-
 reveal_type(Shape().nested_type())  # revealed: list[Shape]
 reveal_type(Shape().nested_func())  # revealed: Shape
 
@@ -53,6 +48,104 @@ class Outer:
             return self
 ```
 
+## Detection of implicit Self
+
+In instance methods, the first parameter (regardless of its name) is assumed to have type
+`typing.Self` unless it has an explicit annotation. This does not apply to `@classmethod` and
+`@staticmethod`.
+
+```toml
+[environment]
+python-version = "3.11"
+```
+
+```py
+from typing import Self
+
+class A:
+    def implicit_self(self) -> Self:
+        # TODO: first argument in a method should be considered as "typing.Self"
+        reveal_type(self)  # revealed: Unknown
+        return self
+
+    def foo(self) -> int:
+        def first_arg_is_not_self(a: int) -> int:
+            return a
+        return first_arg_is_not_self(1)
+
+    @classmethod
+    def bar(cls): ...
+    @staticmethod
+    def static(x): ...
+
+a = A()
+# TODO: Should reveal Self@implicit_self. Requires implicit self in method body(https://github.com/astral-sh/ruff/pull/18473)
+reveal_type(a.implicit_self())  # revealed: A
+reveal_type(a.implicit_self)  # revealed: bound method A.implicit_self() -> A
+```
+
+If the method is a class or static method then first argument is not self:
+
+```py
+A.bar()
+a.static(1)
+```
+
+"self" name is not special; any first parameter name is treated as Self.
+
+```py
+from typing import Self, Generic, TypeVar
+
+T = TypeVar("T")
+
+class B:
+    def implicit_this(this) -> Self:
+        # TODO: Should reveal Self@implicit_this
+        reveal_type(this)  # revealed: Unknown
+        return this
+
+    def ponly(self, /, x: int) -> None:
+        # TODO: Should reveal Self@ponly
+        reveal_type(self)  # revealed: Unknown
+
+    def kwonly(self, *, x: int) -> None:
+        # TODO: Should reveal Self@kwonly
+        reveal_type(self)  # revealed: Unknown
+
+    @property
+    def name(self) -> str:
+        # TODO: Should reveal Self@name
+        reveal_type(self)  # revealed: Unknown
+        return "b"
+
+B.ponly(B(), 1)
+B.name
+B.kwonly(B(), x=1)
+
+class G(Generic[T]):
+    def id(self) -> Self:
+        # TODO: Should reveal Self@id
+        reveal_type(self)  # revealed: Unknown
+        return self
+
+g = G[int]()
+
+# TODO: Should reveal Self@id Requires implicit self in method body(https://github.com/astral-sh/ruff/pull/18473)
+reveal_type(G[int].id(g))  # revealed: G[int]
+```
+
+Free functions and nested functions do not use implicit `Self`:
+
+```py
+def not_a_method(self):
+    reveal_type(self)  # revealed: Unknown
+
+class C:
+    def outer(self) -> None:
+        def inner(self):
+            reveal_type(self)  # revealed: Unknown
+```
+
 ## typing_extensions
 
 ```toml
@@ -208,6 +301,53 @@ class MyMetaclass(type):
         return super().__new__(cls)
 ```
 
+## Explicit Annotation Overrides Implicit `Self`
+
+If the first parameter is explicitly annotated, that annotation takes precedence over the implicit
+`Self` treatment.
+
+```toml
+[environment]
+python-version = "3.11"
+```
+
+```py
+class Explicit:
+    # TODO: Should warn the user if self is overriden with a type that is not subtype of the class
+    def bad(self: int) -> None:
+        reveal_type(self)  # revealed: int
+
+    def forward(self: "Explicit") -> None:
+        reveal_type(self)  # revealed: Explicit
+
+e = Explicit()
+# error: [invalid-argument-type] "Argument to bound method `bad` is incorrect: Expected `int`, found `Explicit`"
+e.bad()
+```
+
+## Type of Implicit Self
+
+The assigned type to self argument depends on the method signature. When the method is defined in a
+non-generic class and has no other mention of `typing.Self` (for example in return type) then type
+of `self` is instance of the class.
+
+```py
+from typing import Self
+
+class C:
+    def f(self) -> Self:
+        return self
+
+    def z(self) -> None: ...
+
+C.z(1)  # error: [invalid-argument-type] "Argument to function `z` is incorrect: Expected `C`, found `Literal[1]`"
+```
+
+```py
+# error: [invalid-argument-type] "Argument to function `f` is incorrect: Argument type `Literal[1]` does not satisfy upper bound `C` of type variable `Self`"
+C.f(1)
+```
+
 ## Binding a method fixes `Self`
 
 When a method is bound, any instances of `Self` in its signature are "fixed", since we now know the

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

@@ -69,7 +69,9 @@ reveal_type(bound_method(1))  # revealed: str
 When we call the function object itself, we need to pass the `instance` explicitly:
 
 ```py
-C.f(1)  # error: [missing-argument]
+# error: [invalid-argument-type] "Argument to function `f` is incorrect: Expected `C`, found `Literal[1]`"
+# error: [missing-argument]
+C.f(1)
 
 reveal_type(C.f(C(), 1))  # revealed: str
 ```

crates/ty_python_semantic/resources/mdtest/descriptor_protocol.md

@@ -431,6 +431,8 @@ def _(flag: bool):
     reveal_type(C7.union_of_class_data_descriptor_and_attribute)  # revealed: Literal["data", 2]
 
     C7.union_of_metaclass_attributes = 2 if flag else 1
+    # TODO: https://github.com/astral-sh/ty/issues/1163
+    # error: [invalid-assignment]
     C7.union_of_metaclass_data_descriptor_and_attribute = 2 if flag else 100
     C7.union_of_class_attributes = 2 if flag else 1
     C7.union_of_class_data_descriptor_and_attribute = 2 if flag else DataDescriptor()

crates/ty_python_semantic/resources/mdtest/generics/legacy/classes.md

@@ -562,17 +562,17 @@ class C(Generic[T]):
         return u
 
 reveal_type(generic_context(C))  # revealed: tuple[T@C]
-reveal_type(generic_context(C.method))  # revealed: None
+reveal_type(generic_context(C.method))  # revealed: tuple[Self@method]
-reveal_type(generic_context(C.generic_method))  # revealed: tuple[U@generic_method]
+reveal_type(generic_context(C.generic_method))  # revealed: tuple[Self@generic_method, U@generic_method]
 reveal_type(generic_context(C[int]))  # revealed: None
-reveal_type(generic_context(C[int].method))  # revealed: None
+reveal_type(generic_context(C[int].method))  # revealed: tuple[Self@method]
-reveal_type(generic_context(C[int].generic_method))  # revealed: tuple[U@generic_method]
+reveal_type(generic_context(C[int].generic_method))  # revealed: tuple[Self@generic_method, U@generic_method]
 
 c: C[int] = C[int]()
 reveal_type(c.generic_method(1, "string"))  # revealed: Literal["string"]
 reveal_type(generic_context(c))  # revealed: None
-reveal_type(generic_context(c.method))  # revealed: None
+reveal_type(generic_context(c.method))  # revealed: tuple[Self@method]
-reveal_type(generic_context(c.generic_method))  # revealed: tuple[U@generic_method]
+reveal_type(generic_context(c.generic_method))  # revealed: tuple[Self@generic_method, U@generic_method]
 ```
 
 ## Specializations propagate

crates/ty_python_semantic/resources/mdtest/generics/pep695/classes.md

@@ -504,17 +504,17 @@ class C[T]:
     def cannot_shadow_class_typevar[T](self, t: T): ...
 
 reveal_type(generic_context(C))  # revealed: tuple[T@C]
-reveal_type(generic_context(C.method))  # revealed: None
+reveal_type(generic_context(C.method))  # revealed: tuple[Self@method]
-reveal_type(generic_context(C.generic_method))  # revealed: tuple[U@generic_method]
+reveal_type(generic_context(C.generic_method))  # revealed: tuple[Self@generic_method, U@generic_method]
 reveal_type(generic_context(C[int]))  # revealed: None
-reveal_type(generic_context(C[int].method))  # revealed: None
+reveal_type(generic_context(C[int].method))  # revealed: tuple[Self@method]
-reveal_type(generic_context(C[int].generic_method))  # revealed: tuple[U@generic_method]
+reveal_type(generic_context(C[int].generic_method))  # revealed: tuple[Self@generic_method, U@generic_method]
 
 c: C[int] = C[int]()
 reveal_type(c.generic_method(1, "string"))  # revealed: Literal["string"]
 reveal_type(generic_context(c))  # revealed: None
-reveal_type(generic_context(c.method))  # revealed: None
+reveal_type(generic_context(c.method))  # revealed: tuple[Self@method]
-reveal_type(generic_context(c.generic_method))  # revealed: tuple[U@generic_method]
+reveal_type(generic_context(c.generic_method))  # revealed: tuple[Self@generic_method, U@generic_method]
 ```
 
 ## Specializations propagate

crates/ty_python_semantic/resources/mdtest/generics/pep695/functions.md

@@ -534,6 +534,5 @@ class C:
 def _(x: int):
     reveal_type(C().explicit_self(x))  # revealed: tuple[C, int]
 
-    # TODO: this should be `tuple[C, int]` as well, once we support implicit `self`
+    reveal_type(C().implicit_self(x))  # revealed: tuple[C, int]
-    reveal_type(C().implicit_self(x))  # revealed: tuple[Unknown, int]
 ```

crates/ty_python_semantic/resources/mdtest/generics/scoping.md

@@ -117,6 +117,7 @@ reveal_type(bound_method.__func__)  # revealed: def f(self, x: int) -> str
 reveal_type(C[int]().f(1))  # revealed: str
 reveal_type(bound_method(1))  # revealed: str
 
+# error: [invalid-argument-type] "Argument to function `f` is incorrect: Argument type `Literal[1]` does not satisfy upper bound `C[Unknown]` of type variable `Self`"
 C[int].f(1)  # error: [missing-argument]
 reveal_type(C[int].f(C[int](), 1))  # revealed: str
 
@@ -154,7 +155,7 @@ from ty_extensions import generic_context
 legacy.m("string", None)  # error: [invalid-argument-type]
 reveal_type(legacy.m)  # revealed: bound method Legacy[int].m[S](x: int, y: S@m) -> S@m
 reveal_type(generic_context(Legacy))  # revealed: tuple[T@Legacy]
-reveal_type(generic_context(legacy.m))  # revealed: tuple[S@m]
+reveal_type(generic_context(legacy.m))  # revealed: tuple[Self@m, S@m]
 ```
 
 With PEP 695 syntax, it is clearer that the method uses a separate typevar:

crates/ty_python_semantic/resources/mdtest/named_tuple.md

@@ -277,9 +277,9 @@ reveal_type(Person._make(("Alice", 42)))  # revealed: Unknown
 
 person = Person("Alice", 42)
 
+# error: [invalid-argument-type] "Argument to bound method `_asdict` is incorrect: Expected `NamedTupleFallback`, found `Person`"
 reveal_type(person._asdict())  # revealed: dict[str, Any]
-# TODO: should be `Person` once we support implicit type of `self`
+reveal_type(person._replace(name="Bob"))  # revealed: Person
-reveal_type(person._replace(name="Bob"))  # revealed: Unknown
 ```
 
 When accessing them on child classes of generic `NamedTuple`s, the return type is specialized
@@ -296,8 +296,7 @@ class Box(NamedTuple, Generic[T]):
 class IntBox(Box[int]):
     pass
 
-# TODO: should be `IntBox` once we support the implicit type of `self`
+reveal_type(IntBox(1)._replace(content=42))  # revealed: IntBox
-reveal_type(IntBox(1)._replace(content=42))  # revealed: Unknown
 ```
 
 ## `collections.namedtuple`

crates/ty_python_semantic/resources/mdtest/narrow/isinstance.md

@@ -363,8 +363,12 @@ class Invariant[T]:
 def _(x: object):
     if isinstance(x, Invariant):
         reveal_type(x)  # revealed: Top[Invariant[Unknown]]
+        # error: [invalid-argument-type] "Argument to bound method `get` is incorrect: Expected `Self@get`, found `Top[Invariant[Unknown]]`"
+        # error: [invalid-argument-type] "Argument to bound method `get` is incorrect: Argument type `Top[Invariant[Unknown]]` does not satisfy upper bound `Bottom[Invariant[Unknown]]` of type variable `Self`"
         reveal_type(x.get())  # revealed: object
         # error: [invalid-argument-type] "Argument to bound method `push` is incorrect: Expected `Never`, found `Literal[42]`"
+        # error: [invalid-argument-type] "Argument to bound method `push` is incorrect: Expected `Self@push`, found `Top[Invariant[Unknown]]`"
+        # error: [invalid-argument-type] "Argument to bound method `push` is incorrect: Argument type `Top[Invariant[Unknown]]` does not satisfy upper bound `Bottom[Invariant[Unknown]]` of type variable `Self`"
         x.push(42)
 ```
 

crates/ty_python_semantic/resources/mdtest/pep695_type_aliases.md

@@ -325,7 +325,7 @@ type A = list[Union["A", str]]
 def f(x: A):
     reveal_type(x)  # revealed: list[A | str]
     for item in x:
-        reveal_type(item)  # revealed: list[A | str] | str
+        reveal_type(item)  # revealed: list[Any | str] | str
 ```
 
 #### With new-style union
@@ -336,7 +336,7 @@ type A = list["A" | str]
 def f(x: A):
     reveal_type(x)  # revealed: list[A | str]
     for item in x:
-        reveal_type(item)  # revealed: list[A | str] | str
+        reveal_type(item)  # revealed: list[Any | str] | str
 ```
 
 #### With Optional
@@ -349,7 +349,7 @@ type A = list[Optional[Union["A", str]]]
 def f(x: A):
     reveal_type(x)  # revealed: list[A | str | None]
     for item in x:
-        reveal_type(item)  # revealed: list[A | str | None] | str | None
+        reveal_type(item)  # revealed: list[Any | str | None] | str | None
 ```
 
 ### Tuple comparison

crates/ty_python_semantic/resources/mdtest/type_properties/is_subtype_of.md

@@ -1948,8 +1948,6 @@ static_assert(is_subtype_of(TypeOf[A.g], Callable[[int], int]))
 static_assert(not is_subtype_of(TypeOf[a.f], Callable[[float], int]))
 static_assert(not is_subtype_of(TypeOf[A.g], Callable[[], int]))
 
-# TODO: This assertion should be true
-# error: [static-assert-error] "Static assertion error: argument of type `ty_extensions.ConstraintSet[never]` is statically known to be falsy"
 static_assert(is_subtype_of(TypeOf[A.f], Callable[[A, int], int]))
 ```
 

crates/ty_python_semantic/resources/mdtest/typed_dict.md

@@ -657,15 +657,13 @@ alice: Employee = {"name": "Alice", "employee_id": 1}
 eve: Employee = {"name": "Eve"}
 
 def combine(p: Person, e: Employee):
-    # TODO: Should be `Person` once we support the implicit type of self
+    reveal_type(p.copy())  # revealed: Person
-    reveal_type(p.copy())  # revealed: Unknown
+    reveal_type(e.copy())  # revealed: Employee
-    # TODO: Should be `Employee` once we support the implicit type of self
-    reveal_type(e.copy())  # revealed: Unknown
 
     reveal_type(p | p)  # revealed: Person
     reveal_type(e | e)  # revealed: Employee
 
-    # TODO: Should be `Person` once we support the implicit type of self and subtyping for TypedDicts
+    # TODO: Should be `Person` once we support subtyping for TypedDicts
     reveal_type(p | e)  # revealed: Employee
 ```
 

crates/ty_python_semantic/resources/mdtest/with/async.md

@@ -254,8 +254,7 @@ async def long_running_task():
 
 async def main():
     async with asyncio.TaskGroup() as tg:
-        # TODO: should be `TaskGroup`
+        reveal_type(tg)  # revealed: TaskGroup
-        reveal_type(tg)  # revealed: Unknown
 
         tg.create_task(long_running_task())
 ```

crates/ty_python_semantic/src/types.rs

@@ -52,8 +52,8 @@ use crate::types::function::{
     DataclassTransformerParams, FunctionSpans, FunctionType, KnownFunction,
 };
 use crate::types::generics::{
-    GenericContext, PartialSpecialization, Specialization, bind_typevar, walk_generic_context,
+    GenericContext, PartialSpecialization, Specialization, bind_typevar, get_self_type,
-    walk_partial_specialization, walk_specialization,
+    walk_generic_context, walk_partial_specialization, walk_specialization,
 };
 pub use crate::types::ide_support::{
     CallSignatureDetails, Member, MemberWithDefinition, all_members, call_signature_details,
@@ -5703,7 +5703,6 @@ impl<'db> Type<'db> {
                     .build()),
 
                 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) else {
                         return Err(InvalidTypeExpressionError {
@@ -5713,42 +5712,8 @@ impl<'db> Type<'db> {
                             ],
                         });
                     };
-
+                    let self_type = get_self_type(db, scope_id, typevar_binding_context, class);
-                    let upper_bound = Type::instance(
+                    Ok(self_type.map(Type::NonInferableTypeVar).unwrap_or(*self))
-                        db,
-                        class.apply_specialization(db, |generic_context| {
-                            let types = generic_context
-                                .variables(db)
-                                .iter()
-                                .map(|typevar| Type::NonInferableTypeVar(*typevar));
-
-                            generic_context.specialize(db, types.collect())
-                        }),
-                    );
-
-                    let class_definition = class.definition(db);
-                    let typevar = TypeVarInstance::new(
-                        db,
-                        ast::name::Name::new_static("Self"),
-                        Some(class_definition),
-                        Some(TypeVarBoundOrConstraints::UpperBound(upper_bound).into()),
-                        // According to the [spec], we can consider `Self`
-                        // equivalent to an invariant type variable
-                        // [spec]: https://typing.python.org/en/latest/spec/generics.html#self
-                        Some(TypeVarVariance::Invariant),
-                        None,
-                        TypeVarKind::TypingSelf,
-                    );
-                    Ok(bind_typevar(
-                        db,
-                        &module,
-                        index,
-                        scope_id.file_scope_id(db),
-                        typevar_binding_context,
-                        typevar,
-                    )
-                    .map(Type::NonInferableTypeVar)
-                    .unwrap_or(*self))
                 }
                 SpecialFormType::TypeAlias => Ok(Type::Dynamic(DynamicType::TodoTypeAlias)),
                 SpecialFormType::TypedDict => Err(InvalidTypeExpressionError {

crates/ty_python_semantic/src/types/display.rs

@@ -1149,11 +1149,13 @@ impl Display for DisplayParameter<'_> {
         if let Some(name) = self.param.display_name() {
             f.write_str(&name)?;
             if let Some(annotated_type) = self.param.annotated_type() {
-                write!(
+                if !self.param.has_synthetic_annotation() {
-                    f,
+                    write!(
-                    ": {}",
+                        f,
-                    annotated_type.display_with(self.db, self.settings)
+                        ": {}",
-                )?;
+                        annotated_type.display_with(self.db, self.settings)
+                    )?;
+                }
             }
             // Default value can only be specified if `name` is given.
             if let Some(default_ty) = self.param.default_type() {
@@ -1166,7 +1168,9 @@ impl Display for DisplayParameter<'_> {
         } else if let Some(ty) = self.param.annotated_type() {
             // This case is specifically for the `Callable` signature where name and default value
             // cannot be provided.
-            ty.display_with(self.db, self.settings).fmt(f)?;
+            if !self.param.has_synthetic_annotation() {
+                ty.display_with(self.db, self.settings).fmt(f)?;
+            }
         }
         Ok(())
     }

crates/ty_python_semantic/src/types/generics.rs

@@ -3,13 +3,13 @@ use std::borrow::Cow;
 use crate::types::constraints::ConstraintSet;
 
 use itertools::Itertools;
-use ruff_db::parsed::ParsedModuleRef;
+use ruff_db::parsed::{ParsedModuleRef, parsed_module};
 use ruff_python_ast as ast;
 use rustc_hash::FxHashMap;
 
-use crate::semantic_index::SemanticIndex;
 use crate::semantic_index::definition::Definition;
-use crate::semantic_index::scope::{FileScopeId, NodeWithScopeKind};
+use crate::semantic_index::scope::{FileScopeId, NodeWithScopeKind, ScopeId};
+use crate::semantic_index::{SemanticIndex, semantic_index};
 use crate::types::class::ClassType;
 use crate::types::class_base::ClassBase;
 use crate::types::infer::infer_definition_types;
@@ -17,10 +17,10 @@ use crate::types::instance::{Protocol, ProtocolInstanceType};
 use crate::types::signatures::{Parameter, Parameters, Signature};
 use crate::types::tuple::{TupleSpec, TupleType, walk_tuple_type};
 use crate::types::{
-    ApplyTypeMappingVisitor, BoundTypeVarInstance, FindLegacyTypeVarsVisitor, HasRelationToVisitor,
+    ApplyTypeMappingVisitor, BoundTypeVarInstance, ClassLiteral, FindLegacyTypeVarsVisitor,
-    IsEquivalentVisitor, KnownClass, KnownInstanceType, MaterializationKind, NormalizedVisitor,
+    HasRelationToVisitor, IsEquivalentVisitor, KnownClass, KnownInstanceType, MaterializationKind,
-    Type, TypeMapping, TypeRelation, TypeVarBoundOrConstraints, TypeVarInstance, TypeVarKind,
+    NormalizedVisitor, Type, TypeMapping, TypeRelation, TypeVarBoundOrConstraints, TypeVarInstance,
-    TypeVarVariance, UnionType, binding_type, declaration_type,
+    TypeVarKind, TypeVarVariance, UnionType, binding_type, declaration_type,
 };
 use crate::{Db, FxOrderSet};
 
@@ -103,6 +103,51 @@ pub(crate) fn bind_typevar<'db>(
         })
 }
 
+pub(crate) fn get_self_type<'db>(
+    db: &'db dyn Db,
+    scope_id: ScopeId,
+    typevar_binding_context: Option<Definition<'db>>,
+    class: ClassLiteral<'db>,
+) -> Option<BoundTypeVarInstance<'db>> {
+    // TODO: remove duplication with Type::in_type_expression
+    let module = parsed_module(db, scope_id.file(db)).load(db);
+    let index = semantic_index(db, scope_id.file(db));
+
+    let upper_bound = Type::instance(
+        db,
+        class.apply_specialization(db, |generic_context| {
+            let types = generic_context
+                .variables(db)
+                .iter()
+                .map(|typevar| Type::NonInferableTypeVar(*typevar));
+
+            generic_context.specialize(db, types.collect())
+        }),
+    );
+
+    let class_definition = class.definition(db);
+    let typevar = TypeVarInstance::new(
+        db,
+        ast::name::Name::new_static("Self"),
+        Some(class_definition),
+        Some(TypeVarBoundOrConstraints::UpperBound(upper_bound).into()),
+        // According to the [spec], we can consider `Self`
+        // equivalent to an invariant type variable
+        // [spec]: https://typing.python.org/en/latest/spec/generics.html#self
+        Some(TypeVarVariance::Invariant),
+        None,
+        TypeVarKind::TypingSelf,
+    );
+    bind_typevar(
+        db,
+        &module,
+        index,
+        scope_id.file_scope_id(db),
+        typevar_binding_context,
+        typevar,
+    )
+}
+
 /// A list of formal type variables for a generic function, class, or type alias.
 ///
 /// TODO: Handle nested generic contexts better, with actual parent links to the lexically

crates/ty_python_semantic/src/types/signatures.rs

@@ -15,18 +15,58 @@ use std::{collections::HashMap, slice::Iter};
 use itertools::{EitherOrBoth, Itertools};
 use smallvec::{SmallVec, smallvec_inline};
 
-use super::{DynamicType, Type, TypeVarVariance, definition_expression_type};
+use super::{
+    DynamicType, Type, TypeVarVariance, definition_expression_type, infer_definition_types,
+    semantic_index,
+};
 use crate::semantic_index::definition::Definition;
 use crate::types::constraints::{ConstraintSet, IteratorConstraintsExtension};
-use crate::types::generics::{GenericContext, walk_generic_context};
+use crate::types::function::FunctionType;
+use crate::types::generics::{GenericContext, get_self_type, walk_generic_context};
+use crate::types::infer::nearest_enclosing_class;
 use crate::types::{
-    ApplyTypeMappingVisitor, BindingContext, BoundTypeVarInstance, FindLegacyTypeVarsVisitor,
+    ApplyTypeMappingVisitor, BindingContext, BoundTypeVarInstance, ClassType,
-    HasRelationToVisitor, IsEquivalentVisitor, KnownClass, MaterializationKind, NormalizedVisitor,
+    FindLegacyTypeVarsVisitor, HasRelationToVisitor, IsEquivalentVisitor, KnownClass,
-    TypeMapping, TypeRelation, VarianceInferable, todo_type,
+    MaterializationKind, NormalizedVisitor, TypeMapping, TypeRelation, TypeVarKind,
+    VarianceInferable, todo_type,
 };
 use crate::{Db, FxOrderSet};
 use ruff_python_ast::{self as ast, name::Name};
 
+#[derive(Clone, Copy, Debug)]
+struct MethodInformation<'db> {
+    method_type: FunctionType<'db>,
+    class_type: ClassType<'db>,
+}
+
+fn infer_method_information<'db>(
+    db: &'db dyn Db,
+    definition: Definition<'db>,
+) -> Option<MethodInformation<'db>> {
+    let class_scope_id = definition.scope(db);
+    let file = class_scope_id.file(db);
+    let index = semantic_index(db, file);
+
+    let class_scope = index.scope(class_scope_id.file_scope_id(db));
+    let class_node = class_scope.node().as_class()?;
+
+    let method_type = infer_definition_types(db, definition)
+        .declaration_type(definition)
+        .inner_type()
+        .into_function_literal()?;
+
+    let class_def = index.expect_single_definition(class_node);
+    let class_literal = infer_definition_types(db, class_def)
+        .declaration_type(class_def)
+        .inner_type();
+    let class_type = class_literal.to_class_type(db)?;
+
+    Some(MethodInformation {
+        method_type,
+        class_type,
+    })
+}
+
 /// The signature of a single callable. If the callable is overloaded, there is a separate
 /// [`Signature`] for each overload.
 #[derive(Clone, Debug, PartialEq, Eq, Hash, salsa::Update, get_size2::GetSize)]
@@ -1219,17 +1259,77 @@ impl<'db> Parameters<'db> {
                     .map(pos_only_param),
             );
         }
+        let method_info = infer_method_information(db, definition);
+        let is_classmethod = method_info.is_some_and(|f| f.method_type.is_classmethod(db));
+        let is_staticmethod = method_info.is_some_and(|f| f.method_type.is_staticmethod(db));
 
         let positional_or_keyword = pos_or_keyword_iter.map(|arg| {
-            Parameter::from_node_and_kind(
+            if let Some(MethodInformation {
-                db,
+                method_type: method,
-                definition,
+                class_type: class,
-                &arg.parameter,
+            }) = method_info
-                ParameterKind::PositionalOrKeyword {
+            {
-                    name: arg.parameter.name.id.clone(),
+                if !is_staticmethod
-                    default_type: default_type(arg),
+                    && !is_classmethod
-                },
+                    && arg.parameter.annotation().is_none()
-            )
+                    && parameters.index(arg.name().id()) == Some(0)
+                {
+                    let method_has_self_in_generic_context =
+                        method.signature(db).overloads.iter().any(|s| {
+                            if let Some(context) = s.generic_context {
+                                context
+                                    .variables(db)
+                                    .iter()
+                                    .any(|v| v.typevar(db).kind(db) == TypeVarKind::TypingSelf);
+                                true
+                            } else {
+                                false
+                            }
+                        });
+                    let implicit_annotation = if !method_has_self_in_generic_context
+                        && class.is_not_generic()
+                    {
+                        Type::instance(db, class)
+                    } else {
+                        let scope_id = definition.scope(db);
+                        let typevar_binding_context = Some(definition);
+                        let index = semantic_index(db, scope_id.file(db));
+                        let class = nearest_enclosing_class(db, index, scope_id).unwrap();
+                        Type::TypeVar(
+                            get_self_type(db, scope_id, typevar_binding_context, class).unwrap(),
+                        )
+                    };
+                    Parameter {
+                        annotated_type: Some(implicit_annotation),
+                        synthetic_annotation: true,
+                        kind: ParameterKind::PositionalOrKeyword {
+                            name: arg.parameter.name.id.clone(),
+                            default_type: default_type(arg),
+                        },
+                        form: ParameterForm::Value,
+                    }
+                } else {
+                    Parameter::from_node_and_kind(
+                        db,
+                        definition,
+                        &arg.parameter,
+                        ParameterKind::PositionalOrKeyword {
+                            name: arg.parameter.name.id.clone(),
+                            default_type: default_type(arg),
+                        },
+                    )
+                }
+            } else {
+                Parameter::from_node_and_kind(
+                    db,
+                    definition,
+                    &arg.parameter,
+                    ParameterKind::PositionalOrKeyword {
+                        name: arg.parameter.name.id.clone(),
+                        default_type: default_type(arg),
+                    },
+                )
+            }
         });
 
         let variadic = vararg.as_ref().map(|arg| {
@@ -1402,6 +1502,10 @@ pub(crate) struct Parameter<'db> {
     /// Annotated type of the parameter.
     annotated_type: Option<Type<'db>>,
 
+    /// If the type of parameter was inferred e.g. the first argument of a method has type
+    /// `typing.Self`.
+    synthetic_annotation: bool,
+
     kind: ParameterKind<'db>,
     pub(crate) form: ParameterForm,
 }
@@ -1410,6 +1514,7 @@ impl<'db> Parameter<'db> {
     pub(crate) fn positional_only(name: Option<Name>) -> Self {
         Self {
             annotated_type: None,
+            synthetic_annotation: false,
             kind: ParameterKind::PositionalOnly {
                 name,
                 default_type: None,
@@ -1421,6 +1526,7 @@ impl<'db> Parameter<'db> {
     pub(crate) fn positional_or_keyword(name: Name) -> Self {
         Self {
             annotated_type: None,
+            synthetic_annotation: false,
             kind: ParameterKind::PositionalOrKeyword {
                 name,
                 default_type: None,
@@ -1432,6 +1538,7 @@ impl<'db> Parameter<'db> {
     pub(crate) fn variadic(name: Name) -> Self {
         Self {
             annotated_type: None,
+            synthetic_annotation: false,
             kind: ParameterKind::Variadic { name },
             form: ParameterForm::Value,
         }
@@ -1440,6 +1547,7 @@ impl<'db> Parameter<'db> {
     pub(crate) fn keyword_only(name: Name) -> Self {
         Self {
             annotated_type: None,
+            synthetic_annotation: false,
             kind: ParameterKind::KeywordOnly {
                 name,
                 default_type: None,
@@ -1451,6 +1559,7 @@ impl<'db> Parameter<'db> {
     pub(crate) fn keyword_variadic(name: Name) -> Self {
         Self {
             annotated_type: None,
+            synthetic_annotation: false,
             kind: ParameterKind::KeywordVariadic { name },
             form: ParameterForm::Value,
         }
@@ -1489,6 +1598,7 @@ impl<'db> Parameter<'db> {
                 .annotated_type
                 .map(|ty| ty.apply_type_mapping_impl(db, type_mapping, visitor)),
             kind: self.kind.apply_type_mapping_impl(db, type_mapping, visitor),
+            synthetic_annotation: self.synthetic_annotation,
             form: self.form,
         }
     }
@@ -1504,6 +1614,7 @@ impl<'db> Parameter<'db> {
     ) -> Self {
         let Parameter {
             annotated_type,
+            synthetic_annotation,
             kind,
             form,
         } = self;
@@ -1547,6 +1658,7 @@ impl<'db> Parameter<'db> {
 
         Self {
             annotated_type: Some(annotated_type),
+            synthetic_annotation: *synthetic_annotation,
             kind,
             form: *form,
         }
@@ -1569,6 +1681,7 @@ impl<'db> Parameter<'db> {
             }),
             kind,
             form: ParameterForm::Value,
+            synthetic_annotation: false,
         }
     }
 
@@ -1623,6 +1736,11 @@ impl<'db> Parameter<'db> {
         &self.kind
     }
 
+    /// Whether the type of the parameter was inferred.
+    pub(crate) fn has_synthetic_annotation(&self) -> bool {
+        self.synthetic_annotation
+    }
+
     /// Name of the parameter (if it has one).
     pub(crate) fn name(&self) -> Option<&ast::name::Name> {
         match &self.kind {