[ty] Implement equivalence for protocols with method members

Author: AlexWaygood

crates/ty_python_semantic/resources/mdtest/protocols.md

@@ -1476,8 +1476,7 @@ class P1(Protocol):
 class P2(Protocol):
     def x(self, y: int) -> None: ...
 
-# TODO: this should pass
-static_assert(is_equivalent_to(P1, P2))  # error: [static-assert-error]
+static_assert(is_equivalent_to(P1, P2))
 ```
 
 As with protocols that only have non-method members, this also holds true when they appear in
@@ -1487,8 +1486,7 @@ differently ordered unions:
 class A: ...
 class B: ...
 
-# TODO: this should pass
-static_assert(is_equivalent_to(A | B | P1, P2 | B | A))  # error: [static-assert-error]
+static_assert(is_equivalent_to(A | B | P1, P2 | B | A))
 ```
 
 ## Narrowing of protocols
@@ -1881,6 +1879,86 @@ if isinstance(obj, (B, A)):
     reveal_type(obj)  # revealed: (Unknown & B) | (Unknown & A)
 ```
 
+### Protocols that use `Self`
+
+`Self` is a `TypeVar` with an upper bound of the class in which it is defined. This means that
+`Self` annotations in protocols can also be tricky to handle without infinite recursion and stack
+overflows.
+
+```toml
+[environment]
+python-version = "3.12"
+```
+
+```py
+from typing_extensions import Protocol, Self
+from ty_extensions import static_assert
+
+class _HashObject(Protocol):
+    def copy(self) -> Self: ...
+
+class Foo: ...
+
+# Attempting to build this union caused us to overflow on an early version of
+# <https://github.com/astral-sh/ruff/pull/18659>
+x: Foo | _HashObject
+```
+
+Some other similar cases that caused issues in our early `Protocol` implementation:
+
+`a.py`:
+
+```py
+from typing_extensions import Protocol, Self
+
+class PGconn(Protocol):
+    def connect(self) -> Self: ...
+
+class Connection:
+    pgconn: PGconn
+
+def is_crdb(conn: PGconn) -> bool:
+    return isinstance(conn, Connection)
+```
+
+and:
+
+`b.py`:
+
+```py
+from typing_extensions import Protocol
+
+class PGconn(Protocol):
+    def connect[T: PGconn](self: T) -> T: ...
+
+class Connection:
+    pgconn: PGconn
+
+def f(x: PGconn):
+    isinstance(x, Connection)
+```
+
+### Recursive protocols used as the first argument to `cast()`
+
+These caused issues in an early version of our `Protocol` implementation due to the fact that we use
+a recursive function in our `cast()` implementation to check whether a type contains `Unknown` or
+`Todo`. Recklessly recursing into a type causes stack overflows if the type is recursive:
+
+```toml
+[environment]
+python-version = "3.12"
+```
+
+```py
+from typing import cast, Protocol
+
+class Iterator[T](Protocol):
+    def __iter__(self) -> Iterator[T]: ...
+
+def f(value: Iterator):
+    cast(Iterator, value)  # error: [redundant-cast]
+```
+
 ## TODO
 
 Add tests for:

crates/ty_python_semantic/src/types.rs

@@ -1350,8 +1350,7 @@ impl<'db> Type<'db> {
                 .is_some_and(|instance| instance.has_relation_to(db, target, relation)),
 
             (Type::FunctionLiteral(self_function_literal), Type::Callable(_)) => {
-                self_function_literal
-                    .into_callable_type(db)
+                Type::Callable(self_function_literal.into_callable_type(db))
                     .has_relation_to(db, target, relation)
             }
 

crates/ty_python_semantic/src/types/function.rs

@@ -767,8 +767,8 @@ impl<'db> FunctionType<'db> {
     }
 
     /// Convert the `FunctionType` into a [`Type::Callable`].
-    pub(crate) fn into_callable_type(self, db: &'db dyn Db) -> Type<'db> {
-        Type::Callable(CallableType::new(db, self.signature(db), false))
+    pub(crate) fn into_callable_type(self, db: &'db dyn Db) -> CallableType<'db> {
+        CallableType::new(db, self.signature(db), false)
     }
 
     /// Convert the `FunctionType` into a [`Type::BoundMethod`].

crates/ty_python_semantic/src/types/protocol_class.rs

@@ -259,7 +259,7 @@ impl<'db> ProtocolMemberData<'db> {
 
 #[derive(Debug, Copy, Clone, PartialEq, Eq, salsa::Update, Hash)]
 enum ProtocolMemberKind<'db> {
-    Method(Type<'db>), // TODO: use CallableType
+    Method(CallableType<'db>),
     Property(PropertyInstanceType<'db>),
     Other(Type<'db>),
 }
@@ -334,7 +334,7 @@ fn walk_protocol_member<'db, V: super::visitor::TypeVisitor<'db> + ?Sized>(
     visitor: &mut V,
 ) {
     match member.kind {
-        ProtocolMemberKind::Method(method) => visitor.visit_type(db, method),
+        ProtocolMemberKind::Method(method) => visitor.visit_callable_type(db, method),
         ProtocolMemberKind::Property(property) => {
             visitor.visit_property_instance_type(db, property);
         }
@@ -353,7 +353,7 @@ impl<'a, 'db> ProtocolMember<'a, 'db> {
 
     fn ty(&self) -> Type<'db> {
         match &self.kind {
-            ProtocolMemberKind::Method(callable) => *callable,
+            ProtocolMemberKind::Method(callable) => Type::Callable(*callable),
             ProtocolMemberKind::Property(property) => Type::PropertyInstance(*property),
             ProtocolMemberKind::Other(ty) => *ty,
         }
@@ -500,13 +500,10 @@ fn cached_protocol_interface<'db>(
                         (Type::Callable(callable), BoundOnClass::Yes)
                             if callable.is_function_like(db) =>
                         {
-                            ProtocolMemberKind::Method(ty)
+                            ProtocolMemberKind::Method(callable)
                         }
-                        // TODO: method members that have `FunctionLiteral` types should be upcast
-                        // to `CallableType` so that two protocols with identical method members
-                        // are recognized as equivalent.
-                        (Type::FunctionLiteral(_function), BoundOnClass::Yes) => {
-                            ProtocolMemberKind::Method(ty)
+                        (Type::FunctionLiteral(function), BoundOnClass::Yes) => {
+                            ProtocolMemberKind::Method(function.into_callable_type(db))
                         }
                         _ => ProtocolMemberKind::Other(ty),
                     };