[ty] make Type::BoundMethod include instances of same-named methods bound to a subclass (astral-sh#24039)

Fixes astral-sh/ty#2428.

Author: oconnor663

crates/ty_python_semantic/resources/mdtest/intersection_types.md

@@ -1324,5 +1324,58 @@ def f(c: C):
     reveal_type(c.x)  # revealed: ~AlwaysFalsy
 ```
 
+## Methods on intersections
+
+### The same method from a common base
+
+```py
+from ty_extensions import Intersection
+
+class Base:
+    def f(self) -> int:
+        return 0
+
+class X(Base):
+    pass
+
+class Y(Base):
+    pass
+
+def test(x: Intersection[X, Y]) -> None:
+    reveal_type(x.f)  # revealed: (bound method X.f() -> int) & (bound method Y.f() -> int)
+    reveal_type(x.f())  # revealed: int
+
+# Example subclass that inhabits that intersection:
+class Z(X, Y):
+    pass
+
+test(Z())
+```
+
+### A method that could be compatible with a protocol (without violating Liskov)
+
+```py
+from typing import Protocol
+from ty_extensions import Intersection
+
+class Proto(Protocol):
+    def method(self) -> int: ...
+
+class X:
+    # This `method` isn't compatible with `Proto`, but a subclass could refine it...
+    def method(self) -> int | str:
+        return "hello"
+
+def test(x: Intersection[X, Proto]) -> None:
+    reveal_type(x.method())  # revealed: int
+
+# ...like this. This subclass inhabits the intersection above.
+class Y(X):
+    def method(self) -> int:
+        return 42
+
+test(Y())
+```
+
 [complement laws]: https://en.wikipedia.org/wiki/Complement_(set_theory)
 [de morgan's laws]: https://en.wikipedia.org/wiki/De_Morgan%27s_laws

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

@@ -472,6 +472,88 @@ static_assert(not is_disjoint_from(TypeOf[f], FunctionType))
 static_assert(not is_disjoint_from(TypeOf[f], object))
 ```
 
+### Bound methods
+
+```py
+from typing import final
+from ty_extensions import TypeOf, is_disjoint_from, static_assert
+
+class A:
+    def foo(self) -> None: ...
+    def bar(self) -> None: ...
+
+class B:
+    def foo(self) -> None: ...
+
+# Bound methods with different names are disjoint.
+static_assert(is_disjoint_from(TypeOf[A().foo], TypeOf[A().bar]))
+
+# Bound methods with the same name but different self types are not disjoint, because a subclass
+# could inherit from both...
+static_assert(not is_disjoint_from(TypeOf[A().foo], TypeOf[B().foo]))
+
+@final
+class F1:
+    def foo(self) -> None: ...
+
+@final
+class F2:
+    def foo(self) -> None: ...
+
+# ...unless one or both of those classes are `@final`.
+static_assert(is_disjoint_from(TypeOf[A().foo], TypeOf[F2().foo]))
+static_assert(is_disjoint_from(TypeOf[B().foo], TypeOf[F2().foo]))
+static_assert(is_disjoint_from(TypeOf[F1().foo], TypeOf[F2().foo]))
+```
+
+### Bound `@final` methods
+
+Two different `@final` methods are disjoint, even if they share the same name and neither class is
+`@final`, because no subclass could satisfy both without overriding one:
+
+```py
+from typing import final
+from ty_extensions import TypeOf, is_disjoint_from, static_assert
+
+class C:
+    @final
+    def foo(self) -> None: ...
+
+class D:
+    @final
+    def foo(self) -> None: ...
+
+static_assert(is_disjoint_from(TypeOf[C().foo], TypeOf[D().foo]))
+```
+
+We do have to be careful not to get confused when the same `@final` method has different (but
+compatible) bound self types:
+
+```py
+class E(C): ...
+
+# `E.foo` and `C.foo` are the same method, so `E().foo` satisfies both `BoundMethod` types.
+static_assert(not is_disjoint_from(TypeOf[E().foo], TypeOf[C().foo]))
+static_assert(is_disjoint_from(TypeOf[E().foo], TypeOf[D().foo]))
+```
+
+Also, we can't establish disjointness when only one of the methods is `@final`. Consider this tricky
+multiple inheritance case:
+
+```py
+class F:
+    def foo(self) -> None: ...
+
+static_assert(not is_disjoint_from(TypeOf[C().foo], TypeOf[F().foo]))
+static_assert(not is_disjoint_from(TypeOf[D().foo], TypeOf[F().foo]))
+
+class G(C, F): ...
+
+static_assert(not is_disjoint_from(TypeOf[C().foo], TypeOf[G().foo]))
+static_assert(is_disjoint_from(TypeOf[D().foo], TypeOf[G().foo]))
+static_assert(not is_disjoint_from(TypeOf[F().foo], TypeOf[G().foo]))
+```
+
 ### `AlwaysTruthy` and `AlwaysFalsy`
 
 ```py

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

@@ -47,7 +47,8 @@ static_assert(not is_single_valued(Callable[[int, str], None]))
 class A:
     def method(self): ...
 
-static_assert(is_single_valued(TypeOf[A().method]))
+# Binding the same method to different instances yields different objects: `[].sort != [].sort`
+static_assert(not is_single_valued(TypeOf[A().method]))
 static_assert(is_single_valued(TypeOf[types.FunctionType.__get__]))
 static_assert(is_single_valued(TypeOf[A.method.__get__]))
 ```

crates/ty_python_semantic/src/types.rs

@@ -2085,7 +2085,6 @@ impl<'db> Type<'db> {
     pub(crate) fn is_single_valued(self, db: &'db dyn Db) -> bool {
         match self {
             Type::FunctionLiteral(..)
-            | Type::BoundMethod(_)
             | Type::WrapperDescriptor(_)
             | Type::KnownBoundMethod(_)
             | Type::ModuleLiteral(..)
@@ -2140,6 +2139,11 @@ impl<'db> Type<'db> {
                 false
             }
 
+            Type::BoundMethod(_) => {
+                // Binding the same method to different instances yields different objects: `[].sort != [].sort`
+                false
+            }
+
             Type::TypeIs(type_is) => type_is.is_bound(db),
             Type::TypeGuard(type_guard) => type_guard.is_bound(db),
 

crates/ty_python_semantic/src/types/relation.rs

@@ -8,6 +8,7 @@ use crate::types::constraints::{
 };
 use crate::types::cyclic::PairVisitor;
 use crate::types::enums::is_single_member_enum;
+use crate::types::function::FunctionDecorators;
 use crate::types::set_theoretic::RecursivelyDefined;
 use crate::types::{
     CallableType, ClassBase, ClassType, CycleDetector, DynamicType, KnownBoundMethodType,
@@ -1877,15 +1878,13 @@ impl<'a, 'c, 'db> DisjointnessChecker<'a, 'c, 'db> {
             (
                 // note `LiteralString` is not single-valued, but we handle the special case above
                 left @ (Type::FunctionLiteral(..)
-                | Type::BoundMethod(..)
                 | Type::KnownBoundMethod(..)
                 | Type::WrapperDescriptor(..)
                 | Type::ModuleLiteral(..)
                 | Type::ClassLiteral(..)
                 | Type::SpecialForm(..)
                 | Type::KnownInstance(..)),
                 right @ (Type::FunctionLiteral(..)
-                | Type::BoundMethod(..)
                 | Type::KnownBoundMethod(..)
                 | Type::WrapperDescriptor(..)
                 | Type::ModuleLiteral(..)
@@ -2197,6 +2196,58 @@ impl<'a, 'c, 'db> DisjointnessChecker<'a, 'c, 'db> {
                     .negate(db, self.constraints)
             }
 
+            // A `BoundMethod` type includes instances of the same method bound to a
+            // subtype/subclass of the self type.
+            (Type::BoundMethod(a), Type::BoundMethod(b)) => {
+                if a.function(db).name(db) != b.function(db).name(db) {
+                    // We typically ask about `BoundMethod` disjointness when we're looking at a
+                    // method call on an intersection type like `A & B`. In that case, the same
+                    // method name would show up on both sides of this check. However for
+                    // completeness, if we're ever comparing `BoundMethod` types with different
+                    // method names, then they're clearly disjoint.
+                    self.always()
+                } else if a.function(db) != b.function(db)
+                    && a.function(db)
+                        .has_known_decorator(db, FunctionDecorators::FINAL)
+                    && b.function(db)
+                        .has_known_decorator(db, FunctionDecorators::FINAL)
+                {
+                    // If *both* methods are `@final` (and they're not literally the same
+                    // definition), they must be disjoint.
+                    //
+                    // Note that we can't establish disjointness when only one side is `@final`,
+                    // because we have to worry about cases like this:
+                    //
+                    // ```
+                    // class A:
+                    //      def f(self): ...
+                    // class B:
+                    //      @final
+                    //      def f(self): ...
+                    // # Valid in this order, though `C(A, B)` would be invalid.
+                    // class C(B, A): ...
+                    // ```
+                    self.always()
+                } else {
+                    // The names match, so `BoundMethod` disjointness depends on whether the bound
+                    // self types are disjoint. Note that this can produce confusing results in the
+                    // face of Liskov violations. For example:
+                    // ```
+                    // class A:
+                    //     def f(self) -> int: ...
+                    // class B:
+                    //     def f(self) -> str: ...
+                    // def _(x: Intersection[A, B]):
+                    //     x.f()
+                    // ```
+                    // `class C(A, B)` could inhabit that intersection, but `int` and `str` are
+                    // disjoint, so the type of `x.f()` there is going to be inferred as `Never`.
+                    // That's probably not correct in practice, but the right way to address it is
+                    // to emit a diagnostic on the definition of `C.f`.
+                    self.check_type_pair(db, a.self_instance(db), b.self_instance(db))
+                }
+            }
+
             (Type::BoundMethod(_), other) | (other, Type::BoundMethod(_)) => {
                 self.check_type_pair(db, KnownClass::MethodType.to_instance(db), other)
             }