[ty] Extend Liskov checks to also cover classmethods and staticmethods (#21598)

## Summary Building on https://github.com/astral-sh/ruff/pull/21436. There's nothing conceptually more complicated about this, it just requires its own set of tests and its own subdiagnostic hint. I also uncovered another inconsistency between mypy/pyright/pyrefly, which is fun. In this case, I suggest we go with pyright's behaviour. ## Test Plan mdtests/snapshots
2025-11-24 23:14:06 +00:00
parent bfd65c4215
commit adf095e889
7 changed files with 386 additions and 86 deletions
--- a/crates/ty_python_semantic/resources/mdtest/liskov.md
+++ b/crates/ty_python_semantic/resources/mdtest/liskov.md
@@ -523,3 +523,63 @@ class Baz(NamedTuple):
 class Spam(Baz):
    def _asdict(self) -> tuple[int, ...]: ...  # error: [invalid-method-override]
 ```
+
+## Staticmethods and classmethods
+
+Methods decorated with `@staticmethod` or `@classmethod` are checked in much the same way as other
+methods.
+
+<!-- snapshot-diagnostics -->
+
+```pyi
+class Parent:
+    def instance_method(self, x: int) -> int: ...
+    @classmethod
+    def class_method(cls, x: int) -> int: ...
+    @staticmethod
+    def static_method(x: int) -> int: ...
+
+class BadChild1(Parent):
+    @staticmethod
+    def instance_method(self, x: int) -> int: ...  # error: [invalid-method-override]
+    # TODO: we should emit `invalid-method-override` here.
+    # Although the method has the same signature as `Parent.class_method`
+    # when accessed on instances, it does not have the same signature as
+    # `Parent.class_method` when accessed on the class object itself
+    def class_method(cls, x: int) -> int: ...
+    def static_method(x: int) -> int: ...  # error: [invalid-method-override]
+
+class BadChild2(Parent):
+    # TODO: we should emit `invalid-method-override` here.
+    # Although the method has the same signature as `Parent.class_method`
+    # when accessed on instances, it does not have the same signature as
+    # `Parent.class_method` when accessed on the class object itself.
+    #
+    # Note that whereas `BadChild1.class_method` is reported as a Liskov violation by
+    # mypy, pyright and pyrefly, pyright is the only one of those three to report a
+    # Liskov violation on this method as of 2025-11-23.
+    @classmethod
+    def instance_method(self, x: int) -> int: ...
+    @staticmethod
+    def class_method(cls, x: int) -> int: ...  # error: [invalid-method-override]
+    @classmethod
+    def static_method(x: int) -> int: ...  # error: [invalid-method-override]
+
+class BadChild3(Parent):
+    @classmethod
+    def class_method(cls, x: bool) -> object: ...  # error: [invalid-method-override]
+    @staticmethod
+    def static_method(x: bool) -> object: ...  # error: [invalid-method-override]
+
+class GoodChild1(Parent):
+    @classmethod
+    def class_method(cls, x: int) -> int: ...
+    @staticmethod
+    def static_method(x: int) -> int: ...
+
+class GoodChild2(Parent):
+    @classmethod
+    def class_method(cls, x: object) -> bool: ...
+    @staticmethod
+    def static_method(x: object) -> bool: ...
+```
--- a/crates/ty_python_semantic/resources/mdtest/snapshots/liskov.md_-_The_Liskov_Substitut…_-_Staticmethods_and_cl…_(49e28aae6fdd1291).snap
+++ b/crates/ty_python_semantic/resources/mdtest/snapshots/liskov.md_-_The_Liskov_Substitut…_-_Staticmethods_and_cl…_(49e28aae6fdd1291).snap
@@ -0,0 +1,220 @@
+---
+source: crates/ty_test/src/lib.rs
+expression: snapshot
+---
+---
+mdtest name: liskov.md - The Liskov Substitution Principle - Staticmethods and classmethods
+mdtest path: crates/ty_python_semantic/resources/mdtest/liskov.md
+---
+
+# Python source files
+
+## mdtest_snippet.pyi
+
+```
+ 1 | class Parent:
+ 2 |     def instance_method(self, x: int) -> int: ...
+ 3 |     @classmethod
+ 4 |     def class_method(cls, x: int) -> int: ...
+ 5 |     @staticmethod
+ 6 |     def static_method(x: int) -> int: ...
+ 7 | 
+ 8 | class BadChild1(Parent):
+ 9 |     @staticmethod
+10 |     def instance_method(self, x: int) -> int: ...  # error: [invalid-method-override]
+11 |     # TODO: we should emit `invalid-method-override` here.
+12 |     # Although the method has the same signature as `Parent.class_method`
+13 |     # when accessed on instances, it does not have the same signature as
+14 |     # `Parent.class_method` when accessed on the class object itself
+15 |     def class_method(cls, x: int) -> int: ...
+16 |     def static_method(x: int) -> int: ...  # error: [invalid-method-override]
+17 | 
+18 | class BadChild2(Parent):
+19 |     # TODO: we should emit `invalid-method-override` here.
+20 |     # Although the method has the same signature as `Parent.class_method`
+21 |     # when accessed on instances, it does not have the same signature as
+22 |     # `Parent.class_method` when accessed on the class object itself.
+23 |     #
+24 |     # Note that whereas `BadChild1.class_method` is reported as a Liskov violation by
+25 |     # mypy, pyright and pyrefly, pyright is the only one of those three to report a
+26 |     # Liskov violation on this method as of 2025-11-23.
+27 |     @classmethod
+28 |     def instance_method(self, x: int) -> int: ...
+29 |     @staticmethod
+30 |     def class_method(cls, x: int) -> int: ...  # error: [invalid-method-override]
+31 |     @classmethod
+32 |     def static_method(x: int) -> int: ...  # error: [invalid-method-override]
+33 | 
+34 | class BadChild3(Parent):
+35 |     @classmethod
+36 |     def class_method(cls, x: bool) -> object: ...  # error: [invalid-method-override]
+37 |     @staticmethod
+38 |     def static_method(x: bool) -> object: ...  # error: [invalid-method-override]
+39 | 
+40 | class GoodChild1(Parent):
+41 |     @classmethod
+42 |     def class_method(cls, x: int) -> int: ...
+43 |     @staticmethod
+44 |     def static_method(x: int) -> int: ...
+45 | 
+46 | class GoodChild2(Parent):
+47 |     @classmethod
+48 |     def class_method(cls, x: object) -> bool: ...
+49 |     @staticmethod
+50 |     def static_method(x: object) -> bool: ...
+```
+
+# Diagnostics
+
+```
+error[invalid-method-override]: Invalid override of method `instance_method`
+  --> src/mdtest_snippet.pyi:10:9
+   |
+ 8 | class BadChild1(Parent):
+ 9 |     @staticmethod
+10 |     def instance_method(self, x: int) -> int: ...  # error: [invalid-method-override]
+   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Definition is incompatible with `Parent.instance_method`
+11 |     # TODO: we should emit `invalid-method-override` here.
+12 |     # Although the method has the same signature as `Parent.class_method`
+   |
+  ::: src/mdtest_snippet.pyi:2:9
+   |
+ 1 | class Parent:
+ 2 |     def instance_method(self, x: int) -> int: ...
+   |         ------------------------------------ `Parent.instance_method` defined here
+ 3 |     @classmethod
+ 4 |     def class_method(cls, x: int) -> int: ...
+   |
+info: `BadChild1.instance_method` is a staticmethod but `Parent.instance_method` is an instance method
+info: This violates the Liskov Substitution Principle
+info: rule `invalid-method-override` is enabled by default
+
+```
+
+```
+error[invalid-method-override]: Invalid override of method `static_method`
+  --> src/mdtest_snippet.pyi:16:9
+   |
+14 |     # `Parent.class_method` when accessed on the class object itself
+15 |     def class_method(cls, x: int) -> int: ...
+16 |     def static_method(x: int) -> int: ...  # error: [invalid-method-override]
+   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Definition is incompatible with `Parent.static_method`
+17 |
+18 | class BadChild2(Parent):
+   |
+  ::: src/mdtest_snippet.pyi:6:9
+   |
+ 4 |     def class_method(cls, x: int) -> int: ...
+ 5 |     @staticmethod
+ 6 |     def static_method(x: int) -> int: ...
+   |         ---------------------------- `Parent.static_method` defined here
+ 7 |
+ 8 | class BadChild1(Parent):
+   |
+info: `BadChild1.static_method` is an instance method but `Parent.static_method` is a staticmethod
+info: This violates the Liskov Substitution Principle
+info: rule `invalid-method-override` is enabled by default
+
+```
+
+```
+error[invalid-method-override]: Invalid override of method `class_method`
+  --> src/mdtest_snippet.pyi:30:9
+   |
+28 |     def instance_method(self, x: int) -> int: ...
+29 |     @staticmethod
+30 |     def class_method(cls, x: int) -> int: ...  # error: [invalid-method-override]
+   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Definition is incompatible with `Parent.class_method`
+31 |     @classmethod
+32 |     def static_method(x: int) -> int: ...  # error: [invalid-method-override]
+   |
+  ::: src/mdtest_snippet.pyi:4:9
+   |
+ 2 |     def instance_method(self, x: int) -> int: ...
+ 3 |     @classmethod
+ 4 |     def class_method(cls, x: int) -> int: ...
+   |         -------------------------------- `Parent.class_method` defined here
+ 5 |     @staticmethod
+ 6 |     def static_method(x: int) -> int: ...
+   |
+info: `BadChild2.class_method` is a staticmethod but `Parent.class_method` is a classmethod
+info: This violates the Liskov Substitution Principle
+info: rule `invalid-method-override` is enabled by default
+
+```
+
+```
+error[invalid-method-override]: Invalid override of method `static_method`
+  --> src/mdtest_snippet.pyi:32:9
+   |
+30 |     def class_method(cls, x: int) -> int: ...  # error: [invalid-method-override]
+31 |     @classmethod
+32 |     def static_method(x: int) -> int: ...  # error: [invalid-method-override]
+   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Definition is incompatible with `Parent.static_method`
+33 |
+34 | class BadChild3(Parent):
+   |
+  ::: src/mdtest_snippet.pyi:6:9
+   |
+ 4 |     def class_method(cls, x: int) -> int: ...
+ 5 |     @staticmethod
+ 6 |     def static_method(x: int) -> int: ...
+   |         ---------------------------- `Parent.static_method` defined here
+ 7 |
+ 8 | class BadChild1(Parent):
+   |
+info: `BadChild2.static_method` is a classmethod but `Parent.static_method` is a staticmethod
+info: This violates the Liskov Substitution Principle
+info: rule `invalid-method-override` is enabled by default
+
+```
+
+```
+error[invalid-method-override]: Invalid override of method `class_method`
+  --> src/mdtest_snippet.pyi:36:9
+   |
+34 | class BadChild3(Parent):
+35 |     @classmethod
+36 |     def class_method(cls, x: bool) -> object: ...  # error: [invalid-method-override]
+   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Definition is incompatible with `Parent.class_method`
+37 |     @staticmethod
+38 |     def static_method(x: bool) -> object: ...  # error: [invalid-method-override]
+   |
+  ::: src/mdtest_snippet.pyi:4:9
+   |
+ 2 |     def instance_method(self, x: int) -> int: ...
+ 3 |     @classmethod
+ 4 |     def class_method(cls, x: int) -> int: ...
+   |         -------------------------------- `Parent.class_method` defined here
+ 5 |     @staticmethod
+ 6 |     def static_method(x: int) -> int: ...
+   |
+info: This violates the Liskov Substitution Principle
+info: rule `invalid-method-override` is enabled by default
+
+```
+
+```
+error[invalid-method-override]: Invalid override of method `static_method`
+  --> src/mdtest_snippet.pyi:38:9
+   |
+36 |     def class_method(cls, x: bool) -> object: ...  # error: [invalid-method-override]
+37 |     @staticmethod
+38 |     def static_method(x: bool) -> object: ...  # error: [invalid-method-override]
+   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Definition is incompatible with `Parent.static_method`
+39 |
+40 | class GoodChild1(Parent):
+   |
+  ::: src/mdtest_snippet.pyi:6:9
+   |
+ 4 |     def class_method(cls, x: int) -> int: ...
+ 5 |     @staticmethod
+ 6 |     def static_method(x: int) -> int: ...
+   |         ---------------------------- `Parent.static_method` defined here
+ 7 |
+ 8 | class BadChild1(Parent):
+   |
+info: This violates the Liskov Substitution Principle
+info: rule `invalid-method-override` is enabled by default
+
+```
--- a/crates/ty_python_semantic/src/types.rs
+++ b/crates/ty_python_semantic/src/types.rs
@@ -1107,13 +1107,6 @@ impl<'db> Type<'db> {
        }
    }

-    pub(crate) const fn as_bound_method(self) -> Option<BoundMethodType<'db>> {
-        match self {
-            Type::BoundMethod(bound_method) => Some(bound_method),
-            _ => None,
-        }
-    }
-
    #[track_caller]
    pub(crate) const fn expect_class_literal(self) -> ClassLiteral<'db> {
        self.as_class_literal()
--- a/crates/ty_python_semantic/src/types/class.rs
+++ b/crates/ty_python_semantic/src/types/class.rs
@@ -1264,6 +1264,14 @@ impl MethodDecorator {
            (false, false) => Ok(Self::None),
        }
    }
+
+    pub(crate) const fn description(self) -> &'static str {
+        match self {
+            MethodDecorator::None => "an instance method",
+            MethodDecorator::ClassMethod => "a classmethod",
+            MethodDecorator::StaticMethod => "a staticmethod",
+        }
+    }
 }

 /// Kind-specific metadata for different types of fields
--- a/crates/ty_python_semantic/src/types/diagnostic.rs
+++ b/crates/ty_python_semantic/src/types/diagnostic.rs
@@ -8,13 +8,13 @@ use super::{
 use crate::diagnostic::did_you_mean;
 use crate::diagnostic::format_enumeration;
 use crate::lint::{Level, LintRegistryBuilder, LintStatus};
+use crate::place::Place;
 use crate::semantic_index::definition::{Definition, DefinitionKind};
 use crate::semantic_index::place::{PlaceTable, ScopedPlaceId};
 use crate::semantic_index::{global_scope, place_table, use_def_map};
 use crate::suppression::FileSuppressionId;
-use crate::types::KnownInstanceType;
 use crate::types::call::CallError;
-use crate::types::class::{DisjointBase, DisjointBaseKind, Field};
+use crate::types::class::{DisjointBase, DisjointBaseKind, Field, MethodDecorator};
 use crate::types::function::{FunctionType, KnownFunction};
 use crate::types::liskov::{MethodKind, SynthesizedMethodKind};
 use crate::types::string_annotation::{
@@ -27,6 +27,7 @@ use crate::types::{
    ProtocolInstanceType, SpecialFormType, SubclassOfInner, Type, TypeContext, binding_type,
    infer_isolated_expression, protocol_class::ProtocolClass,
 };
+use crate::types::{KnownInstanceType, MemberLookupPolicy};
 use crate::{Db, DisplaySettings, FxIndexMap, Module, ModuleName, Program, declare_lint};
 use itertools::Itertools;
 use ruff_db::{
@@ -3519,6 +3520,27 @@ pub(super) fn report_invalid_method_override<'db>(
        "Definition is incompatible with `{overridden_method}`"
    ));

+    let class_member = |cls: ClassType<'db>| {
+        cls.class_member(db, member, MemberLookupPolicy::default())
+            .place
+    };
+
+    if let Place::Defined(Type::FunctionLiteral(subclass_function), _, _) = class_member(subclass)
+        && let Place::Defined(Type::FunctionLiteral(superclass_function), _, _) =
+            class_member(superclass)
+        && let Ok(superclass_function_kind) =
+            MethodDecorator::try_from_fn_type(db, superclass_function)
+        && let Ok(subclass_function_kind) = MethodDecorator::try_from_fn_type(db, subclass_function)
+        && superclass_function_kind != subclass_function_kind
+    {
+        diagnostic.info(format_args!(
+            "`{class_name}.{member}` is {subclass_function_kind} \
+            but `{overridden_method}` is {superclass_function_kind}",
+            superclass_function_kind = superclass_function_kind.description(),
+            subclass_function_kind = subclass_function_kind.description(),
+        ));
+    }
+
    diagnostic.info("This violates the Liskov Substitution Principle");

    if !subclass_definition_kind.is_function_def()
@@ -3545,9 +3567,11 @@ pub(super) fn report_invalid_method_override<'db>(
                        .full_range(db, &parsed_module(db, superclass_scope.file(db)).load(db)),
                );

-                let superclass_function_span = superclass_type
-                    .as_bound_method()
-                    .and_then(|method| signature_span(method.function(db)));
+                let superclass_function_span = match superclass_type {
+                    Type::FunctionLiteral(function) => signature_span(function),
+                    Type::BoundMethod(method) => signature_span(method.function(db)),
+                    _ => None,
+                };

                let superclass_definition_kind = definition.kind(db);

--- a/crates/ty_python_semantic/src/types/liskov.rs
+++ b/crates/ty_python_semantic/src/types/liskov.rs
@@ -49,11 +49,6 @@ fn check_class_declaration<'db>(
        return;
    };

-    // TODO: classmethods and staticmethods
-    if function.is_classmethod(db) || function.is_staticmethod(db) {
-        return;
-    }
-
    // Constructor methods are not checked for Liskov compliance
    if matches!(
        &*member.name,