[ty] Add support for @staticmethods (#18809)

## Summary

Add support for `@staticmethod`s. Overall, the changes are very similar
to #16305.

#18587 will be dependent on this PR for a potential fix of
https://github.com/astral-sh/ty/issues/207.

mypy_primer will look bad since the new code allows ty to check more
code.

## Test Plan

Added new markdown tests. Please comment if there's any missing tests
that I should add in, thank you.

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

@@ -430,4 +430,112 @@ reveal_type(C.f2(1))  # revealed: str
 reveal_type(C().f2(1))  # revealed: str
 ```
 
+## `@staticmethod`
+
+### Basic
+
+When a `@staticmethod` attribute is accessed, it returns the underlying function object. This is
+true whether it's accessed on the class or on an instance of the class.
+
+```py
+from __future__ import annotations
+
+class C:
+    @staticmethod
+    def f(x: int) -> str:
+        return "a"
+
+reveal_type(C.f)  # revealed: def f(x: int) -> str
+reveal_type(C().f)  # revealed: def f(x: int) -> str
+```
+
+The method can then be called like a regular function from either the class or an instance, with no
+implicit first argument passed.
+
+```py
+reveal_type(C.f(1))  # revealed: str
+reveal_type(C().f(1))  # revealed: str
+```
+
+When the static method is called incorrectly, we detect it:
+
+```py
+C.f("incorrect")  # error: [invalid-argument-type]
+C.f()  # error: [missing-argument]
+C.f(1, 2)  # error: [too-many-positional-arguments]
+```
+
+When a static method is accessed on a derived class, it behaves identically:
+
+```py
+class Derived(C):
+    pass
+
+reveal_type(Derived.f)  # revealed: def f(x: int) -> str
+reveal_type(Derived().f)  # revealed: def f(x: int) -> str
+
+reveal_type(Derived.f(1))  # revealed: str
+reveal_type(Derived().f(1))  # revealed: str
+```
+
+### Accessing the staticmethod as a static member
+
+```py
+from inspect import getattr_static
+
+class C:
+    @staticmethod
+    def f(): ...
+```
+
+Accessing the staticmethod as a static member. This will reveal the raw function, as `staticmethod`
+is transparent when accessed via `getattr_static`.
+
+```py
+reveal_type(getattr_static(C, "f"))  # revealed: def f() -> Unknown
+```
+
+The `__get__` of a `staticmethod` object simply returns the underlying function. It ignores both the
+instance and owner arguments.
+
+```py
+reveal_type(getattr_static(C, "f").__get__(None, C))  # revealed: def f() -> Unknown
+reveal_type(getattr_static(C, "f").__get__(C(), C))  # revealed: def f() -> Unknown
+reveal_type(getattr_static(C, "f").__get__(C()))  # revealed: def f() -> Unknown
+reveal_type(getattr_static(C, "f").__get__("dummy", C))  # revealed: def f() -> Unknown
+```
+
+### Staticmethods mixed with other decorators
+
+```toml
+[environment]
+python-version = "3.12"
+```
+
+When a `@staticmethod` is additionally decorated with another decorator, it is still treated as a
+static method:
+
+```py
+from __future__ import annotations
+
+def does_nothing[T](f: T) -> T:
+    return f
+
+class C:
+    @staticmethod
+    @does_nothing
+    def f1(x: int) -> str:
+        return "a"
+
+    @does_nothing
+    @staticmethod
+    def f2(x: int) -> str:
+        return "a"
+
+reveal_type(C.f1(1))  # revealed: str
+reveal_type(C().f1(1))  # revealed: str
+reveal_type(C.f2(1))  # revealed: str
+reveal_type(C().f2(1))  # revealed: str
+```
+
 [functions and methods]: https://docs.python.org/3/howto/descriptor.html#functions-and-methods

crates/ty_python_semantic/resources/mdtest/descriptor_protocol.md

@@ -549,6 +549,19 @@ reveal_type(C.get_name())  # revealed: str
 reveal_type(C("42").get_name())  # revealed: str
 ```
 
+### Built-in `staticmethod` descriptor
+
+```py
+class C:
+    @staticmethod
+    def helper(value: str) -> str:
+        return value
+
+reveal_type(C.helper("42"))  # revealed: str
+c = C()
+reveal_type(c.helper("string"))  # revealed: str
+```
+
 ### Functions as descriptors
 
 Functions are descriptors because they implement a `__get__` method. This is crucial in making sure

crates/ty_python_semantic/resources/mdtest/overloads.md

@@ -449,30 +449,32 @@ from __future__ import annotations
 from typing import overload
 
 class CheckStaticMethod:
-    # TODO: error because `@staticmethod` does not exist on all overloads
     @overload
     def method1(x: int) -> int: ...
     @overload
     def method1(x: str) -> str: ...
     @staticmethod
+    # error: [invalid-overload] "Overloaded function `method1` does not use the `@staticmethod` decorator consistently"
     def method1(x: int | str) -> int | str:
         return x
-    # TODO: error because `@staticmethod` does not exist on all overloads
+
     @overload
     def method2(x: int) -> int: ...
     @overload
     @staticmethod
     def method2(x: str) -> str: ...
     @staticmethod
+    # error: [invalid-overload]
     def method2(x: int | str) -> int | str:
         return x
-    # TODO: error because `@staticmethod` does not exist on the implementation
+
     @overload
     @staticmethod
     def method3(x: int) -> int: ...
     @overload
     @staticmethod
     def method3(x: str) -> str: ...
+    # error: [invalid-overload]
     def method3(x: int | str) -> int | str:
         return x
 

crates/ty_python_semantic/src/types/call/bind.rs

@@ -284,6 +284,9 @@ impl<'db> Bindings<'db> {
                                 }
                                 _ => {}
                             }
+                        } else if function.has_known_decorator(db, FunctionDecorators::STATICMETHOD)
+                        {
+                            overload.set_return_type(Type::FunctionLiteral(function));
                         } else if let [Some(first), _] = overload.parameter_types() {
                             if first.is_none(db) {
                                 overload.set_return_type(Type::FunctionLiteral(function));
@@ -319,6 +322,10 @@ impl<'db> Bindings<'db> {
 
                                     _ => {}
                                 }
+                            } else if function
+                                .has_known_decorator(db, FunctionDecorators::STATICMETHOD)
+                            {
+                                overload.set_return_type(*function_ty);
                             } else {
                                 match overload.parameter_types() {
                                     [_, Some(instance), _] if instance.is_none(db) => {

crates/ty_python_semantic/src/types/class.rs

@@ -2119,6 +2119,7 @@ pub enum KnownClass {
     Exception,
     BaseExceptionGroup,
     ExceptionGroup,
+    Staticmethod,
     Classmethod,
     Super,
     // enum
@@ -2249,6 +2250,7 @@ impl<'db> KnownClass {
             // and raises a `TypeError` in Python >=3.14
             // (see https://docs.python.org/3/library/constants.html#NotImplemented)
             | Self::NotImplementedType
+            | Self::Staticmethod
             | Self::Classmethod
             | Self::Field
             | Self::KwOnly
@@ -2293,6 +2295,7 @@ impl<'db> KnownClass {
             | Self::BaseExceptionGroup
             | Self::Exception
             | Self::ExceptionGroup
+            | Self::Staticmethod
             | Self::Classmethod
             | Self::GenericAlias
             | Self::GeneratorType
@@ -2355,6 +2358,7 @@ impl<'db> KnownClass {
             Self::BaseExceptionGroup => "BaseExceptionGroup",
             Self::Exception => "Exception",
             Self::ExceptionGroup => "ExceptionGroup",
+            Self::Staticmethod => "staticmethod",
             Self::Classmethod => "classmethod",
             Self::GenericAlias => "GenericAlias",
             Self::ModuleType => "ModuleType",
@@ -2578,6 +2582,7 @@ impl<'db> KnownClass {
             | Self::BaseExceptionGroup
             | Self::Exception
             | Self::ExceptionGroup
+            | Self::Staticmethod
             | Self::Classmethod
             | Self::Slice
             | Self::Super
@@ -2672,6 +2677,7 @@ impl<'db> KnownClass {
             | Self::BaseExceptionGroup
             | Self::Exception
             | Self::ExceptionGroup
+            | Self::Staticmethod
             | Self::Classmethod
             | Self::GenericAlias
             | Self::ModuleType
@@ -2754,6 +2760,7 @@ impl<'db> KnownClass {
             | Self::BaseExceptionGroup
             | Self::Exception
             | Self::ExceptionGroup
+            | Self::Staticmethod
             | Self::Classmethod
             | Self::TypeVar
             | Self::ParamSpec
@@ -2801,6 +2808,7 @@ impl<'db> KnownClass {
             "BaseExceptionGroup" => Self::BaseExceptionGroup,
             "Exception" => Self::Exception,
             "ExceptionGroup" => Self::ExceptionGroup,
+            "staticmethod" => Self::Staticmethod,
             "classmethod" => Self::Classmethod,
             "GenericAlias" => Self::GenericAlias,
             "NoneType" => Self::NoneType,
@@ -2885,6 +2893,7 @@ impl<'db> KnownClass {
             | Self::ExceptionGroup
             | Self::EllipsisType
             | Self::BaseExceptionGroup
+            | Self::Staticmethod
             | Self::Classmethod
             | Self::FunctionType
             | Self::MethodType

crates/ty_python_semantic/src/types/function.rs

@@ -103,6 +103,8 @@ bitflags! {
         const ABSTRACT_METHOD = 1 << 3;
         /// `@typing.final`
         const FINAL = 1 << 4;
+        /// `@staticmethod`
+        const STATICMETHOD = 1 << 5;
         /// `@typing.override`
         const OVERRIDE = 1 << 6;
     }

crates/ty_python_semantic/src/types/infer.rs

@@ -1277,8 +1277,10 @@ impl<'db, 'ast> TypeInferenceBuilder<'db, 'ast> {
                 }
             }
 
-            // TODO: Add `@staticmethod`
-            for (decorator, name) in [(FunctionDecorators::CLASSMETHOD, "classmethod")] {
+            for (decorator, name) in [
+                (FunctionDecorators::CLASSMETHOD, "classmethod"),
+                (FunctionDecorators::STATICMETHOD, "staticmethod"),
+            ] {
                 let mut decorator_present = false;
                 let mut decorator_missing = vec![];
 
@@ -2195,12 +2197,17 @@ impl<'db, 'ast> TypeInferenceBuilder<'db, 'ast> {
                         _ => {}
                     }
                 }
-                Type::ClassLiteral(class) => {
-                    if class.is_known(self.db(), KnownClass::Classmethod) {
+                Type::ClassLiteral(class) => match class.known(self.db()) {
+                    Some(KnownClass::Classmethod) => {
                         function_decorators |= FunctionDecorators::CLASSMETHOD;
                         continue;
                     }
-                }
+                    Some(KnownClass::Staticmethod) => {
+                        function_decorators |= FunctionDecorators::STATICMETHOD;
+                        continue;
+                    }
+                    _ => {}
+                },
                 Type::DataclassTransformer(params) => {
                     dataclass_transformer_params = Some(params);
                 }