diff --git a/crates/ty_python_semantic/resources/mdtest/binary/in.md b/crates/ty_python_semantic/resources/mdtest/binary/in.md
new file mode 100644
index 0000000000..508ccdb67c
--- /dev/null
+++ b/crates/ty_python_semantic/resources/mdtest/binary/in.md
@@ -0,0 +1,53 @@
+# Static binary operations using `in`
+
+## Basic functionality
+
+This demonstrates type inference support for `<str-literal> in <tuple>`:
+
+```py
+from ty_extensions import static_assert
+
+static_assert("foo" in ("quux", "foo", "baz"))
+static_assert("foo" not in ("quux", "bar", "baz"))
+```
+
+## With variables
+
+```py
+from ty_extensions import static_assert
+
+x = ("quux", "foo", "baz")
+static_assert("foo" in x)
+
+x = ("quux", "bar", "baz")
+static_assert("foo" not in x)
+```
+
+## Statically unknown results in a type error
+
+```py
+from ty_extensions import static_assert
+
+def _(a: str, b: str):
+    static_assert("foo" in (a, b))  # error: [static-assert-error]
+```
+
+## Values being unknown doesn't mean the result is unknown
+
+For example, when the types are completely disjoint:
+
+```py
+from ty_extensions import static_assert
+
+def _(a: int, b: int):
+    static_assert("foo" not in (a, b))
+```
+
+## Failure cases
+
+```py
+from ty_extensions import static_assert
+
+# We don't support byte strings.
+static_assert(b"foo" not in (b"quux", b"foo", b"baz"))  # error: [static-assert-error]
+```
diff --git a/crates/ty_python_semantic/resources/mdtest/ide_support/all_members.md b/crates/ty_python_semantic/resources/mdtest/ide_support/all_members.md
new file mode 100644
index 0000000000..63a0c21c30
--- /dev/null
+++ b/crates/ty_python_semantic/resources/mdtest/ide_support/all_members.md
@@ -0,0 +1,488 @@
+# List all members
+
+## Basic functionality
+
+<!-- snapshot-diagnostics -->
+
+The `ty_extensions.all_members` function allows access to a list of accessible members/attributes on
+a given object. For example, all member functions of `str` are available on `"a"`:
+
+```py
+from ty_extensions import all_members, static_assert
+
+members_of_str = all_members("a")
+
+static_assert("replace" in members_of_str)
+static_assert("startswith" in members_of_str)
+static_assert("isupper" in members_of_str)
+```
+
+Similarly, special members such as `__add__` are also available:
+
+```py
+static_assert("__add__" in members_of_str)
+static_assert("__gt__" in members_of_str)
+```
+
+Members of base classes are also included (these dunder methods are defined on `object`):
+
+```py
+static_assert("__doc__" in members_of_str)
+static_assert("__repr__" in members_of_str)
+```
+
+Non-existent members are not included:
+
+```py
+static_assert("non_existent" not in members_of_str)
+```
+
+Note: The full list of all members is relatively long, but `reveal_type` can theoretically be used
+to see them all:
+
+```py
+from typing_extensions import reveal_type
+
+reveal_type(members_of_str)  # error: [revealed-type]
+```
+
+## Kinds of types
+
+### Class instances
+
+For instances of classes, `all_members` returns class members and implicit instance members of all
+classes in the MRO:
+
+```py
+from ty_extensions import all_members, static_assert
+
+class Base:
+    base_class_attr: int = 1
+
+    def f_base(self):
+        self.base_instance_attr: str = "Base"
+
+class Intermediate(Base):
+    intermediate_attr: int = 2
+
+    def f_intermediate(self):
+        self.intermediate_instance_attr: str = "Intermediate"
+
+class C(Intermediate):
+    class_attr: int = 3
+
+    def f_c(self):
+        self.instance_attr = "C"
+
+    @property
+    def property_attr(self) -> int:
+        return 1
+
+    @classmethod
+    def class_method(cls) -> int:
+        return 1
+
+    @staticmethod
+    def static_method() -> int:
+        return 1
+
+members_of_instance = all_members(C())
+
+static_assert("base_class_attr" in members_of_instance)
+static_assert("intermediate_attr" in members_of_instance)
+static_assert("class_attr" in members_of_instance)
+
+static_assert("base_instance_attr" in members_of_instance)
+static_assert("intermediate_instance_attr" in members_of_instance)
+static_assert("instance_attr" in members_of_instance)
+
+static_assert("f_base" in members_of_instance)
+static_assert("f_intermediate" in members_of_instance)
+static_assert("f_c" in members_of_instance)
+
+static_assert("property_attr" in members_of_instance)
+static_assert("class_method" in members_of_instance)
+static_assert("static_method" in members_of_instance)
+
+static_assert("non_existent" not in members_of_instance)
+```
+
+### Class objects
+
+Class-level attributes can also be accessed through the class itself:
+
+```py
+from ty_extensions import all_members, static_assert
+
+class Base:
+    base_attr: int = 1
+
+class C(Base):
+    class_attr: str = "c"
+
+    def f(self):
+        self.instance_attr = True
+
+members_of_class = all_members(C)
+
+static_assert("class_attr" in members_of_class)
+static_assert("base_attr" in members_of_class)
+
+static_assert("non_existent" not in members_of_class)
+```
+
+But instance attributes can not be accessed this way:
+
+```py
+static_assert("instance_attr" not in members_of_class)
+```
+
+When a class has a metaclass, members of that metaclass (and bases of that metaclass) are also
+accessible:
+
+```py
+class MetaBase(type):
+    meta_base_attr = 1
+
+class Meta(MetaBase):
+    meta_attr = 2
+
+class D(Base, metaclass=Meta):
+    class_attr = 3
+
+static_assert("meta_base_attr" in all_members(D))
+static_assert("meta_attr" in all_members(D))
+static_assert("base_attr" in all_members(D))
+static_assert("class_attr" in all_members(D))
+```
+
+### Generic classes
+
+```py
+from ty_extensions import all_members, static_assert
+from typing import Generic, TypeVar
+
+T = TypeVar("T")
+
+class C(Generic[T]):
+    base_attr: T
+
+static_assert("base_attr" in all_members(C[int]))
+static_assert("base_attr" in all_members(C[int]()))
+```
+
+### Other instance-like types
+
+```py
+from ty_extensions import all_members, static_assert
+from typing_extensions import LiteralString
+
+static_assert("__xor__" in all_members(True))
+static_assert("bit_length" in all_members(1))
+static_assert("startswith" in all_members("a"))
+static_assert("__buffer__" in all_members(b"a"))
+static_assert("is_integer" in all_members(3.14))
+
+def _(literal_string: LiteralString):
+    static_assert("startswith" in all_members(literal_string))
+
+static_assert("count" in all_members(("some", "tuple", 1, 2)))
+
+static_assert("__doc__" in all_members(len))
+static_assert("__doc__" in all_members("a".startswith))
+```
+
+### Unions
+
+For unions, `all_members` will only return members that are available on all elements of the union.
+
+```py
+from ty_extensions import all_members, static_assert
+
+class A:
+    on_both: int = 1
+    only_on_a: str = "a"
+
+class B:
+    on_both: int = 2
+    only_on_b: str = "b"
+
+def f(union: A | B):
+    static_assert("on_both" in all_members(union))
+    static_assert("only_on_a" not in all_members(union))
+    static_assert("only_on_b" not in all_members(union))
+```
+
+### Intersections
+
+#### Only positive types
+
+Conversely, for intersections, `all_members` will list members that are available on any of the
+elements:
+
+```py
+from ty_extensions import all_members, static_assert
+
+class A:
+    on_both: int = 1
+    only_on_a: str = "a"
+
+class B:
+    on_both: int = 2
+    only_on_b: str = "b"
+
+def f(intersection: object):
+    if isinstance(intersection, A):
+        if isinstance(intersection, B):
+            static_assert("on_both" in all_members(intersection))
+            static_assert("only_on_a" in all_members(intersection))
+            static_assert("only_on_b" in all_members(intersection))
+```
+
+#### With negative types
+
+It also works when negative types are introduced:
+
+```py
+from ty_extensions import all_members, static_assert
+
+class A:
+    on_all: int = 1
+    only_on_a: str = "a"
+    only_on_ab: str = "a"
+    only_on_ac: str = "a"
+
+class B:
+    on_all: int = 2
+    only_on_b: str = "b"
+    only_on_ab: str = "b"
+    only_on_bc: str = "b"
+
+class C:
+    on_all: int = 3
+    only_on_c: str = "c"
+    only_on_ac: str = "c"
+    only_on_bc: str = "c"
+
+def f(intersection: object):
+    if isinstance(intersection, A):
+        if isinstance(intersection, B):
+            if not isinstance(intersection, C):
+                reveal_type(intersection)  # revealed: A & B & ~C
+                static_assert("on_all" in all_members(intersection))
+                static_assert("only_on_a" in all_members(intersection))
+                static_assert("only_on_b" in all_members(intersection))
+                static_assert("only_on_c" not in all_members(intersection))
+                static_assert("only_on_ab" in all_members(intersection))
+                static_assert("only_on_ac" in all_members(intersection))
+                static_assert("only_on_bc" in all_members(intersection))
+```
+
+## Modules
+
+### Basic support with sub-modules
+
+`all_members` can also list attributes on modules:
+
+```py
+from ty_extensions import all_members, static_assert
+import math
+
+static_assert("pi" in all_members(math))
+static_assert("cos" in all_members(math))
+```
+
+This also works for submodules:
+
+```py
+import os
+
+static_assert("path" in all_members(os))
+
+import os.path
+
+static_assert("join" in all_members(os.path))
+```
+
+Special members available on all modules are also included:
+
+```py
+static_assert("__name__" in all_members(math))
+static_assert("__doc__" in all_members(math))
+```
+
+### `__all__` is not respected for direct module access
+
+`foo.py`:
+
+```py
+from ty_extensions import all_members, static_assert
+
+import bar
+
+static_assert("lion" in all_members(bar))
+static_assert("tiger" in all_members(bar))
+```
+
+`bar.py`:
+
+```py
+__all__ = ["lion"]
+
+lion = 1
+tiger = 1
+```
+
+### `__all__` is respected for glob imports
+
+`foo.py`:
+
+```py
+from ty_extensions import all_members, static_assert
+
+import bar
+
+static_assert("lion" in all_members(bar))
+static_assert("tiger" not in all_members(bar))
+```
+
+`bar.py`:
+
+```py
+from quux import *
+```
+
+`quux.py`:
+
+```py
+__all__ = ["lion"]
+
+lion = 1
+tiger = 1
+```
+
+### `__all__` is respected for stub files
+
+`module.py`:
+
+```py
+def evaluate(x=None):
+    if x is None:
+        return 0
+    return x
+```
+
+`module.pyi`:
+
+```pyi
+from typing import Optional
+
+__all__ = ["evaluate"]
+
+def evaluate(x: Optional[int] = None) -> int: ...
+```
+
+`play.py`:
+
+```py
+from ty_extensions import all_members, static_assert
+
+import module
+
+static_assert("evaluate" in all_members(module))
+static_assert("Optional" not in all_members(module))
+```
+
+## Conditionally available members
+
+Some members are only conditionally available. For example, `int.bit_count` was only introduced in
+Python 3.10:
+
+### 3.9
+
+```toml
+[environment]
+python-version = "3.9"
+```
+
+```py
+from ty_extensions import all_members, static_assert
+
+static_assert("bit_count" not in all_members(42))
+```
+
+### 3.10
+
+```toml
+[environment]
+python-version = "3.10"
+```
+
+```py
+from ty_extensions import all_members, static_assert
+
+static_assert("bit_count" in all_members(42))
+```
+
+## Failures cases
+
+### Dynamically added members
+
+Dynamically added members can not be accessed:
+
+```py
+from ty_extensions import all_members, static_assert
+
+class C:
+    static_attr = 1
+
+    def __setattr__(self, name: str, value: str) -> None:
+        pass
+
+    def __getattr__(self, name: str) -> str:
+        return "a"
+
+c = C()
+c.dynamic_attr = "a"
+
+static_assert("static_attr" in all_members(c))
+static_assert("dynamic_attr" not in all_members(c))
+```
+
+### Dataclasses
+
+So far, we do not include synthetic members of dataclasses.
+
+```py
+from ty_extensions import all_members, static_assert
+from dataclasses import dataclass
+
+@dataclass(order=True)
+class Person:
+    name: str
+    age: int
+
+static_assert("name" in all_members(Person))
+static_assert("age" in all_members(Person))
+
+# These are always available, since they are also defined on `object`:
+static_assert("__init__" in all_members(Person))
+static_assert("__repr__" in all_members(Person))
+static_assert("__eq__" in all_members(Person))
+
+# TODO: this should ideally be available:
+static_assert("__lt__" in all_members(Person))  # error: [static-assert-error]
+```
+
+### Attributes not available at runtime
+
+Typeshed includes some attributes in `object` that are not available for some (builtin) types. For
+example, `__annotations__` does not exist on `int` at runtime, but it is available as an attribute
+on `object` in typeshed:
+
+```py
+from ty_extensions import all_members, static_assert
+
+# TODO: this should ideally not be available:
+static_assert("__annotations__" not in all_members(3))  # error: [static-assert-error]
+```
diff --git a/crates/ty_python_semantic/resources/mdtest/snapshots/all_members.md_-_List_all_members_-_Basic_functionality_(6b9531a70334bfad).snap b/crates/ty_python_semantic/resources/mdtest/snapshots/all_members.md_-_List_all_members_-_Basic_functionality_(6b9531a70334bfad).snap
new file mode 100644
index 0000000000..55a377e0c3
--- /dev/null
+++ b/crates/ty_python_semantic/resources/mdtest/snapshots/all_members.md_-_List_all_members_-_Basic_functionality_(6b9531a70334bfad).snap
@@ -0,0 +1,44 @@
+---
+source: crates/ty_test/src/lib.rs
+expression: snapshot
+---
+---
+mdtest name: all_members.md - List all members - Basic functionality
+mdtest path: crates/ty_python_semantic/resources/mdtest/ide_support/all_members.md
+---
+
+# Python source files
+
+## mdtest_snippet.py
+
+```
+ 1 | from ty_extensions import all_members, static_assert
+ 2 | 
+ 3 | members_of_str = all_members("a")
+ 4 | 
+ 5 | static_assert("replace" in members_of_str)
+ 6 | static_assert("startswith" in members_of_str)
+ 7 | static_assert("isupper" in members_of_str)
+ 8 | static_assert("__add__" in members_of_str)
+ 9 | static_assert("__gt__" in members_of_str)
+10 | static_assert("__doc__" in members_of_str)
+11 | static_assert("__repr__" in members_of_str)
+12 | static_assert("non_existent" not in members_of_str)
+13 | from typing_extensions import reveal_type
+14 | 
+15 | reveal_type(members_of_str)  # error: [revealed-type]
+```
+
+# Diagnostics
+
+```
+info[revealed-type]: Revealed type
+  --> src/mdtest_snippet.py:15:13
+   |
+13 | from typing_extensions import reveal_type
+14 |
+15 | reveal_type(members_of_str)  # error: [revealed-type]
+   |             ^^^^^^^^^^^^^^ `tuple[Literal["__add__"], Literal["__annotations__"], Literal["__class__"], Literal["__contains__"], Literal["__delattr__"], Literal["__dict__"], Literal["__dir__"], Literal["__doc__"], Literal["__eq__"], Literal["__format__"], Literal["__ge__"], Literal["__getattribute__"], Literal["__getitem__"], Literal["__getnewargs__"], Literal["__gt__"], Literal["__hash__"], Literal["__init__"], Literal["__init_subclass__"], Literal["__iter__"], Literal["__le__"], Literal["__len__"], Literal["__lt__"], Literal["__mod__"], Literal["__module__"], Literal["__mul__"], Literal["__ne__"], Literal["__new__"], Literal["__reduce__"], Literal["__reduce_ex__"], Literal["__repr__"], Literal["__reversed__"], Literal["__rmul__"], Literal["__setattr__"], Literal["__sizeof__"], Literal["__str__"], Literal["__subclasshook__"], Literal["capitalize"], Literal["casefold"], Literal["center"], Literal["count"], Literal["encode"], Literal["endswith"], Literal["expandtabs"], Literal["find"], Literal["format"], Literal["format_map"], Literal["index"], Literal["isalnum"], Literal["isalpha"], Literal["isascii"], Literal["isdecimal"], Literal["isdigit"], Literal["isidentifier"], Literal["islower"], Literal["isnumeric"], Literal["isprintable"], Literal["isspace"], Literal["istitle"], Literal["isupper"], Literal["join"], Literal["ljust"], Literal["lower"], Literal["lstrip"], Literal["maketrans"], Literal["partition"], Literal["removeprefix"], Literal["removesuffix"], Literal["replace"], Literal["rfind"], Literal["rindex"], Literal["rjust"], Literal["rpartition"], Literal["rsplit"], Literal["rstrip"], Literal["split"], Literal["splitlines"], Literal["startswith"], Literal["strip"], Literal["swapcase"], Literal["title"], Literal["translate"], Literal["upper"], Literal["zfill"]]`
+   |
+
+```
diff --git a/crates/ty_python_semantic/src/semantic_index.rs b/crates/ty_python_semantic/src/semantic_index.rs
index 7f23fab163..6d57a8515c 100644
--- a/crates/ty_python_semantic/src/semantic_index.rs
+++ b/crates/ty_python_semantic/src/semantic_index.rs
@@ -99,7 +99,9 @@ pub(crate) fn use_def_map<'db>(db: &'db dyn Db, scope: ScopeId<'db>) -> Arc<UseD
     index.use_def_map(scope.file_scope_id(db))
 }
 
-/// Returns all attribute assignments (and their method scope IDs) for a specific class body scope.
+/// Returns all attribute assignments (and their method scope IDs) with a symbol name matching
+/// the one given for a specific class body scope.
+///
 /// Only call this when doing type inference on the same file as `class_body_scope`, otherwise it
 /// introduces a direct dependency on that file's AST.
 pub(crate) fn attribute_assignments<'db, 's>(
@@ -109,6 +111,28 @@ pub(crate) fn attribute_assignments<'db, 's>(
 ) -> impl Iterator<Item = (BindingWithConstraintsIterator<'db, 'db>, FileScopeId)> + use<'s, 'db> {
     let file = class_body_scope.file(db);
     let index = semantic_index(db, file);
+
+    attribute_scopes(db, class_body_scope).filter_map(|function_scope_id| {
+        let attribute_table = index.instance_attribute_table(function_scope_id);
+        let symbol = attribute_table.symbol_id_by_name(name)?;
+        let use_def = &index.use_def_maps[function_scope_id];
+        Some((
+            use_def.instance_attribute_bindings(symbol),
+            function_scope_id,
+        ))
+    })
+}
+
+/// Returns all attribute assignments as scope IDs for a specific class body scope.
+///
+/// Only call this when doing type inference on the same file as `class_body_scope`, otherwise it
+/// introduces a direct dependency on that file's AST.
+pub(crate) fn attribute_scopes<'db, 's>(
+    db: &'db dyn Db,
+    class_body_scope: ScopeId<'db>,
+) -> impl Iterator<Item = FileScopeId> + use<'s, 'db> {
+    let file = class_body_scope.file(db);
+    let index = semantic_index(db, file);
     let class_scope_id = class_body_scope.file_scope_id(db);
 
     ChildrenIter::new(index, class_scope_id).filter_map(|(child_scope_id, scope)| {
@@ -124,13 +148,7 @@ pub(crate) fn attribute_assignments<'db, 's>(
             };
 
         function_scope.node().as_function()?;
-        let attribute_table = index.instance_attribute_table(function_scope_id);
-        let symbol = attribute_table.symbol_id_by_name(name)?;
-        let use_def = &index.use_def_maps[function_scope_id];
-        Some((
-            use_def.instance_attribute_bindings(symbol),
-            function_scope_id,
-        ))
+        Some(function_scope_id)
     })
 }
 
@@ -519,7 +537,7 @@ pub struct ChildrenIter<'a> {
 }
 
 impl<'a> ChildrenIter<'a> {
-    fn new(module_symbol_table: &'a SemanticIndex, parent: FileScopeId) -> Self {
+    pub(crate) fn new(module_symbol_table: &'a SemanticIndex, parent: FileScopeId) -> Self {
         let descendants = DescendantsIter::new(module_symbol_table, parent);
 
         Self {
diff --git a/crates/ty_python_semantic/src/types.rs b/crates/ty_python_semantic/src/types.rs
index 6665ddb7a8..f885b45f58 100644
--- a/crates/ty_python_semantic/src/types.rs
+++ b/crates/ty_python_semantic/src/types.rs
@@ -65,6 +65,7 @@ mod context;
 mod diagnostic;
 mod display;
 mod generics;
+mod ide_support;
 mod infer;
 mod instance;
 mod mro;
@@ -7662,6 +7663,8 @@ pub enum KnownFunction {
     GenericContext,
     /// `ty_extensions.dunder_all_names`
     DunderAllNames,
+    /// `ty_extensions.all_members`
+    AllMembers,
 }
 
 impl KnownFunction {
@@ -7721,7 +7724,8 @@ impl KnownFunction {
             | Self::IsSubtypeOf
             | Self::GenericContext
             | Self::DunderAllNames
-            | Self::StaticAssert => module.is_ty_extensions(),
+            | Self::StaticAssert
+            | Self::AllMembers => module.is_ty_extensions(),
         }
     }
 }
@@ -9390,7 +9394,8 @@ pub(crate) mod tests {
                 | KnownFunction::IsSingleValued
                 | KnownFunction::IsAssignableTo
                 | KnownFunction::IsEquivalentTo
-                | KnownFunction::IsGradualEquivalentTo => KnownModule::TyExtensions,
+                | KnownFunction::IsGradualEquivalentTo
+                | KnownFunction::AllMembers => KnownModule::TyExtensions,
             };
 
             let function_definition = known_module_symbol(&db, module, function_name)
diff --git a/crates/ty_python_semantic/src/types/call/bind.rs b/crates/ty_python_semantic/src/types/call/bind.rs
index fa52bcd960..51ab005c1b 100644
--- a/crates/ty_python_semantic/src/types/call/bind.rs
+++ b/crates/ty_python_semantic/src/types/call/bind.rs
@@ -3,6 +3,7 @@
 //! [signatures][crate::types::signatures], we have to handle the fact that the callable might be a
 //! union of types, each of which might contain multiple overloads.
 
+use itertools::Itertools;
 use smallvec::{SmallVec, smallvec};
 
 use super::{
@@ -22,7 +23,8 @@ use crate::types::signatures::{Parameter, ParameterForm};
 use crate::types::{
     BoundMethodType, DataclassParams, DataclassTransformerParams, FunctionDecorators, FunctionType,
     KnownClass, KnownFunction, KnownInstanceType, MethodWrapperKind, PropertyInstanceType,
-    SpecialFormType, TupleType, TypeMapping, UnionType, WrapperDescriptorKind, todo_type,
+    SpecialFormType, TupleType, TypeMapping, UnionType, WrapperDescriptorKind, ide_support,
+    todo_type,
 };
 use ruff_db::diagnostic::{Annotation, Diagnostic, Severity, SubDiagnostic};
 use ruff_python_ast as ast;
@@ -656,6 +658,18 @@ impl<'db> Bindings<'db> {
                             }
                         }
 
+                        Some(KnownFunction::AllMembers) => {
+                            if let [Some(ty)] = overload.parameter_types() {
+                                overload.set_return_type(TupleType::from_elements(
+                                    db,
+                                    ide_support::all_members(db, *ty)
+                                        .into_iter()
+                                        .sorted()
+                                        .map(|member| Type::string_literal(db, &member)),
+                                ));
+                            }
+                        }
+
                         Some(KnownFunction::Len) => {
                             if let [Some(first_arg)] = overload.parameter_types() {
                                 if let Some(len_ty) = first_arg.len(db) {
diff --git a/crates/ty_python_semantic/src/types/ide_support.rs b/crates/ty_python_semantic/src/types/ide_support.rs
new file mode 100644
index 0000000000..2ba4606d6a
--- /dev/null
+++ b/crates/ty_python_semantic/src/types/ide_support.rs
@@ -0,0 +1,183 @@
+use crate::Db;
+use crate::semantic_index::symbol::ScopeId;
+use crate::semantic_index::{
+    attribute_scopes, global_scope, semantic_index, symbol_table, use_def_map,
+};
+use crate::symbol::{imported_symbol, symbol_from_bindings, symbol_from_declarations};
+use crate::types::{ClassBase, ClassLiteral, KnownClass, Type};
+use ruff_python_ast::name::Name;
+use rustc_hash::FxHashSet;
+
+struct AllMembers {
+    members: FxHashSet<Name>,
+}
+
+impl AllMembers {
+    fn of<'db>(db: &'db dyn Db, ty: Type<'db>) -> Self {
+        let mut all_members = Self {
+            members: FxHashSet::default(),
+        };
+        all_members.extend_with_type(db, ty);
+        all_members
+    }
+
+    fn extend_with_type<'db>(&mut self, db: &'db dyn Db, ty: Type<'db>) {
+        match ty {
+            Type::Union(union) => self.members.extend(
+                union
+                    .elements(db)
+                    .iter()
+                    .map(|ty| AllMembers::of(db, *ty).members)
+                    .reduce(|acc, members| acc.intersection(&members).cloned().collect())
+                    .unwrap_or_default(),
+            ),
+
+            Type::Intersection(intersection) => self.members.extend(
+                intersection
+                    .positive(db)
+                    .iter()
+                    .map(|ty| AllMembers::of(db, *ty).members)
+                    .reduce(|acc, members| acc.union(&members).cloned().collect())
+                    .unwrap_or_default(),
+            ),
+
+            Type::NominalInstance(instance) => {
+                let (class_literal, _specialization) = instance.class.class_literal(db);
+                self.extend_with_class_members(db, class_literal);
+                self.extend_with_instance_members(db, class_literal);
+            }
+
+            Type::ClassLiteral(class_literal) => {
+                self.extend_with_class_members(db, class_literal);
+
+                if let Type::ClassLiteral(meta_class_literal) = ty.to_meta_type(db) {
+                    self.extend_with_class_members(db, meta_class_literal);
+                }
+            }
+
+            Type::GenericAlias(generic_alias) => {
+                let class_literal = generic_alias.origin(db);
+                self.extend_with_class_members(db, class_literal);
+            }
+
+            Type::SubclassOf(subclass_of_type) => {
+                if let Some(class_literal) = subclass_of_type.subclass_of().into_class() {
+                    self.extend_with_class_members(db, class_literal.class_literal(db).0);
+                }
+            }
+
+            Type::Dynamic(_) | Type::Never | Type::AlwaysTruthy | Type::AlwaysFalsy => {}
+
+            Type::IntLiteral(_)
+            | Type::BooleanLiteral(_)
+            | Type::StringLiteral(_)
+            | Type::BytesLiteral(_)
+            | Type::LiteralString
+            | Type::Tuple(_)
+            | Type::PropertyInstance(_)
+            | Type::FunctionLiteral(_)
+            | Type::BoundMethod(_)
+            | Type::MethodWrapper(_)
+            | Type::WrapperDescriptor(_)
+            | Type::DataclassDecorator(_)
+            | Type::DataclassTransformer(_)
+            | Type::Callable(_)
+            | Type::ProtocolInstance(_)
+            | Type::SpecialForm(_)
+            | Type::KnownInstance(_)
+            | Type::TypeVar(_)
+            | Type::BoundSuper(_) => {
+                if let Type::ClassLiteral(class_literal) = ty.to_meta_type(db) {
+                    self.extend_with_class_members(db, class_literal);
+                }
+            }
+
+            Type::ModuleLiteral(literal) => {
+                self.extend_with_type(db, KnownClass::ModuleType.to_instance(db));
+
+                let Some(file) = literal.module(db).file() else {
+                    return;
+                };
+
+                let module_scope = global_scope(db, file);
+                let use_def_map = use_def_map(db, module_scope);
+                let symbol_table = symbol_table(db, module_scope);
+
+                for (symbol_id, _) in use_def_map.all_public_declarations() {
+                    let symbol_name = symbol_table.symbol(symbol_id).name();
+                    if !imported_symbol(db, file, symbol_name, None)
+                        .symbol
+                        .is_unbound()
+                    {
+                        self.members
+                            .insert(symbol_table.symbol(symbol_id).name().clone());
+                    }
+                }
+            }
+        }
+    }
+
+    fn extend_with_declarations_and_bindings(&mut self, db: &dyn Db, scope_id: ScopeId) {
+        let use_def_map = use_def_map(db, scope_id);
+        let symbol_table = symbol_table(db, scope_id);
+
+        for (symbol_id, declarations) in use_def_map.all_public_declarations() {
+            if symbol_from_declarations(db, declarations)
+                .is_ok_and(|result| !result.symbol.is_unbound())
+            {
+                self.members
+                    .insert(symbol_table.symbol(symbol_id).name().clone());
+            }
+        }
+
+        for (symbol_id, bindings) in use_def_map.all_public_bindings() {
+            if !symbol_from_bindings(db, bindings).is_unbound() {
+                self.members
+                    .insert(symbol_table.symbol(symbol_id).name().clone());
+            }
+        }
+    }
+
+    fn extend_with_class_members<'db>(
+        &mut self,
+        db: &'db dyn Db,
+        class_literal: ClassLiteral<'db>,
+    ) {
+        for parent in class_literal
+            .iter_mro(db, None)
+            .filter_map(ClassBase::into_class)
+            .map(|class| class.class_literal(db).0)
+        {
+            let parent_scope = parent.body_scope(db);
+            self.extend_with_declarations_and_bindings(db, parent_scope);
+        }
+    }
+
+    fn extend_with_instance_members<'db>(
+        &mut self,
+        db: &'db dyn Db,
+        class_literal: ClassLiteral<'db>,
+    ) {
+        for parent in class_literal
+            .iter_mro(db, None)
+            .filter_map(ClassBase::into_class)
+            .map(|class| class.class_literal(db).0)
+        {
+            let class_body_scope = parent.body_scope(db);
+            let file = class_body_scope.file(db);
+            let index = semantic_index(db, file);
+            for function_scope_id in attribute_scopes(db, class_body_scope) {
+                let attribute_table = index.instance_attribute_table(function_scope_id);
+                for symbol in attribute_table.symbols() {
+                    self.members.insert(symbol.name().clone());
+                }
+            }
+        }
+    }
+}
+
+/// List all members of a given type: anything that would be valid when accessed
+/// as an attribute on an object of the given type.
+pub(crate) fn all_members<'db>(db: &'db dyn Db, ty: Type<'db>) -> FxHashSet<Name> {
+    AllMembers::of(db, ty).members
+}
diff --git a/crates/ty_python_semantic/src/types/infer.rs b/crates/ty_python_semantic/src/types/infer.rs
index 50964ded3d..73aefafb3c 100644
--- a/crates/ty_python_semantic/src/types/infer.rs
+++ b/crates/ty_python_semantic/src/types/infer.rs
@@ -6719,6 +6719,37 @@ impl<'db> TypeInferenceBuilder<'db> {
         right: Type<'db>,
         range: TextRange,
     ) -> Result<Type<'db>, CompareUnsupportedError<'db>> {
+        let is_str_literal_in_tuple = |literal: Type<'db>, tuple: TupleType<'db>| {
+            // Protect against doing a lot of work for pathologically large
+            // tuples.
+            //
+            // Ref: https://github.com/astral-sh/ruff/pull/18251#discussion_r2115909311
+            if tuple.len(self.db()) > 1 << 12 {
+                return None;
+            }
+
+            let mut definitely_true = false;
+            let mut definitely_false = true;
+            for element in tuple.elements(self.db()) {
+                if element.is_string_literal() {
+                    if literal == *element {
+                        definitely_true = true;
+                        definitely_false = false;
+                    }
+                } else if !literal.is_disjoint_from(self.db(), *element) {
+                    definitely_false = false;
+                }
+            }
+
+            if definitely_true {
+                Some(true)
+            } else if definitely_false {
+                Some(false)
+            } else {
+                None
+            }
+        };
+
         // Note: identity (is, is not) for equal builtin types is unreliable and not part of the
         // language spec.
         // - `[ast::CompOp::Is]`: return `false` if unequal, `bool` if equal
@@ -6850,6 +6881,30 @@ impl<'db> TypeInferenceBuilder<'db> {
                     }
                 }
             }
+            (Type::StringLiteral(_), Type::Tuple(tuple)) if op == ast::CmpOp::In => {
+                if let Some(answer) = is_str_literal_in_tuple(left, tuple) {
+                    return Ok(Type::BooleanLiteral(answer));
+                }
+
+                self.infer_binary_type_comparison(
+                    KnownClass::Str.to_instance(self.db()),
+                    op,
+                    right,
+                    range,
+                )
+            }
+            (Type::StringLiteral(_), Type::Tuple(tuple)) if op == ast::CmpOp::NotIn => {
+                if let Some(answer) = is_str_literal_in_tuple(left, tuple) {
+                    return Ok(Type::BooleanLiteral(!answer));
+                }
+
+                self.infer_binary_type_comparison(
+                    KnownClass::Str.to_instance(self.db()),
+                    op,
+                    right,
+                    range,
+                )
+            }
             (Type::StringLiteral(_), _) => self.infer_binary_type_comparison(
                 KnownClass::Str.to_instance(self.db()),
                 op,
diff --git a/crates/ty_vendored/ty_extensions/ty_extensions.pyi b/crates/ty_vendored/ty_extensions/ty_extensions.pyi
index 127f97c3c5..0b98b7a55e 100644
--- a/crates/ty_vendored/ty_extensions/ty_extensions.pyi
+++ b/crates/ty_vendored/ty_extensions/ty_extensions.pyi
@@ -43,3 +43,13 @@ def generic_context(type: Any) -> Any: ...
 # Returns the `__all__` names of a module as a tuple of sorted strings, or `None` if
 # either the module does not have `__all__` or it has invalid elements.
 def dunder_all_names(module: Any) -> Any: ...
+
+# Returns a tuple of all members of the given object, similar to `dir(obj)` and
+# `inspect.getmembers(obj)`, with at least the following differences:
+#
+# * `dir` and `inspect.getmembers` may use runtime mutable state to construct
+# the list of attributes returned. In contrast, this routine is limited to
+# static information only.
+# * `dir` will respect an object's `__dir__` implementation, if present, but
+# this method (currently) does not.
+def all_members(obj: Any) -> tuple[str, ...]: ...