[cxx-interop] Document features that shipped in 6.1 and earlier

* [cxx-interop] Document virtual method support

* [cxx-interop] Document `std::unique_ptr` support

* [cxx-interop] Update stale link to known issues

* [cxx-interop] Document `char8_t` support

* [cxx-interop] Document nested struct support

* [cxx-interop] Update revision history

Author: egorzhdan

documentation/cxx-interop/index.md

@@ -492,7 +492,8 @@ because they return a reference that points into the `Forest` object.
 
 #### Virtual Member Functions
 
-Currently virtual member functions are not available in Swift.
+Virtual methods of C++ value types cannot be called from Swift. This is similar
+to C++, where you can only call virtual methods on a pointer or a reference.
 
 #### Static Member Functions
 
@@ -1254,6 +1255,14 @@ object.doSomething()
 // `object` will be released here.
 ```
 
+### Inheritance and Virtual Member Functions
+
+Similar to value types, casting an instance of a derived reference type to a
+base reference type, or vice versa, is not yet supported by Swift.
+
+If a reference type has virtual methods, you can call those methods from Swift.
+This includes pure virtual methods.
+
 #### Exposing C++ Shared Reference Types back from Swift
 
 C++ can call into Swift APIs that take or return C++ Shared Reference Types. Objects of these types are always created on the C++ side,
@@ -1276,11 +1285,6 @@ The C++ caller of this function is responsible for releasing the object.
 The `SWIFT_UNSAFE_REFERENCE` annotation macro has the same effect as `SWIFT_IMMORTAL_REFERENCE`
 annotation macro. However, it communicates different semantics: the type is intended to be used unsafely, rather than living for the duration of the program.
 
-### Unique Reference Types
-
-Unique reference types, such as types passed around by `std::unique_ptr` are
-not yet supported by Swift.
-
 ## Using C++ Standard Library from Swift
 
 This section describes how to import the C++ standard library, and how
@@ -1982,6 +1986,11 @@ that are outlined in the documentation above.
 
 This section lists the recent changes made to this reference guide.
 
+**2025-04-07**
+
+- C++ virtual methods of reference types are now available in Swift.
+- `std::unique_ptr` is now supported in Swift.
+
 **2024-08-12**
 
 - Added several customization macros from `<swift/bridging>` to the list.

documentation/cxx-interop/status/index.md

@@ -196,10 +196,11 @@ The following C++ standard library types are supported in Swift:
 - Specializations of `std::set`, `std::multiset` and `std::unordered_set`
 - Specializations of `std::optional`
 - Specializations of `std::shared_ptr`
+- Specializations of `std::unique_ptr`
 - Specializations of `std::array`
 
-Other standard library types, like `std::unique_ptr`, `std::function` and
-`std::variant` are not yet supported in Swift.
+Other standard library types, like `std::function` and `std::variant`, are not
+yet supported in Swift.
 
 ### Other C++ Features Handled by Swift
 
@@ -229,16 +230,14 @@ header.
 
 ### Swift Structures Supported by C++
 
-Swift can generate C++ representation for most top-level Swift structures. The
-following Swift structures are not yet supported:
+Swift can generate C++ representation for most Swift structures. The following
+Swift structures are not yet supported:
 
 - Zero-sized structures that don't have any stored properties
 - Non-copyable structures
 - Generic structures with generic constraints, or with more than three generic
   parameters, or that have variadic generics
 
-Swift currently does not expose nested structures to C++.
-
 ### Swift Classes and Actors Supported by C++
 
 Swift can generate C++ representation for most top-level Swift classes and
@@ -247,8 +246,6 @@ following Swift classes are not yet supported:
 
 - Generic classes and actors
 
-Swift currently does not expose nested classes and actors to C++.
-
 ### Swift Enumerations Supported by C++
 
 Swift can generate C++ representation for most top-level Swift enumerations
@@ -351,6 +348,7 @@ standard library that can be represented in C++:
 | `CBool`   | `bool`       |
 | `CChar`   | `char`       |
 | `CWideChar`   | `wchar_t`   |
+| `CChar8`    | `char8_t`        |
 | `CChar16`   | `char16_t`       |
 | `CChar32`   | `char32_t`   |
 | `CSignedChar`   | `signed char`   |
@@ -369,7 +367,7 @@ standard library that can be represented in C++:
 ## Constraints and Limitations
 
 Swift has some known limitations related to C++ interoperability support.
-They're currently [listed on GitHub](https://github.com/swiftlang/swift/issues/66159).
+They are currently [tracked on GitHub](https://github.com/swiftlang/swift/issues?q=is%3Aissue%20state%3Aopen%20label%3A%22c%2B%2B%20interop%22).
 
 ### Swift Package Manager Constraints