Allow classic COM interfaces with get_self (#1314)

* Allow classic COM interfaces with get_self

Fixes #1312

* Fix mingw builds

---------

Co-authored-by: Kenny Kerr <kenny@kennykerr.ca>

Author: sylveon

strings/base_implements.h

@@ -242,6 +242,12 @@ WINRT_EXPORT namespace winrt
         return &static_cast<impl::produce<D, default_interface<I>>*>(get_abi(from))->shim();
     }
 
+    template <typename D, typename I>
+    D* get_self(com_ptr<I> const& from) noexcept
+    {
+        return static_cast<D*>(static_cast<impl::producer<D, I>*>(from.get()));
+    }
+
     template <typename D, typename I>
     [[deprecated]] D* from_abi(I const& from) noexcept
     {

strings/base_meta.h

@@ -13,6 +13,9 @@ WINRT_EXPORT namespace winrt
     template <typename T>
     struct com_ptr;
 
+    template <typename D, typename I>
+    D* get_self(com_ptr<I> const& from) noexcept;
+
     namespace param
     {
         template <typename T>

test/old_tests/UnitTests/interop.cpp

@@ -7,6 +7,10 @@ using namespace Windows::Foundation;
 
 namespace
 {
+    struct IClassicComInterface : ::IUnknown {};
+
+    struct ClassicCom : implements<ClassicCom, IClassicComInterface> {};
+
     struct Stringable : implements<Stringable, IStringable>
     {
         Stringable(std::wstring_view const& value = L"Stringable") : m_value(value)
@@ -30,8 +34,16 @@ namespace
         object->AddRef();
         return object->Release();
     }
+
+    template <typename T>
+    uint32_t get_ref_count(com_ptr<T> const& object)
+    {
+        return get_ref_count(object.get());
+    }
 }
 
+template <> inline constexpr winrt::guid winrt::impl::guid_v<IClassicComInterface>{ 0xc136bb75, 0xbc03, 0x41a6, { 0xa5, 0xdc, 0x5e, 0xfa, 0x67, 0x92, 0x4e, 0xbf } };
+
 TEST_CASE("interop")
 {
     uint32_t const before = get_module_lock();
@@ -108,6 +120,43 @@ TEST_CASE("self")
     REQUIRE(get_ref_count(object) == 1);
     object = nullptr;
 
+    strong = weak.get();
+    REQUIRE(!strong);
+}
+
+TEST_CASE("self_classic_com")
+{
+    com_ptr<ClassicCom> strong = make_self<ClassicCom>();
+
+    REQUIRE(get_ref_count(strong.get()) == 1);
+
+    com_ptr<IClassicComInterface> object = strong.as<IClassicComInterface>();
+
+    REQUIRE(get_ref_count(strong.get()) == 2);
+
+    ClassicCom* ptr = get_self<ClassicCom>(object);
+    REQUIRE(ptr == strong.get());
+
+    REQUIRE(get_ref_count(strong.get()) == 2);
+    strong = nullptr;
+    REQUIRE(get_ref_count(object) == 1);
+
+    strong = get_self<ClassicCom>(object)->get_strong();
+    REQUIRE(get_ref_count(object) == 2);
+    strong = nullptr;
+    REQUIRE(get_ref_count(object) == 1);
+
+    weak_ref<ClassicCom> weak = get_self<ClassicCom>(object)->get_weak();
+    REQUIRE(get_ref_count(object) == 1); // <-- still just one!
+
+    strong = weak.get();
+    REQUIRE(strong);
+    REQUIRE(get_ref_count(object) == 2);
+
+    strong = nullptr;
+    REQUIRE(get_ref_count(object) == 1);
+    object = nullptr;
+
     strong = weak.get();
     REQUIRE(!strong);
 }