Add a clang-specific impl->projection conversion operator (#1274)

Author: DHowett

strings/base_implements.h

@@ -110,10 +110,25 @@ namespace winrt::impl
     template <typename D, typename I, typename Enable>
     struct producer_convert : producer<D, typename default_interface<I>::type>
     {
+#ifdef __clang__
+        // This is sub-optimal in that it requires an AddRef and Release of the
+        // implementation type for every conversion, but it works around an
+        // issue where Clang ignores the conversion of producer_ref<I> const
+        // to I&& (an rvalue ref that cannot bind a const rvalue).
+        // See CWG rev. 110 active issue 2077, "Overload resolution and invalid
+        // rvalue-reference initialization"
+        operator I() const noexcept
+        {
+            I result{ nullptr };
+            copy_from_abi(result, (produce<D, typename default_interface<I>::type>*)this);
+            return result;
+        }
+#else
         operator producer_ref<I> const() const noexcept
         {
             return { (produce<D, typename default_interface<I>::type>*)this };
         }
+#endif
 
         operator producer_vtable<I> const() const noexcept
         {

test/old_tests/UnitTests/as_implements.cpp

@@ -138,3 +138,13 @@ TEST_CASE("as_implements_inheritance")
         REQUIRE(bar.get() == foo2.get());
     }
 }
+
+TEST_CASE("convert_to_implements_via_uniform_initialization")
+{
+    // uniform initialization relies on IStringable(IStringable&&),
+    // which requires non-const rvalue semantics.
+    com_ptr<Foo> foo = make_self<Foo>();
+    IStringable stringable{ *foo };
+    com_ptr<Foo> foo2 = stringable.as<Foo>();
+    REQUIRE(foo.get() == foo2.get());
+}