Use a checked cast where the ABI type is a real base

The previous change assumed every ABI type derives from unknown_abi, which
is false for classic COM interfaces: they derive from ::IUnknown and are
layout compatible but unrelated, so a static_cast does not compile. That
broke the test projects, which implement ICapture and IMoreDerived.

impl::abi_cast picks static_cast when a conversion exists and falls back to
reinterpret_cast otherwise. C4946 only fires for related classes, so the
fallback is silent and the warning is still gone. Applied to both to_abi
overloads as well, which have the same problem in reverse: producer derives
from the interface for classic COM but holds its vtable by value for WinRT.

Also brace the name_v terminator array, which only compiles on that path.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
Copilot-Session: b396c800-f1bf-4ba4-a50f-a7f91d4d4f94
Hostname: ManJump9

Author: DanielJump

strings/base_identity.h

@@ -468,7 +468,7 @@ WINRT_EXPORT namespace winrt::impl
         combine
         (
             to_array<wchar_t>(guid_of<T>()),
-            std::array<wchar_t, 1>{ L'\0' }
+            std::array<wchar_t, 1>{{ L'\0' }}
         )
     };
 

strings/base_implements.h

@@ -232,6 +232,24 @@ WINRT_EXPORT namespace winrt::impl
             std::memset(ptr, 0, sizeof(T));
         }
     }
+
+    // Converts between an implementation type and the ABI type it is layout compatible with.
+    // Classic COM interfaces derive from ::IUnknown rather than from unknown_abi, and producer
+    // holds its vtable by value for WinRT interfaces, so the two types are often unrelated and
+    // only a reinterpret_cast will do. Where they are related, a reinterpret_cast is both
+    // unnecessary and reported as C4946, so prefer the checked conversion when one exists.
+    template <typename To, typename From>
+    To* abi_cast(From* from) noexcept
+    {
+        if constexpr (std::is_convertible_v<From*, To*>)
+        {
+            return static_cast<To*>(from);
+        }
+        else
+        {
+            return reinterpret_cast<To*>(from);
+        }
+    }
 }
 
 WINRT_EXPORT namespace winrt
@@ -257,13 +275,13 @@ WINRT_EXPORT namespace winrt
     template <typename I, typename D>
     impl::abi_t<I>* to_abi(impl::producer<D, I> const* from) noexcept
     {
-        return reinterpret_cast<impl::abi_t<I>*>(const_cast<impl::producer<D, I>*>(from));
+        return impl::abi_cast<impl::abi_t<I>>(const_cast<impl::producer<D, I>*>(from));
     }
 
     template <typename I, typename D>
     impl::abi_t<I>* to_abi(impl::producer_convert<D, I> const* from) noexcept
     {
-        return reinterpret_cast<impl::abi_t<I>*>((impl::producer<D, default_interface<I>>*)from);
+        return impl::abi_cast<impl::abi_t<I>>(const_cast<impl::producer<D, default_interface<I>>*>(static_cast<impl::producer<D, default_interface<I>> const*>(from)));
     }
 }
 
@@ -1568,7 +1586,7 @@ WINRT_EXPORT namespace winrt
 
         impl::unknown_abi* get_unknown() const noexcept override
         {
-            return static_cast<impl::unknown_abi*>(to_abi<typename impl::implements_default_interface<D>::type>(this));
+            return impl::abi_cast<impl::unknown_abi>(to_abi<typename impl::implements_default_interface<D>::type>(this));
         }
 
         hstring GetRuntimeClassName() const override