Enable classic COM on mingw-w64 (#1215)

Author: alvinhochun

strings/base_macros.h

@@ -59,6 +59,14 @@
 #define WINRT_IMPL_NOVTABLE
 #endif
 
+#if defined(__clang__)
+#define WINRT_IMPL_HAS_DECLSPEC_UUID __has_declspec_attribute(uuid)
+#elif defined(_MSC_VER)
+#define WINRT_IMPL_HAS_DECLSPEC_UUID 1
+#else
+#define WINRT_IMPL_HAS_DECLSPEC_UUID 0
+#endif
+
 #ifdef __IUnknown_INTERFACE_DEFINED__
 #define WINRT_IMPL_IUNKNOWN_DEFINED
 #else

strings/base_meta.h

@@ -120,27 +120,17 @@ namespace winrt::impl
     template <typename T>
     struct classic_com_guid_error
     {
-#ifdef __clang__
-#if !__has_declspec_attribute(uuid)
+#if !defined(__MINGW32__) && defined(__clang__) && !WINRT_IMPL_HAS_DECLSPEC_UUID
         static_assert(std::is_void_v<T> /* dependent_false */, "To use classic COM interfaces, you must compile with -fms-extensions.");
-#endif
-
-#ifndef WINRT_IMPL_IUNKNOWN_DEFINED
+#elif !defined(WINRT_IMPL_IUNKNOWN_DEFINED)
         static_assert(std::is_void_v<T> /* dependent_false */, "To use classic COM interfaces, you must include <unknwn.h> before including C++/WinRT headers.");
-#endif
 #else // MSVC won't hit this struct, so we can safely assume everything that isn't Clang isn't supported
         static_assert(std::is_void_v<T> /* dependent_false */, "Classic COM interfaces are not supported with this compiler.");
 #endif
     };
 
     template <typename T>
-#ifdef __clang__
-#if __has_declspec_attribute(uuid) && defined(WINRT_IMPL_IUNKNOWN_DEFINED)
-    inline constexpr guid guid_v{ __uuidof(T) };
-#else
-    inline constexpr guid guid_v = classic_com_guid_error<T>::value;
-#endif
-#elif defined(_MSC_VER)
+#if (defined(_MSC_VER) && !defined(__clang__)) || ((WINRT_IMPL_HAS_DECLSPEC_UUID || defined(__MINGW32__)) && defined(WINRT_IMPL_IUNKNOWN_DEFINED))
     inline constexpr guid guid_v{ __uuidof(T) };
 #else
     inline constexpr guid guid_v = classic_com_guid_error<T>::value;