From d2bd72e7956a6ba03fe64e7a1a7012a6364b8807 Mon Sep 17 00:00:00 2001
From: Raymond Chen <oldnewthing@users.noreply.github.com>
Date: Sun, 13 Jun 2021 12:13:09 -0700
Subject: [PATCH 1/2] Improve diagnostics when implements<> is used incorectly

A common error is forgetting to provide the name of the derived
class as the first `implements<>` template parameter. This results
in a strange `implements_type is not defined` error. Detect this
case with a `static_assert` with a more help error message.

Note that we have to put the `static_assert` inside the implementation
of `GetRuntimeClassName` rather than putting it in the class definition
of `implements`. That's because `implements` is sometimes used with
an incomplete type, and we cannot use `is_base_of` with incomplete types.
Wait until `GetRuntimeClassName` is generated, at which point the class
is complete.
---
 strings/base_implements.h | 1 +
 1 file changed, 1 insertion(+)

diff --git a/strings/base_implements.h b/strings/base_implements.h
index a763da424..8202360d6 100644
--- a/strings/base_implements.h
+++ b/strings/base_implements.h
@@ -1437,6 +1437,7 @@ WINRT_EXPORT namespace winrt
 
         hstring GetRuntimeClassName() const override
         {
+            static_assert(std::is_base_of_v<implements_type, D>, "First template parameter to implements<> or ClassT<> must be derived class name, e.g. struct X : implements<X, ...>");
             return impl::runtime_class_name<typename impl::implements_default_interface<D>::type>::get();
         }
 

From b4ad42c593a4eeae664a27623c8eca8ed9f3be84 Mon Sep 17 00:00:00 2001
From: Raymond Chen <oldnewthing@users.noreply.github.com>
Date: Sun, 13 Jun 2021 16:03:18 -0700
Subject: [PATCH 2/2] Another common error is forgetting to derive from
 implements at all

---
 strings/base_implements.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/strings/base_implements.h b/strings/base_implements.h
index 8202360d6..7bc85220b 100644
--- a/strings/base_implements.h
+++ b/strings/base_implements.h
@@ -1437,7 +1437,7 @@ WINRT_EXPORT namespace winrt
 
         hstring GetRuntimeClassName() const override
         {
-            static_assert(std::is_base_of_v<implements_type, D>, "First template parameter to implements<> or ClassT<> must be derived class name, e.g. struct X : implements<X, ...>");
+            static_assert(std::is_base_of_v<implements_type, D>, "Class must derive from implements<> or ClassT<> where the first template parameter is the derived class name, e.g. struct D : implements<D, ...>");
             return impl::runtime_class_name<typename impl::implements_default_interface<D>::type>::get();
         }