[clang][DR] Add test for DR1227 and mark it as complete

DR: http://www.open-std.org/jtc1/sc22/wg21/docs/cwg_defects.html#1227

This DR has been implemented in Clang as far back as I could check. This patch simply adds a test and comment to mark it as complete.

Differential Revision: https://reviews.llvm.org/D122682

clang/test/CXX/drs/dr12xx.cpp

@@ -27,6 +27,21 @@ namespace dr1213 { // dr1213: 7
 #endif
 }
 
+#if __cplusplus >= 201103L
+namespace dr1227 { // dr1227: yes
+template <class T> struct A { using X = typename T::X; }; // expected-error {{type 'int' cannot be used prior to '::' because it has no members}}
+template <class T> typename T::X f(typename A<T>::X);
+template <class T> void f(...) { }
+template <class T> auto g(typename A<T>::X) -> typename T::X; // expected-note {{in instantiation of template class 'dr1227::A<int>' requested here}}
+template <class T> void g(...) { }
+
+void h() {
+  f<int>(0); // OK, substituting return type causes deduction to fail
+  g<int>(0); // expected-note {{while substituting explicitly-specified template arguments into function template 'g'}}
+}
+}
+#endif
+
 namespace dr1250 { // dr1250: 3.9
 struct Incomplete;
 

clang/www/cxx_dr_status.html

@@ -7176,7 +7176,7 @@ and <I>POD class</I></td>
     <td><a href="https://wg21.link/cwg1227">1227</a></td>
     <td>CD3</td>
     <td>Mixing immediate and non-immediate contexts in deduction failure</td>
-    <td class="none" align="center">Unknown</td>
+    <td class="full" align="center">Yes</td>
   </tr>
   <tr id="1228">
     <td><a href="https://wg21.link/cwg1228">1228</a></td>