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

[zero9178]

Summary

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.

[ChuanqiXu]

LGTM.

clang/test/CXX/drs/dr12xx.cpp

#if __cplusplus >= 201103L
   // We apply the same rule to vector subscripting.
   typedef int V4Int __attribute__((__vector_size__(sizeof(int) * 4)));
   typedef int EV4Int __attribute__((__ext_vector_type__(4)));
   using U = decltype(V4Int()[0]);
   using U = decltype(EV4Int()[0]);
 #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;
 
 struct Base {
   virtual const Incomplete *meow() = 0;
 };
 
 struct Derived : Base {

clang/www/cxx_dr_status.html

<tr id="1226">
     <td>CD3</td>
     <td>Converting a <I>braced-init-list</I> default argument</td>
     <td class="none" align="center">Unknown</td>
   </tr>
   <tr id="1227">
     <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>
     <td>NAD</td>
     <td>Copy-list-initialization and <TT>explicit</TT> constructors</td>
     <td class="none" align="center">Unknown</td>
   </tr>
   <tr id="1229">