[clang] Add test for CWG1894 and CWG2199

[[https://wg21.link/p1787 | P1787]]: CWG1894 and its duplicate CWG2199 are resolved per Richard’s proposal for [[ https://listarchives.isocpp.org/cgi-bin/wg21/message?wg=core&msg=28415 | “dr407 still leaves open questions about typedef / tag hiding” ]], using generic conflicting-declaration rules even for typedef, and discarding a redundant typedef-name when looking up an elaborated-type-specifier.
Wording: See changes to [dcl.typedef], [basic.lookup.elab], and [basic.lookup]/4.

Generic conflicting-declaration rules are specified in changes to [basic.scope.scope]. [[ https://cplusplus.github.io/CWG/issues/407.html | CWG407]], [[ https://cplusplus.github.io/CWG/issues/1894.html | CWG1894 ]], and [[ https://cplusplus.github.io/CWG/issues/2199.html | CWG2199 ]] discuss how elaborated type specifiers interact with typedefs, using directives, and using declarations. Since existing test for CWG407 covers examples provided in CWG1894 and CWG2199, and does it in accordance with P1787, I reused parts of it.

Reviewed By: #clang-language-wg, cor3ntin

Differential Revision: https://reviews.llvm.org/D148136

clang/test/CXX/drs/dr18xx.cpp

@@ -168,3 +168,31 @@ void dr1891() { // dr1891: 4
   b = static_cast<B&&>(b); // expected-error {{copy assignment operator is implicitly deleted}}
 #endif
 }
+
+namespace dr1894 { // dr1894: 3.8
+                   // NB: reusing part of dr407 test
+namespace A {
+  struct S {};
+}
+namespace B {
+  typedef int S;
+}
+namespace E {
+  typedef A::S S;
+  using A::S;
+  struct S s;
+}
+namespace F {
+  typedef A::S S;
+}
+namespace G {
+  using namespace A;
+  using namespace F;
+  struct S s;
+}
+namespace H {
+  using namespace F;
+  using namespace A;
+  struct S s;
+}
+}

clang/test/CXX/drs/dr21xx.cpp

@@ -1,7 +1,9 @@
 // RUN: %clang_cc1 -std=c++98 -triple x86_64-unknown-unknown %s -verify -fexceptions -Wno-deprecated-builtins -fcxx-exceptions -pedantic-errors
 // RUN: %clang_cc1 -std=c++11 -triple x86_64-unknown-unknown %s -verify -fexceptions -Wno-deprecated-builtins -fcxx-exceptions -pedantic-errors
 // RUN: %clang_cc1 -std=c++14 -triple x86_64-unknown-unknown %s -verify -fexceptions -Wno-deprecated-builtins -fcxx-exceptions -pedantic-errors
-// RUN: %clang_cc1 -std=c++1z -triple x86_64-unknown-unknown %s -verify -fexceptions -Wno-deprecated-builtins -fcxx-exceptions -pedantic-errors
+// RUN: %clang_cc1 -std=c++17 -triple x86_64-unknown-unknown %s -verify -fexceptions -Wno-deprecated-builtins -fcxx-exceptions -pedantic-errors
+// RUN: %clang_cc1 -std=c++20 -triple x86_64-unknown-unknown %s -verify -fexceptions -Wno-deprecated-builtins -fcxx-exceptions -pedantic-errors
+// RUN: %clang_cc1 -std=c++2b -triple x86_64-unknown-unknown %s -verify -fexceptions -Wno-deprecated-builtins -fcxx-exceptions -pedantic-errors
 
 #if __cplusplus < 201103L
 // expected-error@+1 {{variadic macro}}
@@ -188,3 +190,31 @@ namespace dr2180 { // dr2180: yes
   B &B::operator=(B&&) = default; // expected-error {{would delete}} expected-note@-10{{inaccessible move assignment}}
 #endif
 }
+
+namespace dr2199 { // dr2199: 3.8
+                   // NB: reusing part of dr407 test
+namespace A {
+  struct S {};
+}
+namespace B {
+  typedef int S;
+}
+namespace E {
+  typedef A::S S;
+  using A::S;
+  struct S s;
+}
+namespace F {
+  typedef A::S S;
+}
+namespace G {
+  using namespace A;
+  using namespace F;
+  struct S s;
+}
+namespace H {
+  using namespace F;
+  using namespace A;
+  struct S s;
+}
+}

clang/test/CXX/drs/dr4xx.cpp

@@ -124,6 +124,7 @@ namespace dr406 { // dr406: yes
 }
 
 namespace dr407 { // dr407: 3.8
+                  // NB: reused by dr1894 and dr2199
   struct S;
   typedef struct S S;
   void f() {

clang/www/cxx_dr_status.html

@@ -11171,7 +11171,7 @@ <h2 id="cxxdr">C++ defect report implementation status</h2>
     <td><a href="https://cplusplus.github.io/CWG/issues/1894.html">1894</a></td>
     <td>CD6</td>
     <td><I>typedef-name</I>s and <I>using-declaration</I>s</td>
-    <td class="none" align="center">Unknown</td>
+    <td class="full" align="center">Clang 3.8</td>
   </tr>
   <tr id="1895">
     <td><a href="https://cplusplus.github.io/CWG/issues/1895.html">1895</a></td>
@@ -13001,7 +13001,7 @@ <h2 id="cxxdr">C++ defect report implementation status</h2>
     <td><a href="https://cplusplus.github.io/CWG/issues/2199.html">2199</a></td>
     <td>CD6</td>
     <td>Typedefs and tags</td>
-    <td class="none" align="center">Unknown</td>
+    <td class="full" align="center">Clang 3.8</td>
   </tr>
   <tr id="2200">
     <td><a href="https://cplusplus.github.io/CWG/issues/2200.html">2200</a></td>