[clang] Add test for CWG873

[Endill]

Summary

Also add missing marking to the test of related issue 621.

https://cplusplus.github.io/CWG/issues/621.html
https://cplusplus.github.io/CWG/issues/873.html

[shafik]

LGTM

clang/test/CXX/drs/dr6xx.cpp

namespace dr619 { // dr619: yes
   void f() {
     extern int x[];
     sizeof(x); // expected-error {{incomplete}}
   }
 }
 
 // dr620: dup 568
 
-namespace dr621 {
+namespace dr621 { // dr621: yes
   template<typename T> T f();
   template<> int f() {} // expected-note {{previous}}
   template<> int f<int>() {} // expected-error {{redefinition}}
 }
 
 // dr623: na
 // FIXME: Add documentation saying we allow invalid pointer values.
 

clang/test/CXX/drs/dr8xx.cpp

+// RUN: %clang_cc1 -std=c++98 -triple x86_64-unknown-unknown %s -verify -fexceptions -fcxx-exceptions -pedantic-errors
+// RUN: %clang_cc1 -std=c++11 -triple x86_64-unknown-unknown %s -verify -fexceptions -fcxx-exceptions -pedantic-errors 2>&1 | FileCheck %s
+// RUN: %clang_cc1 -std=c++14 -triple x86_64-unknown-unknown %s -verify -fexceptions -fcxx-exceptions -pedantic-errors 2>&1 | FileCheck %s
+// RUN: %clang_cc1 -std=c++17 -triple x86_64-unknown-unknown %s -verify -fexceptions -fcxx-exceptions -pedantic-errors 2>&1 | FileCheck %s
+// RUN: %clang_cc1 -std=c++20 -triple x86_64-unknown-unknown %s -verify -fexceptions -fcxx-exceptions -pedantic-errors 2>&1 | FileCheck %s
+// RUN: %clang_cc1 -std=c++23 -triple x86_64-unknown-unknown %s -verify -fexceptions -fcxx-exceptions -pedantic-errors 2>&1 | FileCheck %s
+// RUN: %clang_cc1 -std=c++2c -triple x86_64-unknown-unknown %s -verify -fexceptions -fcxx-exceptions -pedantic-errors 2>&1 | FileCheck %s
+
+// expected-no-diagnostics
+
+namespace dr873 { // dr873: yes
+#if __cplusplus >= 201103L
+template <typename T> void f(T &&);
+template <> void f(int &) {}  // #1
+template <> void f(int &&) {} // #2
+void g(int i) {
+  f(i); // calls f<int&>(int&), i.e., #1
+#pragma clang __debug dump f(i)
+  //      CHECK: CallExpr {{.*}}
+  // CHECK-NEXT: |-ImplicitCastExpr {{.*}}
+  // CHECK-NEXT: | `-DeclRefExpr {{.*}} 'f' 'void (int &)' {{.*}}
+
+  f(0); // calls f<int>(int&&), i.e., #2
+#pragma clang __debug dump f(0)
+  //      CHECK: CallExpr {{.*}}
+  // CHECK-NEXT: |-ImplicitCastExpr {{.*}}
+  // CHECK-NEXT: | `-DeclRefExpr {{.*}} 'f' 'void (int &&)' {{.*}}
+}
+#endif
+} // namespace dr873

clang/www/cxx_dr_status.html

<tr id="620">
     <td>CD1</td>
     <td>Declaration order in layout-compatible POD structs</td>
     <td class="full" align="center">Duplicate of <a href="#568">568</a></td>
   </tr>
   <tr id="621">
     <td><a href="https://cplusplus.github.io/CWG/issues/621.html">621</a></td>
     <td>C++11</td>
     <td>Template argument deduction from function return types</td>
-    <td class="none" align="center">Unknown</td>
+    <td class="full" align="center">Yes</td>
   </tr>
   <tr id="622">
     <td><a href="https://cplusplus.github.io/CWG/issues/622.html">622</a></td>
     <td>NAD</td>
     <td>Relational comparisons of arbitrary pointers</td>
     <td class="none" align="center">Unknown</td>
   </tr>
   <tr id="623">
<tr id="872">
     <td>CD2</td>
     <td>Lexical issues with raw strings</td>
     <td class="none" align="center">Unknown</td>
   </tr>
   <tr id="873">
     <td><a href="https://cplusplus.github.io/CWG/issues/873.html">873</a></td>
     <td>C++11</td>
     <td>Deducing rvalue references in declarative contexts</td>
-    <td class="none" align="center">Unknown</td>
+    <td class="full" align="center">Yes</td>
   </tr>
   <tr id="874">
     <td><a href="https://cplusplus.github.io/CWG/issues/874.html">874</a></td>
     <td>CD2</td>
     <td>Class-scope definitions of enumeration types</td>
     <td class="none" align="center">Unknown</td>
   </tr>
   <tr id="876">