[clang] Add test for CWG2370

[Endill]

Summary

P1787: CWG2370 is resolved by performing a search in (only) the immediate scope of any friend, per the CWG opinion from San Diego.
Wording: In a friend declaration declarator whose declarator-id is a qualified-id whose lookup context is a class or namespace S, lookup for an unqualified name that appears after the declarator-id performs a search in the scope associated with S. If that lookup finds nothing, it undergoes unqualified name lookup. ([basic.lookup.unqual]/6).

Clarification for P1787 description: when applied to the test in this patch, "immediate scope" refers to N, and "(only)" refers to the fact that type is not searched in parent scope of N. See example after the wording if additional clarification is needed. The most relevant line there is friend void A::f(F); // OK.

[shafik]

clang/test/CXX/drs/dr23xx.cpp

182

The implementation seems to all accept this example: https://godbolt.org/z/vE6bEP6xa

but the examples from the p1787 have a decidely mixed conformance: https://godbolt.org/z/dhq7oEKaY

but the A::F(F) you point out in your example clang does get wrong and gcc does not. So at minimum please file bug reports against the examples that clang does not get right from p1787 and we need to dig into why your example above seems to not the same since that is what you intended.

[Endill]

clang/test/CXX/drs/dr23xx.cpp

182

Are you sure behavior is different for my N::g(type) when compared to A::f(F)? Clang is the only one to reject both examples: https://godbolt.org/z/MKb6fE8K5

[Endill]

Bug report filed: https://github.com/llvm/llvm-project/issues/62061

[shafik]

LGTM

clang/test/CXX/drs/dr23xx.cpp

182

Oh wow, I someone how missed the commented line of code and thought you were referring to the uncomment line.

clang/test/CXX/drs/dr23xx.cpp

 // RUN: %clang_cc1 -std=c++98 %s -verify -fexceptions -fcxx-exceptions -pedantic-errors 2>&1
 // RUN: %clang_cc1 -std=c++11 %s -verify -fexceptions -fcxx-exceptions -pedantic-errors 2>&1 | FileCheck %s
 // RUN: %clang_cc1 -std=c++14 %s -verify -fexceptions -fcxx-exceptions -pedantic-errors 2>&1 | FileCheck %s
 // RUN: %clang_cc1 -std=c++17 %s -verify -fexceptions -fcxx-exceptions -pedantic-errors 2>&1 | FileCheck %s
-// RUN: %clang_cc1 -std=c++2a %s -verify -fexceptions -fcxx-exceptions -pedantic-errors 2>&1 | FileCheck %s
+// RUN: %clang_cc1 -std=c++20 %s -verify -fexceptions -fcxx-exceptions -pedantic-errors 2>&1 | FileCheck %s
+// RUN: %clang_cc1 -std=c++2b %s -verify -fexceptions -fcxx-exceptions -pedantic-errors 2>&1 | FileCheck %s
+
 
 #if __cplusplus >= 201103L
 namespace dr2338 { // dr2338: 12
 namespace B {
 enum E : bool { Zero, One };
 static_assert((int)(E)2 == 1, "");
 } // namespace B
 namespace D {
void g() {
   f2(&b);
   f(C{});
   f(D{});
   f(F{}); // expected-error {{ambiguous conversion from derived class}}
 }
 } //namespace dr2303
 #endif
 
+namespace dr2370 { // dr2370: no
+namespace N {
+typedef int type;
+void g(type);
+void h(type);
+} // namespace N
+class C {
+  typedef N::type N_type;
+  // FIXME: `type` should be searched for in N
+  // friend void N::g(type);
+  friend void N::h(N_type);
+};
+} // namespace dr2370
+
 // dr2385: na
 
 namespace dr2394 { // dr2394: 15
 
 struct A {};
 const A a;
 
 // Now allowed to default-init B.

clang/www/cxx_dr_status.html

<tr id="2369">
     <td>CD6</td>
     <td>Ordering between constraints and substitution</td>
     <td class="none" align="center">Unknown</td>
   </tr>
   <tr id="2370">
     <td><a href="https://wg21.link/cwg2370">2370</a></td>
     <td>CD6</td>
     <td><TT>friend</TT> declarations of namespace-scope functions</td>
-    <td class="none" align="center">Unknown</td>
+    <td class="none" align="center">No</td>
   </tr>
   <tr id="2371">
     <td><a href="https://wg21.link/cwg2371">2371</a></td>
     <td>CD5</td>
     <td>Use of the English term &#8220;attributes&#8221; is confusing</td>
     <td class="none" align="center">Unknown</td>
   </tr>
   <tr id="2372">