[clang] [Clang][Sema] Implement approved resolution for CWG2858 (PR #88042)

[Krystian Stasiowski via cfe-commits]

https://github.com/sdkrystian created https://github.com/llvm/llvm-project/pull/88042

The approved resolution for [CWG2858](https://cplusplus.github.io/CWG/issues/2858.html) changes [[expr.prim.id.qual] p2 sentence 2](https://eel.is/c++draft/expr.prim.id.qual#2) to read:
> A declarative _nested-name-specifier_ shall not have a _computed-type-specifier_.

This patch implements the approved resolution. Since we don't consider _nested-name-specifiers_ in friend declarations to be declarative (yet), it currently isn't possible to write a test that would produce this diagnostic (`diagnoseQualifiedDeclaration` is never called if the `DeclContext` can't be computed). Nevertheless, tests were added which will produce the diagnostic once we start calling `diagnoseQualifiedDeclaration` for friend declarations. 

>From 5f06642fa89d517adbb11157fa88352bd86d79bf Mon Sep 17 00:00:00 2001
From: Krystian Stasiowski <sdkrystian at gmail.com>
Date: Mon, 8 Apr 2024 09:46:08 -0400
Subject: [PATCH] [Clang][Sema] Implement accepted resolution for CWG2858

---
 clang/include/clang/Basic/DiagnosticSemaKinds.td |  7 +++----
 clang/lib/Sema/SemaDecl.cpp                      | 13 ++++++-------
 .../expr.prim.id/expr.prim.id.qual/p3.cpp        | 16 ++++++++++++++++
 3 files changed, 25 insertions(+), 11 deletions(-)

diff --git a/clang/include/clang/Basic/DiagnosticSemaKinds.td b/clang/include/clang/Basic/DiagnosticSemaKinds.td
index a1dda2d2461c31..2b9077173aa426 100644
--- a/clang/include/clang/Basic/DiagnosticSemaKinds.td
+++ b/clang/include/clang/Basic/DiagnosticSemaKinds.td
@@ -2402,10 +2402,6 @@ def err_selected_explicit_constructor : Error<
 def note_explicit_ctor_deduction_guide_here : Note<
   "explicit %select{constructor|deduction guide}0 declared here">;
 
-// C++11 decltype
-def err_decltype_in_declarator : Error<
-    "'decltype' cannot be used to name a declaration">;
-
 // C++11 auto
 def warn_cxx98_compat_auto_type_specifier : Warning<
   "'auto' type specifier is incompatible with C++98">,
@@ -8302,6 +8298,9 @@ def ext_template_after_declarative_nns : ExtWarn<
 def ext_alias_template_in_declarative_nns : ExtWarn<
   "a declarative nested name specifier cannot name an alias template">,
   InGroup<DiagGroup<"alias-template-in-declaration-name">>;
+def err_computed_type_in_declarative_nns  : Error<
+  "%select{a pack indexing type|'decltype'}0 cannot be used in "
+  "a declarative nested name specifier">;
 
 def err_no_typeid_with_fno_rtti : Error<
   "use of typeid requires -frtti">;
diff --git a/clang/lib/Sema/SemaDecl.cpp b/clang/lib/Sema/SemaDecl.cpp
index c790dab72dd721..1ba6b3beb1c758 100644
--- a/clang/lib/Sema/SemaDecl.cpp
+++ b/clang/lib/Sema/SemaDecl.cpp
@@ -6335,16 +6335,15 @@ bool Sema::diagnoseQualifiedDeclaration(CXXScopeSpec &SS, DeclContext *DC,
         if (TST->isDependentType() && TST->isTypeAlias())
           Diag(Loc, diag::ext_alias_template_in_declarative_nns)
               << SpecLoc.getLocalSourceRange();
-      } else if (T->isDecltypeType()) {
+      } else if (T->isDecltypeType() || T->getAsAdjusted<PackIndexingType>()) {
         // C++23 [expr.prim.id.qual]p2:
         //   [...] A declarative nested-name-specifier shall not have a
-        //   decltype-specifier.
+        //   computed-type-specifier.
         //
-        // FIXME: This wording appears to be defective as it does not forbid
-        // declarative nested-name-specifiers with pack-index-specifiers.
-        // See https://github.com/cplusplus/CWG/issues/499.
-        Diag(Loc, diag::err_decltype_in_declarator)
-            << SpecLoc.getTypeLoc().getSourceRange();
+        // CWG2858 changed this from 'decltype-specifier' to
+        // 'computed-type-specifier'.
+        Diag(Loc, diag::err_computed_type_in_declarative_nns)
+            << T->isDecltypeType() << SpecLoc.getTypeLoc().getSourceRange();
       }
     }
   } while ((SpecLoc = SpecLoc.getPrefix()));
diff --git a/clang/test/CXX/expr/expr.prim/expr.prim.id/expr.prim.id.qual/p3.cpp b/clang/test/CXX/expr/expr.prim/expr.prim.id/expr.prim.id.qual/p3.cpp
index c73ffa55a26a31..5da13cc22abef2 100644
--- a/clang/test/CXX/expr/expr.prim/expr.prim.id/expr.prim.id.qual/p3.cpp
+++ b/clang/test/CXX/expr/expr.prim/expr.prim.id/expr.prim.id.qual/p3.cpp
@@ -1,4 +1,5 @@
 // RUN: %clang_cc1 -verify %s
+// RUN: %clang_cc1 -std=c++2c -verify %s
 
 template<typename T>
 struct A {
@@ -27,3 +28,18 @@ namespace N {
 
 template<typename T>
 void N::E<T>::f() { } // expected-warning {{a declarative nested name specifier cannot name an alias template}}
+
+#if __cplusplus > 202302L
+template<typename... Ts>
+struct A {
+  // FIXME: The nested-name-specifier in the following friend declarations are declarative,
+  // but we don't treat them as such (yet).
+  friend void Ts...[0]::f();
+  template<typename U>
+  friend void Ts...[0]::g();
+
+  friend struct Ts...[0]::B;
+  template<typename U>
+  friend struct Ts...[0]::C; // expected-warning{{is not supported; ignoring this friend declaration}}
+};
+#endif