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

The approved resolution for CWG2858 changes
[expr.prim.id.qual] p2 sentence 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.

1 files changed, 6 insertions, 7 deletions

diff --git a/clang/lib/Sema/SemaDecl.cpp b/clang/lib/Sema/SemaDecl.cpp
index 5a23179..a4699d6 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()));