[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.

clang/docs/ReleaseNotes.rst

@@ -147,6 +147,9 @@ Resolutions to C++ Defect Reports
   compatibility of two types.
   (`CWG2759: [[no_unique_address] and common initial sequence  <https://cplusplus.github.io/CWG/issues/2759.html>`_).
 
+- Clang now diagnoses declarative nested-name-specifiers with pack-index-specifiers.
+  (`CWG2858: Declarative nested-name-specifiers and pack-index-specifiers <https://cplusplus.github.io/CWG/issues/2858.html>`_).
+
 C Language Changes
 ------------------
 

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">,
@@ -8313,6 +8309,9 @@ 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<
+  "a %select{pack indexing|'decltype'}0 specifier cannot be used in "
+  "a declarative nested name specifier">;
 
 def err_no_typeid_with_fno_rtti : Error<
   "use of typeid requires -frtti">;

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()));

clang/test/CXX/dcl.decl/dcl.meaning/p1-0x.cpp

@@ -6,8 +6,8 @@ class foo {
   void func();
 };
 
-int decltype(foo())::i; // expected-error{{'decltype' cannot be used to name a declaration}}
-void decltype(foo())::func() { // expected-error{{'decltype' cannot be used to name a declaration}}
+int decltype(foo())::i; // expected-error{{a 'decltype' specifier cannot be used in a declarative nested name specifier}}
+void decltype(foo())::func() { // expected-error{{a 'decltype' specifier cannot be used in a declarative nested name specifier}}
 }
 
 

clang/test/CXX/drs/dr28xx.cpp

@@ -58,3 +58,26 @@ void B<int>::g() requires true;
 #endif
 
 } // namespace dr2847
+
+namespace dr2858 { // dr2858: 19
+
+#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;
+  // FIXME: The index of the pack-index-specifier is printed as a memory address in the diagnostic.
+  template<typename U>
+  friend struct Ts...[0]::C;
+  // expected-warning-re@-1 {{dependent nested name specifier 'Ts...[{{.*}}]::' for friend template declaration is not supported; ignoring this friend declaration}}
+};
+
+#endif
+
+} // namespace dr2858

clang/test/Parser/cxx-class.cpp

@@ -59,14 +59,14 @@ typedef union {
   } y;
 } bug3177;
 
-// check that we don't consume the token after the access specifier 
+// check that we don't consume the token after the access specifier
 // when it's not a colon
 class D {
 public // expected-error{{expected ':'}}
   int i;
 };
 
-// consume the token after the access specifier if it's a semicolon 
+// consume the token after the access specifier if it's a semicolon
 // that was meant to be a colon
 class E {
 public; // expected-error{{expected ':'}}
@@ -281,7 +281,7 @@ struct A {} ::PR41192::a; // ok, no missing ';' here  expected-warning {{extra q
 #if __cplusplus >= 201103L
 struct C;
 struct D { static C c; };
-struct C {} decltype(D())::c; // expected-error {{'decltype' cannot be used to name a declaration}}
+struct C {} decltype(D())::c; // expected-error {{a 'decltype' specifier cannot be used in a declarative nested name specifier}}
 #endif
 }