[Clang][Sema] Require BaseClass:: (not other classes) in member using-declaration in C++98 mode (#143492)

[CWG400](https://wg21.link/cwg400) rejects member using-declaration
whose nested-name-specifier doesn't refer to a base class of the current
class.

```cpp
struct A {};
struct B {
  using B::A; // error
};
```

Clang didn't reject this case in C++98 mode. This patch fixes this
issue.

clang/docs/ReleaseNotes.rst

@@ -71,6 +71,11 @@ C++ Specific Potentially Breaking Changes
   if it's out of range. The Boost numeric_conversion library is impacted by
   this; it was fixed in Boost 1.81. (#GH143034)
 
+- Fully implemented `CWG400 Using-declarations and the `
+  `"struct hack" <https://wg21.link/CWG400>`_. Invalid member using-declaration
+  whose nested-name-specifier doesn't refer to a base class such as
+  ``using CurrentClass::Foo;`` is now rejected in C++98 mode.
+
 ABI Changes in This Version
 ---------------------------
 

clang/lib/Sema/SemaDeclCXX.cpp

@@ -13619,82 +13619,40 @@ bool Sema::CheckUsingDeclQualifier(SourceLocation UsingLoc, bool HasTypename,
       RequireCompleteDeclContext(const_cast<CXXScopeSpec&>(SS), NamedContext))
     return true;
 
-  if (getLangOpts().CPlusPlus11) {
-    // C++11 [namespace.udecl]p3:
-    //   In a using-declaration used as a member-declaration, the
-    //   nested-name-specifier shall name a base class of the class
-    //   being defined.
+  // C++26 [namespace.udecl]p3:
+  //   In a using-declaration used as a member-declaration, each
+  //   using-declarator shall either name an enumerator or have a
+  //   nested-name-specifier naming a base class of the current class
+  //   ([expr.prim.this]). ...
+  // "have a nested-name-specifier naming a base class of the current class"
+  // was introduced by CWG400.
 
-    if (cast<CXXRecordDecl>(CurContext)->isProvablyNotDerivedFrom(
-                                 cast<CXXRecordDecl>(NamedContext))) {
+  if (cast<CXXRecordDecl>(CurContext)
+          ->isProvablyNotDerivedFrom(cast<CXXRecordDecl>(NamedContext))) {
 
-      if (Cxx20Enumerator) {
-        Diag(NameLoc, diag::warn_cxx17_compat_using_decl_non_member_enumerator)
-            << SS.getRange();
-        return false;
-      }
-
-      if (CurContext == NamedContext) {
-        Diag(SS.getBeginLoc(),
-             diag::err_using_decl_nested_name_specifier_is_current_class)
-            << SS.getRange();
-        return !getLangOpts().CPlusPlus20;
-      }
-
-      if (!cast<CXXRecordDecl>(NamedContext)->isInvalidDecl()) {
-        Diag(SS.getBeginLoc(),
-             diag::err_using_decl_nested_name_specifier_is_not_base_class)
-            << SS.getScopeRep() << cast<CXXRecordDecl>(CurContext)
-            << SS.getRange();
-      }
-      return true;
+    if (Cxx20Enumerator) {
+      Diag(NameLoc, diag::warn_cxx17_compat_using_decl_non_member_enumerator)
+          << SS.getRange();
+      return false;
     }
 
-    return false;
+    if (CurContext == NamedContext) {
+      Diag(SS.getBeginLoc(),
+           diag::err_using_decl_nested_name_specifier_is_current_class)
+          << SS.getRange();
+      return !getLangOpts().CPlusPlus20;
+    }
+
+    if (!cast<CXXRecordDecl>(NamedContext)->isInvalidDecl()) {
+      Diag(SS.getBeginLoc(),
+           diag::err_using_decl_nested_name_specifier_is_not_base_class)
+          << SS.getScopeRep() << cast<CXXRecordDecl>(CurContext)
+          << SS.getRange();
+    }
+    return true;
   }
 
-  // C++03 [namespace.udecl]p4:
-  //   A using-declaration used as a member-declaration shall refer
-  //   to a member of a base class of the class being defined [etc.].
-
-  // Salient point: SS doesn't have to name a base class as long as
-  // lookup only finds members from base classes.  Therefore we can
-  // diagnose here only if we can prove that can't happen,
-  // i.e. if the class hierarchies provably don't intersect.
-
-  // TODO: it would be nice if "definitely valid" results were cached
-  // in the UsingDecl and UsingShadowDecl so that these checks didn't
-  // need to be repeated.
-
-  llvm::SmallPtrSet<const CXXRecordDecl *, 4> Bases;
-  auto Collect = [&Bases](const CXXRecordDecl *Base) {
-    Bases.insert(Base);
-    return true;
-  };
-
-  // Collect all bases. Return false if we find a dependent base.
-  if (!cast<CXXRecordDecl>(CurContext)->forallBases(Collect))
-    return false;
-
-  // Returns true if the base is dependent or is one of the accumulated base
-  // classes.
-  auto IsNotBase = [&Bases](const CXXRecordDecl *Base) {
-    return !Bases.count(Base);
-  };
-
-  // Return false if the class has a dependent base or if it or one
-  // of its bases is present in the base set of the current context.
-  if (Bases.count(cast<CXXRecordDecl>(NamedContext)) ||
-      !cast<CXXRecordDecl>(NamedContext)->forallBases(IsNotBase))
-    return false;
-
-  Diag(SS.getRange().getBegin(),
-       diag::err_using_decl_nested_name_specifier_is_not_base_class)
-    << SS.getScopeRep()
-    << cast<CXXRecordDecl>(CurContext)
-    << SS.getRange();
-
-  return true;
+  return false;
 }
 
 Decl *Sema::ActOnAliasDeclaration(Scope *S, AccessSpecifier AS,

clang/test/CXX/class.access/class.access.dcl/p1.cpp

@@ -318,26 +318,23 @@ namespace test4 {
     // expected-error@-4 {{ISO C++11 does not allow access declarations; use using declarations instead}}
 #endif
 
-    C::foo; // legal in C++03
+    C::foo;
 #if __cplusplus <= 199711L
     // expected-warning@-2 {{access declarations are deprecated; use using declarations instead}}
 #else
     // expected-error@-4 {{ISO C++11 does not allow access declarations; use using declarations instead}}
-    // expected-error@-5 {{using declaration refers to its own class}}
 #endif
+    // expected-error@-6 {{using declaration refers to its own class}}
 
-    Subclass::foo; // legal in C++03
+    Subclass::foo;
 #if __cplusplus <= 199711L
     // expected-warning@-2 {{access declarations are deprecated; use using declarations instead}}
 #else
     // expected-error@-4 {{ISO C++11 does not allow access declarations; use using declarations instead}}
-    // expected-error@-5 {{using declaration refers into 'Subclass', which is not a base class of 'C'}}
 #endif
+    // expected-error@-6 {{using declaration refers into 'Subclass', which is not a base class of 'C'}}
 
     int bar();
-#if __cplusplus <= 199711L
-    //expected-note@-2 {{target of using declaration}}
-#endif
     C::bar;
 #if __cplusplus <= 199711L
     // expected-warning@-2 {{access declarations are deprecated; use using declarations instead}}

clang/test/CXX/dcl.dcl/basic.namespace/namespace.udecl/p3.cpp

@@ -30,17 +30,10 @@ class D2 : public B {
   using B::x;
   using C::g; // expected-error{{using declaration refers into 'C', which is not a base class of 'D2'}}
 
-  // These are valid in C++98 but not in C++11.
-  using D::f2;
-  using D::E2;
-  using D::e2;
-  using D::x2;
-#if __cplusplus >= 201103L
-  // expected-error@-5 {{using declaration refers into 'D', which is not a base class of 'D2'}}
-  // expected-error@-5 {{using declaration refers into 'D', which is not a base class of 'D2'}}
-  // expected-error@-5 {{using declaration refers into 'D', which is not a base class of 'D2'}}
-  // expected-error@-5 {{using declaration refers into 'D', which is not a base class of 'D2'}}
-#endif
+  using D::f2; // expected-error {{using declaration refers into 'D', which is not a base class of 'D2'}}
+  using D::E2; // expected-error {{using declaration refers into 'D', which is not a base class of 'D2'}}
+  using D::e2; // expected-error {{using declaration refers into 'D', which is not a base class of 'D2'}}
+  using D::x2; // expected-error {{using declaration refers into 'D', which is not a base class of 'D2'}}
 
   using B::EC;
   using B::EC::ec; // expected-warning {{a C++20 extension}} expected-warning 0-1 {{C++11}}
@@ -71,13 +64,7 @@ namespace test1 {
     using Base::bar; // expected-error {{no member named 'bar'}}
     using Unrelated::foo; // expected-error {{not a base class}}
 
-    // In C++98, it's hard to see that these are invalid, because indirect
-    // references to base class members are permitted.
-    using C::foo;
-    using Subclass::foo;
-#if __cplusplus >= 201103L
-    // expected-error@-3 {{refers to its own class}}
-    // expected-error@-3 {{not a base class}}
-#endif
+    using C::foo; // expected-error {{refers to its own class}}
+    using Subclass::foo; // expected-error {{not a base class}}
   };
 }

clang/test/CXX/dcl.dcl/basic.namespace/namespace.udecl/p4.cpp

@@ -206,17 +206,9 @@ namespace test4 {
     using InnerNS::foo; // expected-error {{not a class}}
     using Base::bar; // expected-error {{no member named 'bar'}}
     using Unrelated::foo; // expected-error {{not a base class}}
-    using C::foo; // legal in C++03
-    using Subclass::foo; // legal in C++03
-#if __cplusplus >= 201103L
-    // expected-error@-3 {{refers to its own class}}
-    // expected-error@-3 {{refers into 'Subclass', which is not a base class}}
-#endif
-
+    using C::foo; // expected-error {{refers to its own class}}
+    using Subclass::foo; // expected-error {{refers into 'Subclass', which is not a base class}}
     int bar();
-#if __cplusplus < 201103L
-    // expected-note@-2 {{target of using declaration}}
-#endif
     using C::bar; // expected-error {{refers to its own class}}
   };
 }

clang/test/CXX/drs/cwg4xx.cpp

@@ -36,6 +36,15 @@ namespace cwg400 { // cwg400: 2.7
   // expected-error@-1 {{member 'a' found in multiple base classes of different types}}
   //   expected-note@#cwg400-A {{member type 'cwg400::A::a' found by ambiguous name lookup}}
   //   expected-note@#cwg400-B {{member type 'cwg400::B::a' found by ambiguous name lookup}}
+  struct F : A {};
+  struct G : A {
+    using G::A;
+    // expected-error@-1 {{using declaration refers to its own class}}
+    using G::a;
+    // expected-error@-1 {{using declaration refers to its own class}}
+    using F::a;
+    // expected-error@-1 {{using declaration refers into 'F', which is not a base class of 'G'}}
+  };
 } // namespace cwg400
 
 namespace cwg401 { // cwg401: 2.8