[Clang][Sema] Diagnose friend declarations with enum elaborated-type-…

…specifier in all language modes (#80171)

According to [dcl.type.elab] p4:
> If an _elaborated-type-specifier_ appears with the `friend` specifier
as an entire _member-declaration_, the _member-declaration_ shall have
one of the following forms:
>     `friend` _class-key_ _nested-name-specifier_(opt) _identifier_ `;`
>     `friend` _class-key_ _simple-template-id_ `;`
> `friend` _class-key_ _nested-name-specifier_ `template`(opt)
_simple-template-id_ `;`

Notably absent from this list is the `enum` form of an
_elaborated-type-specifier_ "`enum` _nested-name-specifier_(opt)
_identifier_", which appears to be intentional per the resolution of
CWG2363.

Most major implementations accept these declarations, so the diagnostic
is a pedantic warning across all C++ versions.

In addition to the trivial cases previously diagnosed in C++98, we now
diagnose cases where the _elaborated-type-specifier_ has a dependent
_nested-name-specifier_:
```
template<typename T>
struct A
{
    enum class E;
};

struct B
{
    template<typename T>
    friend enum A<T>::E; // pedantic warning: elaborated enumeration type cannot be a friend
};

template<typename T>
struct C
{
    friend enum T::E;  // pedantic warning: elaborated enumeration type cannot be a friend
};
```

clang/docs/ReleaseNotes.rst

@@ -159,6 +159,8 @@ Improvements to Clang's diagnostics
 - The ``-Wshorten-64-to-32`` diagnostic is now grouped under ``-Wimplicit-int-conversion`` instead
    of ``-Wconversion``. Fixes `#69444 <https://github.com/llvm/llvm-project/issues/69444>`_.
 
+- Clang now diagnoses friend declarations with an ``enum`` elaborated-type-specifier in language modes after C++98.
+
 Improvements to Clang's time-trace
 ----------------------------------
 

clang/include/clang/Basic/DiagnosticSemaKinds.td

@@ -1637,10 +1637,10 @@ def err_inline_namespace_std : Error<
 def err_unexpected_friend : Error<
   "friends can only be classes or functions">;
 def ext_enum_friend : ExtWarn<
-  "befriending enumeration type %0 is a C++11 extension">, InGroup<CXX11>;
-def warn_cxx98_compat_enum_friend : Warning<
-  "befriending enumeration type %0 is incompatible with C++98">,
-  InGroup<CXX98Compat>, DefaultIgnore;
+  "elaborated enum specifier cannot be declared as a friend">,
+  InGroup<DiagGroup<"friend-enum">>;
+def note_enum_friend : Note<
+  "remove 'enum%select{| struct| class}0' to befriend an enum">;
 def ext_nonclass_type_friend : ExtWarn<
   "non-class friend type %0 is a C++11 extension">, InGroup<CXX11>;
 def warn_cxx98_compat_nonclass_type_friend : Warning<

clang/include/clang/Sema/DeclSpec.h

@@ -346,10 +346,7 @@ class DeclSpec {
     // FIXME: Attributes should be included here.
   };
 
-  enum FriendSpecified : bool {
-    No,
-    Yes,
-  };
+  enum FriendSpecified : bool { No, Yes };
 
 private:
   // storage-class-specifier
@@ -400,7 +397,7 @@ class DeclSpec {
 
   // friend-specifier
   LLVM_PREFERRED_TYPE(bool)
-  unsigned Friend_specified : 1;
+  unsigned FriendSpecifiedFirst : 1;
 
   // constexpr-specifier
   LLVM_PREFERRED_TYPE(ConstexprSpecKind)
@@ -491,7 +488,7 @@ class DeclSpec {
         TypeSpecPipe(false), TypeSpecSat(false), ConstrainedAuto(false),
         TypeQualifiers(TQ_unspecified), FS_inline_specified(false),
         FS_forceinline_specified(false), FS_virtual_specified(false),
-        FS_noreturn_specified(false), Friend_specified(false),
+        FS_noreturn_specified(false), FriendSpecifiedFirst(false),
         ConstexprSpecifier(
             static_cast<unsigned>(ConstexprSpecKind::Unspecified)),
         Attrs(attrFactory), writtenBS(), ObjCQualifiers(nullptr) {}
@@ -818,9 +815,11 @@ class DeclSpec {
                         const char *&PrevSpec, unsigned &DiagID);
 
   FriendSpecified isFriendSpecified() const {
-    return static_cast<FriendSpecified>(Friend_specified);
+    return static_cast<FriendSpecified>(FriendLoc.isValid());
   }
 
+  bool isFriendSpecifiedFirst() const { return FriendSpecifiedFirst; }
+
   SourceLocation getFriendSpecLoc() const { return FriendLoc; }
 
   bool isModulePrivateSpecified() const { return ModulePrivateLoc.isValid(); }

clang/include/clang/Sema/Sema.h

@@ -8039,9 +8039,6 @@ class Sema final {
                                      SourceLocation RParenLoc, bool Failed);
   void DiagnoseStaticAssertDetails(const Expr *E);
 
-  FriendDecl *CheckFriendTypeDecl(SourceLocation LocStart,
-                                  SourceLocation FriendLoc,
-                                  TypeSourceInfo *TSInfo);
   Decl *ActOnFriendTypeDecl(Scope *S, const DeclSpec &DS,
                             MultiTemplateParamsArg TemplateParams);
   NamedDecl *ActOnFriendFunctionDecl(Scope *S, Declarator &D,

clang/lib/Parse/ParseTentative.cpp

@@ -79,9 +79,9 @@ bool Parser::isCXXDeclarationStatement(
             getCurScope(), *II, Tok.getLocation(), SS, /*Template=*/nullptr);
         if (Actions.isCurrentClassName(*II, getCurScope(), &SS) ||
             isDeductionGuide) {
-          if (isConstructorDeclarator(/*Unqualified=*/SS.isEmpty(),
-                                      isDeductionGuide,
-                                      DeclSpec::FriendSpecified::No))
+          if (isConstructorDeclarator(
+                  /*Unqualified=*/SS.isEmpty(), isDeductionGuide,
+                  /*IsFriend=*/DeclSpec::FriendSpecified::No))
             return true;
         } else if (SS.isNotEmpty()) {
           // If the scope is not empty, it could alternatively be something like

clang/lib/Sema/DeclSpec.cpp

@@ -1102,18 +1102,13 @@ bool DeclSpec::setFunctionSpecNoreturn(SourceLocation Loc,
 
 bool DeclSpec::SetFriendSpec(SourceLocation Loc, const char *&PrevSpec,
                              unsigned &DiagID) {
-  if (Friend_specified) {
+  if (isFriendSpecified()) {
     PrevSpec = "friend";
-    // Keep the later location, so that we can later diagnose ill-formed
-    // declarations like 'friend class X friend;'. Per [class.friend]p3,
-    // 'friend' must be the first token in a friend declaration that is
-    // not a function declaration.
-    FriendLoc = Loc;
     DiagID = diag::warn_duplicate_declspec;
     return true;
   }
 
-  Friend_specified = true;
+  FriendSpecifiedFirst = isEmpty();
   FriendLoc = Loc;
   return false;
 }

clang/lib/Sema/SemaDecl.cpp

@@ -17264,6 +17264,26 @@ Sema::ActOnTag(Scope *S, unsigned TagSpec, TagUseKind TUK, SourceLocation KWLoc,
       return true;
   }
 
+  if (TUK == TUK_Friend && Kind == TagTypeKind::Enum) {
+    // C++23 [dcl.type.elab]p4:
+    //   If an elaborated-type-specifier appears with the friend specifier as
+    //   an entire member-declaration, the member-declaration shall have one
+    //   of the following forms:
+    //     friend class-key nested-name-specifier(opt) identifier ;
+    //     friend class-key simple-template-id ;
+    //     friend class-key nested-name-specifier template(opt)
+    //       simple-template-id ;
+    //
+    // Since enum is not a class-key, so declarations like "friend enum E;"
+    // are ill-formed. Although CWG2363 reaffirms that such declarations are
+    // invalid, most implementations accept so we issue a pedantic warning.
+    Diag(KWLoc, diag::ext_enum_friend) << FixItHint::CreateRemoval(
+        ScopedEnum ? SourceRange(KWLoc, ScopedEnumKWLoc) : KWLoc);
+    assert(ScopedEnum || !ScopedEnumUsesClassTag);
+    Diag(KWLoc, diag::note_enum_friend)
+        << (ScopedEnum + ScopedEnumUsesClassTag);
+  }
+
   // Figure out the underlying type if this a enum declaration. We need to do
   // this early, because it's needed to detect if this is an incompatible
   // redeclaration.

clang/lib/Sema/SemaDeclCXX.cpp

@@ -17545,79 +17545,6 @@ Decl *Sema::BuildStaticAssertDeclaration(SourceLocation StaticAssertLoc,
   return Decl;
 }
 
-/// Perform semantic analysis of the given friend type declaration.
-///
-/// \returns A friend declaration that.
-FriendDecl *Sema::CheckFriendTypeDecl(SourceLocation LocStart,
-                                      SourceLocation FriendLoc,
-                                      TypeSourceInfo *TSInfo) {
-  assert(TSInfo && "NULL TypeSourceInfo for friend type declaration");
-
-  QualType T = TSInfo->getType();
-  SourceRange TypeRange = TSInfo->getTypeLoc().getSourceRange();
-
-  // C++03 [class.friend]p2:
-  //   An elaborated-type-specifier shall be used in a friend declaration
-  //   for a class.*
-  //
-  //   * The class-key of the elaborated-type-specifier is required.
-  if (!CodeSynthesisContexts.empty()) {
-    // Do not complain about the form of friend template types during any kind
-    // of code synthesis. For template instantiation, we will have complained
-    // when the template was defined.
-  } else {
-    if (!T->isElaboratedTypeSpecifier()) {
-      // If we evaluated the type to a record type, suggest putting
-      // a tag in front.
-      if (const RecordType *RT = T->getAs<RecordType>()) {
-        RecordDecl *RD = RT->getDecl();
-
-        SmallString<16> InsertionText(" ");
-        InsertionText += RD->getKindName();
-
-        Diag(TypeRange.getBegin(),
-             getLangOpts().CPlusPlus11 ?
-               diag::warn_cxx98_compat_unelaborated_friend_type :
-               diag::ext_unelaborated_friend_type)
-          << (unsigned) RD->getTagKind()
-          << T
-          << FixItHint::CreateInsertion(getLocForEndOfToken(FriendLoc),
-                                        InsertionText);
-      } else {
-        Diag(FriendLoc,
-             getLangOpts().CPlusPlus11 ?
-               diag::warn_cxx98_compat_nonclass_type_friend :
-               diag::ext_nonclass_type_friend)
-          << T
-          << TypeRange;
-      }
-    } else if (T->getAs<EnumType>()) {
-      Diag(FriendLoc,
-           getLangOpts().CPlusPlus11 ?
-             diag::warn_cxx98_compat_enum_friend :
-             diag::ext_enum_friend)
-        << T
-        << TypeRange;
-    }
-
-    // C++11 [class.friend]p3:
-    //   A friend declaration that does not declare a function shall have one
-    //   of the following forms:
-    //     friend elaborated-type-specifier ;
-    //     friend simple-type-specifier ;
-    //     friend typename-specifier ;
-    if (getLangOpts().CPlusPlus11 && LocStart != FriendLoc)
-      Diag(FriendLoc, diag::err_friend_not_first_in_declaration) << T;
-  }
-
-  //   If the type specifier in a friend declaration designates a (possibly
-  //   cv-qualified) class type, that class is declared as a friend; otherwise,
-  //   the friend declaration is ignored.
-  return FriendDecl::Create(Context, CurContext,
-                            TSInfo->getTypeLoc().getBeginLoc(), TSInfo,
-                            FriendLoc);
-}
-
 /// Handle a friend tag declaration where the scope specifier was
 /// templated.
 DeclResult Sema::ActOnTemplatedFriendTag(
@@ -17755,6 +17682,7 @@ DeclResult Sema::ActOnTemplatedFriendTag(
 Decl *Sema::ActOnFriendTypeDecl(Scope *S, const DeclSpec &DS,
                                 MultiTemplateParamsArg TempParams) {
   SourceLocation Loc = DS.getBeginLoc();
+  SourceLocation FriendLoc = DS.getFriendSpecLoc();
 
   assert(DS.isFriendSpecified());
   assert(DS.getStorageClassSpec() == DeclSpec::SCS_unspecified);
@@ -17766,9 +17694,10 @@ Decl *Sema::ActOnFriendTypeDecl(Scope *S, const DeclSpec &DS,
   //     friend simple-type-specifier ;
   //     friend typename-specifier ;
   //
-  // Any declaration with a type qualifier does not have that form. (It's
-  // legal to specify a qualified type as a friend, you just can't write the
-  // keywords.)
+  // If the friend keyword isn't first, or if the declarations has any type
+  // qualifiers, then the declaration doesn't have that form.
+  if (getLangOpts().CPlusPlus11 && !DS.isFriendSpecifiedFirst())
+    Diag(FriendLoc, diag::err_friend_not_first_in_declaration);
   if (DS.getTypeQualifiers()) {
     if (DS.getTypeQualifiers() & DeclSpec::TQ_const)
       Diag(DS.getConstSpecLoc(), diag::err_friend_decl_spec) << "const";
@@ -17795,24 +17724,35 @@ Decl *Sema::ActOnFriendTypeDecl(Scope *S, const DeclSpec &DS,
   if (DiagnoseUnexpandedParameterPack(Loc, TSI, UPPC_FriendDeclaration))
     return nullptr;
 
-  // This is definitely an error in C++98.  It's probably meant to
-  // be forbidden in C++0x, too, but the specification is just
-  // poorly written.
-  //
-  // The problem is with declarations like the following:
-  //   template <T> friend A<T>::foo;
-  // where deciding whether a class C is a friend or not now hinges
-  // on whether there exists an instantiation of A that causes
-  // 'foo' to equal C.  There are restrictions on class-heads
-  // (which we declare (by fiat) elaborated friend declarations to
-  // be) that makes this tractable.
-  //
-  // FIXME: handle "template <> friend class A<T>;", which
-  // is possibly well-formed?  Who even knows?
-  if (TempParams.size() && !T->isElaboratedTypeSpecifier()) {
-    Diag(Loc, diag::err_tagless_friend_type_template)
-      << DS.getSourceRange();
-    return nullptr;
+  if (!T->isElaboratedTypeSpecifier()) {
+    if (TempParams.size()) {
+      // C++23 [dcl.pre]p5:
+      //   In a simple-declaration, the optional init-declarator-list can be
+      //   omitted only when declaring a class or enumeration, that is, when
+      //   the decl-specifier-seq contains either a class-specifier, an
+      //   elaborated-type-specifier with a class-key, or an enum-specifier.
+      //
+      // The declaration of a template-declaration or explicit-specialization
+      // is never a member-declaration, so this must be a simple-declaration
+      // with no init-declarator-list. Therefore, this is ill-formed.
+      Diag(Loc, diag::err_tagless_friend_type_template) << DS.getSourceRange();
+      return nullptr;
+    } else if (const RecordDecl *RD = T->getAsRecordDecl()) {
+      SmallString<16> InsertionText(" ");
+      InsertionText += RD->getKindName();
+
+      Diag(Loc, getLangOpts().CPlusPlus11
+                    ? diag::warn_cxx98_compat_unelaborated_friend_type
+                    : diag::ext_unelaborated_friend_type)
+          << (unsigned)RD->getTagKind() << T
+          << FixItHint::CreateInsertion(getLocForEndOfToken(FriendLoc),
+                                        InsertionText);
+    } else {
+      Diag(FriendLoc, getLangOpts().CPlusPlus11
+                          ? diag::warn_cxx98_compat_nonclass_type_friend
+                          : diag::ext_nonclass_type_friend)
+          << T << DS.getSourceRange();
+    }
   }
 
   // C++98 [class.friend]p1: A friend of a class is a function
@@ -17828,12 +17768,11 @@ Decl *Sema::ActOnFriendTypeDecl(Scope *S, const DeclSpec &DS,
 
   Decl *D;
   if (!TempParams.empty())
-    D = FriendTemplateDecl::Create(Context, CurContext, Loc,
-                                   TempParams,
-                                   TSI,
-                                   DS.getFriendSpecLoc());
+    D = FriendTemplateDecl::Create(Context, CurContext, Loc, TempParams, TSI,
+                                   FriendLoc);
   else
-    D = CheckFriendTypeDecl(Loc, DS.getFriendSpecLoc(), TSI);
+    D = FriendDecl::Create(Context, CurContext, TSI->getTypeLoc().getBeginLoc(),
+                           TSI, FriendLoc);
 
   if (!D)
     return nullptr;

clang/lib/Sema/SemaTemplateInstantiateDecl.cpp

@@ -1407,11 +1407,8 @@ Decl *TemplateDeclInstantiator::VisitFriendDecl(FriendDecl *D) {
     if (!InstTy)
       return nullptr;
 
-    FriendDecl *FD = SemaRef.CheckFriendTypeDecl(D->getBeginLoc(),
-                                                 D->getFriendLoc(), InstTy);
-    if (!FD)
-      return nullptr;
-
+    FriendDecl *FD = FriendDecl::Create(
+        SemaRef.Context, Owner, D->getLocation(), InstTy, D->getFriendLoc());
     FD->setAccess(AS_public);
     FD->setUnsupportedFriend(D->isUnsupportedFriend());
     Owner->addDecl(FD);

clang/test/CXX/dcl.dcl/dcl.spec/dcl.type/dcl.type.elab/p3.cpp

@@ -16,10 +16,10 @@ class A1 {
   friend union A; // expected-error {{use of 'A' with tag type that does not match previous declaration}}
 
   friend enum A; // expected-error {{use of 'A' with tag type that does not match previous declaration}}
-  friend enum E; 
-#if __cplusplus <= 199711L // C++03 or earlier modes
-  // expected-warning@-2 {{befriending enumeration type 'enum E' is a C++11 extension}}
-#endif
+                 // expected-warning@-1 {{cannot be declared as a friend}}
+                 // expected-note@-2 {{remove 'enum' to befriend an enum}}
+  friend enum E; // expected-warning {{cannot be declared as a friend}}
+                 // expected-note@-1 {{remove 'enum' to befriend an enum}}
 };
 
 template <class T> struct B { // expected-note {{previous use is here}}

clang/test/CXX/dcl.dcl/dcl.spec/dcl.type/dcl.type.elab/p4.cpp

@@ -0,0 +1,40 @@
+// RUN: %clang_cc1 -verify %s -std=c++11 -pedantic-errors
+
+enum class E;
+
+template<typename T>
+struct A {
+  enum class F;
+};
+
+struct B {
+  template<typename T>
+  friend enum A<T>::F; // expected-error {{elaborated enum specifier cannot be declared as a friend}}
+                       // expected-note@-1 {{remove 'enum' to befriend an enum}}
+
+  // FIXME: Per [temp.expl.spec]p19, a friend declaration cannot be an explicit specialization
+  template<>
+  friend enum A<int>::F; // expected-error {{elaborated enum specifier cannot be declared as a friend}}
+                         // expected-note@-1 {{remove 'enum' to befriend an enum}}
+
+  enum class G;
+
+  friend enum E; // expected-error {{elaborated enum specifier cannot be declared as a friend}}
+                 // expected-note@-1 {{remove 'enum' to befriend an enum}}
+};
+
+template<typename T>
+struct C {
+  friend enum T::G; // expected-error {{elaborated enum specifier cannot be declared as a friend}}
+                    // expected-note@-1 {{remove 'enum' to befriend an enum}}
+  friend enum A<T>::G; // expected-error {{elaborated enum specifier cannot be declared as a friend}}
+                       // expected-note@-1 {{remove 'enum' to befriend an enum}}
+};
+
+struct D {
+  friend enum B::G; // expected-error {{elaborated enum specifier cannot be declared as a friend}}
+                    // expected-note@-1 {{remove 'enum' to befriend an enum}}
+  friend enum class B::G; // expected-error {{elaborated enum specifier cannot be declared as a friend}}
+                          // expected-note@-1 {{remove 'enum class' to befriend an enum}}
+                          // expected-error@-2 {{reference to enumeration must use 'enum' not 'enum class'}}
+};