Don't drop dllimport from qualified friend redeclarations (PR20512)

[hans]

Summary

This matches MSVC's logic, which seems to be that when then friend declaration is qualified, it cannot be a declaration of a new symbol and so the dll linkage doesn't change.

[rnk]

lgtm

[hans]

Closed by commit rL214774 (authored by @hans).

cfe/trunk/lib/Sema/SemaDecl.cpp

S.Diag(NewDecl->getLocation(), diag::err_attribute_dll_redeclaration)
       << NewDecl
       << (NewImportAttr ? (const Attr *)NewImportAttr : NewExportAttr);
     S.Diag(OldDecl->getLocation(), diag::note_previous_declaration);
     NewDecl->setInvalidDecl();
     return;
   }
 
   // A redeclaration is not allowed to drop a dllimport attribute, the only
-  // exceptions being inline function definitions and local extern declarations.
+  // exceptions being inline function definitions, local extern declarations,
+  // and qualified friend declarations.
   // NB: MSVC converts such a declaration to dllexport.
-  bool IsInline = false, IsStaticDataMember = false;
+  bool IsInline = false, IsStaticDataMember = false, IsQualifiedFriend = false;
   if (const auto *VD = dyn_cast<VarDecl>(NewDecl))
     // Ignore static data because out-of-line definitions are diagnosed
     // separately.
     IsStaticDataMember = VD->isStaticDataMember();
-  else if (const auto *FD = dyn_cast<FunctionDecl>(NewDecl))
+  else if (const auto *FD = dyn_cast<FunctionDecl>(NewDecl)) {
     IsInline = FD->isInlined();
+    IsQualifiedFriend = FD->getQualifier() &&
+                        FD->getFriendObjectKind() == Decl::FOK_Declared;
+  }
 
   if (OldImportAttr && !HasNewAttr && !IsInline && !IsStaticDataMember &&
-      !NewDecl->isLocalExternDecl()) {
+      !NewDecl->isLocalExternDecl() && !IsQualifiedFriend) {
     S.Diag(NewDecl->getLocation(),
            diag::warn_redeclaration_without_attribute_prev_attribute_ignored)
       << NewDecl << OldImportAttr;
     S.Diag(OldDecl->getLocation(), diag::note_previous_declaration);
     S.Diag(OldImportAttr->getLocation(), diag::note_previous_attribute);
     OldDecl->dropAttr<DLLImportAttr>();
     NewDecl->dropAttr<DLLImportAttr>();
   }

cfe/trunk/test/CodeGenCXX/dllimport.cpp

 
 // Friend functions
 // MSC-DAG: declare dllimport void @"\01?friend1@@YAXXZ"()
 // GNU-DAG: declare dllimport void @_Z7friend1v()
 // MSC-DAG: declare           void @"\01?friend2@@YAXXZ"()
 // GNU-DAG: declare           void @_Z7friend2v()
 // MSC-DAG: define            void @"\01?friend3@@YAXXZ"()
 // GNU-DAG: define            void @_Z7friend3v()
+// MSC-DAG: declare           void @"\01?friend4@@YAXXZ"()
+// GNU-DAG: declare           void @_Z7friend4v()
+// MSC-DAG: declare dllimport void @"\01?friend5@@YAXXZ"()
+// GNU-DAG: declare dllimport void @_Z7friend5v()
+
 struct FuncFriend {
   friend __declspec(dllimport) void friend1();
   friend __declspec(dllimport) void friend2();
   friend __declspec(dllimport) void friend3();
 };
 __declspec(dllimport) void friend1();
                       void friend2(); // dllimport ignored
                       void friend3() {} // dllimport ignored
+
+__declspec(dllimport) void friend4();
+__declspec(dllimport) void friend5();
+struct FuncFriendRedecl {
+  friend void friend4(); // dllimport ignored
+  friend void ::friend5();
+};
 USE(friend1)
 USE(friend2)
 USE(friend3)
+USE(friend4)
+USE(friend5)
 
 // Implicit declarations can be redeclared with dllimport.
 // MSC-DAG: declare dllimport noalias i8* @"\01??2@{{YAPAXI|YAPEAX_K}}@Z"(
 // GNU-DAG: declare dllimport noalias i8* @_Znw{{[yj]}}(
 __declspec(dllimport) void* operator new(__SIZE_TYPE__ n);
 void UNIQ(use)() { ::operator new(42); }
 
 // MSC-DAG: declare dllimport void @"\01?externalFunc@ns@@YAXXZ"()

cfe/trunk/test/SemaCXX/dllimport.cpp

struct FuncFriend {
   friend                       void friend5(); // expected-note{{previous declaration is here}}
 };
 __declspec(dllimport) void friend1();
                       void friend2(); // expected-warning{{'friend2' redeclared without 'dllimport' attribute: previous 'dllimport' ignored}}
                       void friend3() {} // expected-warning{{'friend3' redeclared without 'dllimport' attribute: previous 'dllimport' ignored}}
 __declspec(dllimport) void friend4(); // expected-error{{redeclaration of 'friend4' cannot add 'dllimport' attribute}}
 __declspec(dllimport) inline void friend5() {} // expected-error{{redeclaration of 'friend5' cannot add 'dllimport' attribute}}
 
+void __declspec(dllimport) friend6(); // expected-note{{previous declaration is here}} expected-note{{previous attribute is here}}
+void __declspec(dllimport) friend7();
+struct FuncFriend2 {
+  friend void friend6(); // expected-warning{{'friend6' redeclared without 'dllimport' attribute: previous 'dllimport' ignored}}
+  friend void ::friend7();
+};
+
 // Implicit declarations can be redeclared with dllimport.
 __declspec(dllimport) void* operator new(__SIZE_TYPE__ n);
 
 // External linkage is required.
 __declspec(dllimport) static int staticFunc(); // expected-error{{'staticFunc' must have external linkage when declared 'dllimport'}}
 __declspec(dllimport) Internal internalRetFunc(); // expected-error{{'internalRetFunc' must have external linkage when declared 'dllimport'}}
 namespace    { __declspec(dllimport) void internalFunc(); } // expected-error{{'(anonymous namespace)::internalFunc' must have external linkage when declared 'dllimport'}}
 namespace ns { __declspec(dllimport) void externalFunc(); }