Non-zero-length bit-fields make a class non-empty.

This implements the rule intended by the standard (see LWG 2358)
and the rule intended by the Itanium C++ ABI (see
https://github.com/itanium-cxx-abi/cxx-abi/pull/51), and makes
Clang match the behavior of GCC, ICC, and MSVC.

A pedantic reading of both the standard and the ABI indicate that Clang
is currently technically correct, but that's not worth much when it's
clear that the wording is wrong in both those places.

This is an ABI break for classes that derive from a class that is empty
other than one or more unnamed non-zero-length bit-fields. Such cases
are expected to be rare, but -fclang-abi-compat=6 restores the old
behavior just in case.

Differential Revision: https://reviews.llvm.org/D45174

llvm-svn: 331620

1 files changed, 13 insertions, 9 deletions

diff --git a/clang/lib/AST/DeclCXX.cpp b/clang/lib/AST/DeclCXX.cpp
index dd653d8f573..f3b3ed0fa29 100644
--- a/clang/lib/AST/DeclCXX.cpp
+++ b/clang/lib/AST/DeclCXX.cpp
@@ -856,10 +856,10 @@ void CXXRecordDecl::addedMember(Decl *D) {
     return;
   }
 
-  ASTContext &Context = getASTContext();
-
   // Handle non-static data members.
   if (const auto *Field = dyn_cast<FieldDecl>(D)) {
+    ASTContext &Context = getASTContext();
+
     // C++2a [class]p7:
     //   A standard-layout class is a class that:
     //    [...]
@@ -872,8 +872,16 @@ void CXXRecordDecl::addedMember(Decl *D) {
     //   A declaration for a bit-field that omits the identifier declares an 
     //   unnamed bit-field. Unnamed bit-fields are not members and cannot be 
     //   initialized.
-    if (Field->isUnnamedBitfield())
+    if (Field->isUnnamedBitfield()) {
+      // C++ [meta.unary.prop]p4: [LWG2358]
+      //   T is a class type [...] with [...] no unnamed bit-fields of non-zero
+      //   length
+      if (data().Empty && !Field->isZeroLengthBitField(Context) &&
+          Context.getLangOpts().getClangABICompat() >
+              LangOptions::ClangABI::Ver6)
+        data().Empty = false;
       return;
+    }
     
     // C++11 [class]p7:
     //   A standard-layout class is a class that:
@@ -1220,12 +1228,8 @@ void CXXRecordDecl::addedMember(Decl *D) {
     }
 
     // C++14 [meta.unary.prop]p4:
-    //   T is a class type [...] with [...] no non-static data members other
-    //   than bit-fields of length 0...
-    if (data().Empty) {
-      if (!Field->isZeroLengthBitField(Context))
-        data().Empty = false;
-    }
+    //   T is a class type [...] with [...] no non-static data members
+    data().Empty = false;
   }
   
   // Handle using declarations of conversion functions.