PR37979: integral promotions in C++ treat enum bit-fields like enums,

not like bit-fields.

We used to get this right "by accident", because conversions for the
selected built-in overloaded operator would convert the enum bit-field
to its corresponding underlying type early. But after DR1687 that no
longer happens.

Technically this change should also apply to C, where bit-fields only
have special promotion rules if the bit-field's declared type is
_Bool, int, signed int, or unsigned int, but for GCC compatibility we
only look at the bit-width and not the underlying type when performing
bit-field integral promotions in C.

llvm-svn: 335925

1 files changed, 13 insertions, 0 deletions

diff --git a/clang/lib/Sema/SemaOverload.cpp b/clang/lib/Sema/SemaOverload.cpp
index e9d2eb6cc5c..6f01f19fc51 100644
--- a/clang/lib/Sema/SemaOverload.cpp
+++ b/clang/lib/Sema/SemaOverload.cpp
@@ -2007,6 +2007,14 @@ bool Sema::IsIntegralPromotion(Expr *From, QualType FromType, QualType ToType) {
         isCompleteType(From->getLocStart(), FromType))
       return Context.hasSameUnqualifiedType(
           ToType, FromEnumType->getDecl()->getPromotionType());
+
+    // C++ [conv.prom]p5:
+    //   If the bit-field has an enumerated type, it is treated as any other
+    //   value of that type for promotion purposes.
+    //
+    // ... so do not fall through into the bit-field checks below in C++.
+    if (getLangOpts().CPlusPlus)
+      return false;
   }
 
   // C++0x [conv.prom]p2:
@@ -2054,6 +2062,11 @@ bool Sema::IsIntegralPromotion(Expr *From, QualType FromType, QualType ToType) {
   // other value of that type for promotion purposes (C++ 4.5p3).
   // FIXME: We should delay checking of bit-fields until we actually perform the
   // conversion.
+  //
+  // FIXME: In C, only bit-fields of types _Bool, int, or unsigned int may be
+  // promoted, per C11 6.3.1.1/2. We promote all bit-fields (including enum
+  // bit-fields and those whose underlying type is larger than int) for GCC
+  // compatibility.
   if (From) {
     if (FieldDecl *MemberDecl = From->getSourceBitField()) {
       llvm::APSInt BitWidth;