Implements CWG 1601 in [over.ics.rank/4.2]

Summary:
The overload resolution for enums with a fixed underlying type has changed in the C++14 standard. This patch implements the new rule.

Patch by Mark de Wever!

Reviewers: rsmith

Reviewed By: rsmith

Subscribers: cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D65695

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@373866 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 42 insertions, 0 deletions

diff --git a/lib/Sema/SemaOverload.cpp b/lib/Sema/SemaOverload.cpp
index 3db8e3d2e5..80bd3562bc 100644
--- a/lib/Sema/SemaOverload.cpp
+++ b/lib/Sema/SemaOverload.cpp
@@ -3765,6 +3765,34 @@ isBetterReferenceBindingKind(const StandardConversionSequence &SCS1,
           !SCS2.IsLvalueReference && SCS2.BindsToFunctionLvalue);
 }
 
+enum class FixedEnumPromotion {
+  None,
+  ToUnderlyingType,
+  ToPromotedUnderlyingType
+};
+
+/// Returns kind of fixed enum promotion the \a SCS uses.
+static FixedEnumPromotion
+getFixedEnumPromtion(Sema &S, const StandardConversionSequence &SCS) {
+
+  if (SCS.Second != ICK_Integral_Promotion)
+    return FixedEnumPromotion::None;
+
+  QualType FromType = SCS.getFromType();
+  if (!FromType->isEnumeralType())
+    return FixedEnumPromotion::None;
+
+  EnumDecl *Enum = FromType->getAs<EnumType>()->getDecl();
+  if (!Enum->isFixed())
+    return FixedEnumPromotion::None;
+
+  QualType UnderlyingType = Enum->getIntegerType();
+  if (S.Context.hasSameType(SCS.getToType(1), UnderlyingType))
+    return FixedEnumPromotion::ToUnderlyingType;
+
+  return FixedEnumPromotion::ToPromotedUnderlyingType;
+}
+
 /// CompareStandardConversionSequences - Compare two standard
 /// conversion sequences to determine whether one is better than the
 /// other or if they are indistinguishable (C++ 13.3.3.2p3).
@@ -3806,6 +3834,20 @@ CompareStandardConversionSequences(Sema &S, SourceLocation Loc,
              ? ImplicitConversionSequence::Better
              : ImplicitConversionSequence::Worse;
 
+  // C++14 [over.ics.rank]p4b2:
+  // This is retroactively applied to C++11 by CWG 1601.
+  //
+  //   A conversion that promotes an enumeration whose underlying type is fixed
+  //   to its underlying type is better than one that promotes to the promoted
+  //   underlying type, if the two are different.
+  FixedEnumPromotion FEP1 = getFixedEnumPromtion(S, SCS1);
+  FixedEnumPromotion FEP2 = getFixedEnumPromtion(S, SCS2);
+  if (FEP1 != FixedEnumPromotion::None && FEP2 != FixedEnumPromotion::None &&
+      FEP1 != FEP2)
+    return FEP1 == FixedEnumPromotion::ToUnderlyingType
+               ? ImplicitConversionSequence::Better
+               : ImplicitConversionSequence::Worse;
+
   // C++ [over.ics.rank]p4b2:
   //
   //   If class B is derived directly or indirectly from class A,