4 files changed, 28 insertions, 2 deletions
diff --git a/clang/lib/Sema/DeclSpec.cpp b/clang/lib/Sema/DeclSpec.cpp
index 0f20d10b076..d6f8f91bbf1 100644
--- a/clang/lib/Sema/DeclSpec.cpp
+++ b/clang/lib/Sema/DeclSpec.cpp
@@ -309,6 +309,7 @@ const char *DeclSpec::getSpecifierName(DeclSpec::TST T) {
   case DeclSpec::TST_typeofExpr:  return "typeof";
   case DeclSpec::TST_auto:        return "auto";
   case DeclSpec::TST_decltype:    return "(decltype)";
+  case DeclSpec::TST_underlying_type: return "__underlying_type";
   case DeclSpec::TST_unknown_anytype: return "__unknown_anytype";
   case DeclSpec::TST_error:       return "(error)";
   }
diff --git a/clang/lib/Sema/SemaDecl.cpp b/clang/lib/Sema/SemaDecl.cpp
index 7c6cde2296b..fd28f2b6ee9 100644
--- a/clang/lib/Sema/SemaDecl.cpp
+++ b/clang/lib/Sema/SemaDecl.cpp
@@ -2864,7 +2864,8 @@ static bool RebuildDeclaratorInCurrentInstantiation(Sema &S, Declarator &D,
   switch (DS.getTypeSpecType()) {
   case DeclSpec::TST_typename:
   case DeclSpec::TST_typeofType:
-  case DeclSpec::TST_decltype: {
+  case DeclSpec::TST_decltype:
+  case DeclSpec::TST_underlying_type: {
     // Grab the type from the parser.
     TypeSourceInfo *TSI = 0;
     QualType T = S.GetTypeFromParser(DS.getRepAsType(), &TSI);
diff --git a/clang/lib/Sema/SemaTemplateVariadic.cpp b/clang/lib/Sema/SemaTemplateVariadic.cpp
index 096d353bccc..100c6a77a06 100644
--- a/clang/lib/Sema/SemaTemplateVariadic.cpp
+++ b/clang/lib/Sema/SemaTemplateVariadic.cpp
@@ -617,7 +617,8 @@ bool Sema::containsUnexpandedParameterPacks(Declarator &D) {
   const DeclSpec &DS = D.getDeclSpec();
   switch (DS.getTypeSpecType()) {
   case TST_typename:
-  case TST_typeofType: {
+  case TST_typeofType:
+  case TST_underlying_type: {
     QualType T = DS.getRepAsType().get();
     if (!T.isNull() && T->containsUnexpandedParameterPack())
       return true;
diff --git a/clang/lib/Sema/SemaType.cpp b/clang/lib/Sema/SemaType.cpp
index 06548a44ddb..dbacb47812a 100644
--- a/clang/lib/Sema/SemaType.cpp
+++ b/clang/lib/Sema/SemaType.cpp
@@ -836,6 +836,29 @@ static QualType ConvertDeclSpecToType(Sema &S, TypeProcessingState &state) {
     }
     break;
   }
+  case DeclSpec::TST_underlying_type:
+    // FIXME: Preserve type source info?
+    Result = S.GetTypeFromParser(DS.getRepAsType());
+    assert(!Result.isNull() && "Didn't get a type for __underlying_type?");
+    if (!Result->isDependentType()) {
+      if (Result->isEnumeralType()) {
+        EnumDecl *ED = Result->getAs<EnumType>()->getDecl();
+        S.DiagnoseUseOfDecl(ED, DS.getTypeSpecTypeLoc());
+        QualType UnderlyingType = ED->getIntegerType();
+        if (UnderlyingType.isNull()) {
+          declarator.setInvalidType(true);
+          Result = Context.IntTy;
+        } else {
+          Result = UnderlyingType;
+        }
+      } else {
+        S.Diag(DS.getTypeSpecTypeLoc(),
+               diag::err_only_enums_have_underlying_types);
+        Result = Context.IntTy;
+      }
+    }
+    break; 
+
   case DeclSpec::TST_auto: {
     // TypeQuals handled by caller.
     Result = Context.getAutoType(QualType());