Correctly handle packs for variadic type traits.

I'm not sure how to write a test for this; the following shows the
difference in -ast-dump:

template <int x> struct A {};
template <class T> struct B { };
template <class ...Args> using C = A<(__is_trivially_constructible(Args...))>;
template <class ...Args> using D = C<B<Args>...>;

However, I can't seem to write a test that triggers a visible difference
in behavior.

llvm-svn: 186726

clang/lib/Sema/TreeTransform.h

@@ -7926,6 +7926,19 @@ TreeTransform<Derived>::TransformTypeTraitExpr(TypeTraitExpr *E) {
       if (To.isNull())
         return ExprError();
 
+      if (To->containsUnexpandedParameterPack()) {
+        To = getDerived().RebuildPackExpansionType(To,
+                                                   PatternTL.getSourceRange(),
+                                                   ExpansionTL.getEllipsisLoc(),
+                                                   NumExpansions);
+        if (To.isNull())
+          return ExprError();
+
+        PackExpansionTypeLoc ToExpansionTL
+          = TLB.push<PackExpansionTypeLoc>(To);
+        ToExpansionTL.setEllipsisLoc(ExpansionTL.getEllipsisLoc());
+      }
+
       Args.push_back(TLB.getTypeSourceInfo(SemaRef.Context, To));
     }