Implement the part of C++0x [temp.arg.explicit]p3 that pertains to

parameter packs. In particular, a parameter pack not otherwise deduced
is deduced to an empty parameter pack.

The C++0x wording here is a bit unfortunate; this should really only
apply to function templates, and it mentions "trailing" parameter
packs, which doesn't really make sense in the context of function
templates. Will file a core issue separately.

llvm-svn: 122463

2 files changed, 38 insertions, 1 deletions

diff --git a/clang/lib/Sema/SemaTemplateDeduction.cpp b/clang/lib/Sema/SemaTemplateDeduction.cpp
index ad649c067c6..90894f22a36 100644
--- a/clang/lib/Sema/SemaTemplateDeduction.cpp
+++ b/clang/lib/Sema/SemaTemplateDeduction.cpp
@@ -1708,9 +1708,10 @@ Sema::FinishTemplateArgumentDeduction(FunctionTemplateDecl *FunctionTemplate,
   //   [...] or if any template argument remains neither deduced nor
   //   explicitly specified, template argument deduction fails.
   llvm::SmallVector<TemplateArgument, 4> Builder;
-  for (unsigned I = 0, N = Deduced.size(); I != N; ++I) {
+  for (unsigned I = 0, N = Deduced.size(); I != N; ++I) {    
     // FIXME: Variadic templates. Unwrap argument packs?
     NamedDecl *Param = FunctionTemplate->getTemplateParameters()->getParam(I);
+    
     if (!Deduced[I].isNull()) {
       if (I < NumExplicitlySpecified) {
         // We have already fully type-checked and converted this
@@ -1777,6 +1778,15 @@ Sema::FinishTemplateArgumentDeduction(FunctionTemplateDecl *FunctionTemplate,
 
       continue;
     }
+    
+    // C++0x [temp.arg.explicit]p3:
+    //    A trailing template parameter pack (14.5.3) not otherwise deduced will 
+    //    be deduced to an empty sequence of template arguments.
+    // FIXME: Where did the word "trailing" come from?
+    if (Param->isTemplateParameterPack()) {
+      Builder.push_back(TemplateArgument(0, 0));
+      continue;
+    }
 
     // Substitute into the default template argument, if available. 
     TemplateArgumentLoc DefArg
diff --git a/clang/test/CXX/temp/temp.fct.spec/temp.arg.explicit/p3-0x.cpp b/clang/test/CXX/temp/temp.fct.spec/temp.arg.explicit/p3-0x.cpp
new file mode 100644
index 00000000000..46d70b671f6
--- /dev/null
+++ b/clang/test/CXX/temp/temp.fct.spec/temp.arg.explicit/p3-0x.cpp
@@ -0,0 +1,27 @@
+// RUN: %clang_cc1 -std=c++0x -fsyntax-only -verify %s
+
+namespace ParameterPacksWithFunctions {
+  template<typename ...> struct count;
+
+  template<typename Head, typename ...Tail>
+  struct count<Head, Tail...> {
+    static const unsigned value = 1 + count<Tail...>::value;
+  };
+
+  template<>
+  struct count<> {
+    static const unsigned value = 0;
+  };
+  
+  template<unsigned> struct unsigned_c { };
+
+  template<typename ... Types>
+  unsigned_c<count<Types...>::value> f();
+
+  void test_f() {
+    unsigned_c<0> uc0a = f(); // okay, deduced to an empty pack
+    unsigned_c<0> uc0b = f<>();
+    unsigned_c<1> uc1 = f<int>();
+    unsigned_c<2> uc2 = f<float, double>();
+  }
+}