[Sema] Diagnose default argument on a parameter pack.

This is ill-formed (and cannot be used anyways).

PR23028.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@233376 91177308-0d34-0410-b5e6-96231b3b80d8

include/clang/Basic/DiagnosticSemaKinds.td

@@ -2808,6 +2808,8 @@ def err_param_default_argument_member_template_redecl : Error<
   "default arguments cannot be added to an out-of-line definition of a member "
   "of a %select{class template|class template partial specialization|nested "
   "class in a template}0">;
+def err_param_default_argument_on_parameter_pack : Error<
+  "parameter pack cannot have a default argument">;
 def err_uninitialized_member_for_assign : Error<
   "cannot define the implicit copy assignment operator for %0, because "
   "non-static %select{reference|const}1 member %2 can't use copy "

lib/Sema/SemaDeclCXX.cpp

@@ -316,8 +316,17 @@ Sema::ActOnParamDefaultArgument(Decl *param, SourceLocation EqualLoc,
   if (DiagnoseUnexpandedParameterPack(DefaultArg, UPPC_DefaultArgument)) {
     Param->setInvalidDecl();
     return;
-  }    
-      
+  }
+
+  // C++11 [dcl.fct.default]p3
+  //   A default argument expression [...] shall not be specified for a
+  //   parameter pack.
+  if (Param->isParameterPack()) {
+    Diag(EqualLoc, diag::err_param_default_argument_on_parameter_pack)
+        << DefaultArg->getSourceRange();
+    return;
+  }
+
   // Check that the default argument is well-formed
   CheckDefaultArgumentVisitor DefaultArgChecker(DefaultArg, this);
   if (DefaultArgChecker.Visit(DefaultArg)) {

test/CXX/dcl.decl/dcl.meaning/dcl.fct.default/p3.cpp

@@ -14,3 +14,6 @@ struct X0 {
   
   void mem8(int (*fp)(int) = (int (*)(int = 17))0); // expected-error{{default arguments can only be specified for parameters in a function declaration}}  
 };
+
+template <typename... Ts>
+void defaultpack(Ts... = 0) {} // expected-error{{parameter pack cannot have a default argument}}